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CONTENTS
- Diagnosing Problems before a Repair
- Oil Consumption
- Oil Leaks
- Fuel Mixture Problems
- Compression Loss
- Engine Noises
- Oil Pressure Problems
- Cooling System Problems
- Electronic Failures/Engine Damage
OBJECTIVES
Upon completion of this chapter, you should be able to:
- Use engine diagnostic tools and equipment safely and properly.
- Diagnose engine and related problems and determine the proper repair procedure.
INTRODUCTION
This chapter focuses on how to troubleshoot problems on a running engine. Also discussed are some of the external causes of engine problems that
can result in a repeat of a previous engine failure, if allowed to continue unresolved. Internal engine parts are shown here to illustrate some of the causes and results of these problems. Internal problem diagnosis after disassembly is also covered in more detail in subsequent chapters. It is very important that you diagnose the cause of a problem before performing a repair. It is not unusual for an inexperienced technician to spend many hours of work only to discover that the repair was unnecessary. Five major diagnosis areas are covered:
- Possible reasons for oil consumption
- Causes of rough running or a loss of engine power
- Engine noises
- Oil pressure problems
- Cooling system problems There are many causes of engine problems. Some are the result of normal wear and tear or a lack of maintenance. Engine problems also might be due to previous work on the engine. Problems that appear to be engine-related can also be caused by other automotive specialty areas, such as the transmission or emission controls. Some- times a problem with a system causes an engine to fail. If the problem is not taken care of, the fail- ure will recur.
Diagnosing Engine
Problems
VINTAGE ENGINES
In the past, gasoline stations provided underhood service, and most of them had an adjoining service facility. Today many of the service facilities have been converted to mini-markets. One result of self-service is that engines are suffering from a lack of maintenance.
C H A P T E R
CHAPTER 3 Diagnosing Engine Problems • 55
inside of the exhaust pipe. Black soot at the exhaust pipe often indicates an overly rich air-fuel mixture, not oil consumption. The rate of normal oil consumption depends on such things as the size of the engine, the weight and shape of the vehicle, the viscosity and service rating of the oil, engine rpm and load during use, engine temperature, and the amount of oxidation and dilu- tion of the oil. Information about oil is covered in detail in Chapter 14. From time to time an owner will complain of an occasional rapid oil loss. This might be a normal condition that sometimes occurs after 1000 or more miles of city driving followed by a highway trip. City driving can result in extra fuel and water dilu- tion in the oil. Before leaving on a long vacation trip, the customer checks the oil and the dipstick regis- ters “full.” But when the diluted oil becomes thor- oughly heated, evaporation of the pollutants gives the appearance of rapid oil consumption as the oil level drops a quart in a few hundred miles.
Bad Valve Guides or Seals The cause of internal oil consumption is often worn valve guides or defective valve guide seals. There might be exhaust smoke during deceleration because of oil leaking into the combustion chamber through the intake valve guides. Deceleration causes very high engine vacuum, which pulls oil into the combustion chamber. A spark plug that is oil fouled on only one side indicates leaking valve guide seals. Carbon depos- its on the necks of the intake valves are another indication ( Figure 3.2 ). Look for carbon deposits when disassembling the cylinder head.
This chapter should serve as a reference for future problems. The descriptions of various prob- lems are listed in the index at the back of the book. More in-depth training in engine diagnosis comes under the overlapping specialty area of engine performance.
DIAGNOSING PROBLEMS BEFORE A REPAIR An engine should be correctly diagnosed before disassembly for two reasons. It should be deter- mined that an overhaul is really necessary. The dam- aged starter motor drive frame shown in Figure 3. resulted in a diagnosis of catastrophic engine failure. Failed belt-driven accessories can also lead you to believe that there is a seized engine. The exact location of a problem should be deter- mined while the engine is running. A thorough discussion of the problem with the owner of the vehicle is also helpful. Sometimes an owner’s driv- ing habits or maintenance procedures can be the cause of the problem.
OIL CONSUMPTION Piston rings are usually the first thing a customer suspects when a car starts to use oil, even though oil can be lost through a variety of other conditions. Oil loss can be due to either external leakage or internal oil consumption. Excessive internal oil consumption can sometimes be spotted as an oily coating on the
FIGURE 3.1 This damaged starter motor was the cause of a “frozen” engine. The starter was repaired, and the engine ran fine. (Courtesy of Tim Gilles)
FIGURE 3.2 Carbon on the neck of a valve. (Courtesy of Tim Gilles)
CHAPTER 3 Diagnosing Engine Problems • 57
consumption might be caused by worn or stuck piston rings (see Figure 4.107).
Excessive Rod Bearing Clearance A high mileage engine will probably have worn crankshaft bearings. Excessive bearing-to- crankshaft journal oil clearance can result in low oil pressure at idle. Oil consumption can increase at higher engine rpm as oil leaks out between the connecting rod journal and the rod bearing. At higher engine speeds, this can result in too much oil being thrown onto the cylinder walls, overwhelm- ing the oil rings, which cannot return all of the oil to the crankcase ( Figure 3.6 ). Whatever oil enters the combustion chamber will be burned with the air- fuel mixture. Even with normal rod bearing clear- ance, high-speed driving can cause increased oil consumption due to extra oil thrown from the rods. In one test, an engine run at 70 mph used seven times the oil that it used at 40 mph.
Incorrect Engine Oil Dipstick It is especially important to make sure that the correct oil dipstick is used after an engine change or short block installation. Manufacturers often install the same engine in different vehicle models. Depending on the vehicle, the engine can be equipped with a different oil pan, which sometimes
A spark plug deposit test can check for internal oil consumption. During this test:
- The engine’s initial spark timing is retarded to 10° after top dead center (ATDC).
- The engine is idled at 500 rpm for 15 minutes.
- Next, the spark plugs are examined for oil wetting.
- The engine should not be run above idle speed until the timing is returned to specification because it will overheat.
NOTE Retarded timing can cause exhaust valves to run dangerously hot.
Spark plugs provide a window to conditions in the cylinder because they extend into the combus- tion chamber. Figure 3.5 shows abnormal spark plug conditions associated with oil consumption.
Increased Oil Consumption after a Valve Job Consider the condition of the entire engine before doing a valve job. If the rest of the engine is worn, sometimes an engine starts to consume oil after a valve job. The valve job increases compres- sion , but it also increases engine vacuum. This can result in more oil being sucked past the worn piston rings into the combustion chamber. When cylinder heads are removed for a valve job, look at the tops of the pistons to see if oil
FIGURE 3.5 Examples of oil-fouled spark plugs. (Courtesy of Tim Gilles)
FIGURE 3.6 Increased oil clearance causes more oil to be thrown onto cylinder walls.
58 • SECTION I Engine Construction, Diagnosis, Disassembly, and Inspection
engine and major service will soon be needed. The oil remains in the valve cover area instead of return- ing to the crankcase; it floods the valve guide, mak- ing the valve stem seal ineffective.
Leaking V-Type Intake Manifold Gasket Intake manifold vacuum can draw oil into the intake ports from the lifter-valley area under some intake manifolds ( Figure 3.8 ). This is a tough prob- lem to find. A smoke test is a good way of finding an intake manifold leak. A cranking vacuum is another way to test for internal air leaks before the engine is disassembled. These procedures are cov- ered later in this chapter. When removing an intake manifold, always visually inspect for the possibility of previous intake gasket leakage. V-type engines equipped with an exhaust gas recirculation ( EGR ) valve on the intake manifold often experience oil-fouling of the spark plugs that are closest to the EGR valve. This is caused when the intake manifold warps or the manifold gasket fails. Replace the gasket with one designed for high temperature applications.
Crankcase Pressure Normally, there is a slight vacuum in the crank- case. One possible reason for excessive oil leakage is a positive crankcase ventilation (PCV) valve that
requires a different length oil dipstick. Excessive oil consumption can result from too short a dipstick. Every time the owner mistakenly adds a quart of oil to the crankcase, the crankshaft throws the oil on the cylinder walls and the overfull engine burns off the excess.
Plugged Cylinder Head
Drainback Holes
When engine oil has not been changed often enough, thick sludge can plug the oil return holes in the cylinder head. These drainback holes allow rocker arm oil to return to the crankcase ( Figure 3.7 ). The problem can be temporarily solved by cleaning out the holes, but it is a symptom of a poorly maintained
VINTAGE ENGINES
Automatic transmissions on many older vehicles use a vacuum modulator. It senses engine load by using intake manifold vacuum to tell the transmission when to shift. If the diaphragm in the modulator leaks, transmission fluid will be sucked into the intake manifold. This can produce smoke that might be confused with engine oil smoke, even though the engine may be in perfect condition. One clue is when the spark plugs nearest the vacuum tap on the intake manifold become oil-fouled with automatic transmission fluid ( ATF ). A leaking modulator diaphragm can also cause rough engine idle because it allows air to leak into the intake manifold. Harsh, late transmission shifts, or no upshifts, are other symptoms. Be sure to question the vehicle owner thoroughly. The combination of any or all of these symptoms can lead an owner to believe that an engine overhaul is needed. The key to diagnosing a faulty vacuum modulator is that the engine is probably consuming ATF and the level drops consistently.
Clogged oil drain passage
No return to oil pan
FIGURE 3.7 Plugged oil drainback holes will cause exhaust smoke.
60 • SECTION I Engine Construction, Diagnosis, Disassembly, and Inspection
- With a long enough hose, the technician can drive the car while watching for pressure on the gauge.
- When running this test at idle, the breather should be plugged, but it should be unplugged during a road test.
A ring seal tester can be used to test an engine’s amount of blowby. It measures airflow out of the crankcase in cubic feet per minute (cfm). Normal airflow is about 5–8 cfm. Above 8 cfm indicates that the rings are not sealing properly. Unfiltered air allows dirt to enter the engine, causing engine wear. This can result from leaking vacuum hoses, vacuum control units, vacuum accessories, or manifold leaks. Crankcase pressure can also cause oil to migrate up the distributor shaft and into the distributor. To locate a leak in the PCV system:
- Seal the breather and PCV valve.
- Use a smoke tester (covered later) or blow (lightly) into the dipstick tube with a rubber- tipped blowgun (regulated to no more than 2– psi). Listen for leaks, using a piece of hose or a stethoscope with the metal end pulled off. A leak is often not readily apparent, especially at the top side of a valve cover gasket or where the intake manifold meets the block at the front or back. Oil might not leak out because of gravity and suc- tion from the crankcase vacuum of the PCV system.
SAFETY NOTE Fuel in the crankcase causes a dangerous condition to develop. A faulty mechanical fuel pump, fuel injector, or fuel pressure regulator will allow the crankcase to collect fuel. Also, fuel enters the crankcase when the engine has been flooded with raw fuel.
CASE HISTORY
A vehicle was brought into a shop for a tune-up. The technician decided that it would be easier to service the distributor if it was removed from the engine. Somehow the ignition switch was left in the “on” position. When the distributor was removed a spark resulted, igniting the air-fuel mixture in the crankcase and causing an explo- sion. The explosion blew the burning mixture out the ignition distributor hole, burning the techni- cian and catching his clothing on fire.
If the PCV system is to be effective, the entire crankcase must remain sealed. A leaking or misplaced gasket can cause enough air leakage to result in fail- ure of the PCV system. This includes the timing cover, oil pan, valve cover gaskets, and crankshaft seals. If the engine is not airtight, suction from the PCV valve will not create sufficient vacuum in the crankcase. An oily air cleaner, or oil in the hose to the air cleaner, often points to a crankcase pressure problem.
NOTE With the correct amount of suction in the crankcase, slight leakage from a gasket or seal should result in outside air leaking into the crankcase, rather than oil leaking out.
CASE HISTORY
A motor home was using excessive oil and leaking from the rear main crankshaft seal. A technician replaced the crankshaft seal, but the new seal con- tinued to leak. After performing some of the tests, the owner of the repair shop determined that the crankcase ventilation system was not working cor- rectly. Motor homes experience extremely high under-hood temperatures. The valve covers are located directly over the exhaust manifolds and are exposed to extreme heat. A valve cover gasket had developed a leak on the intake manifold side. Oil was not leaking out, but air was leaking in, causing the PCV system to be inoperative. Com- pounding the problem, the hose to the air cleaner was supposed to be a molded hose but someone had installed a piece of heater hose instead. The hose had become kinked and did not allow blowby to escape at highway speeds. The combination of these two problems caused internal oil consump- tion and leakage from the crankshaft seals.
SHOP TIP To check for suspected excessive crankcase pressure:
- Remove the oil dipstick.
- Put a pressure/vacuum tester on the oil dipstick tube.
CHAPTER 3 Diagnosing Engine Problems • 61
or torque converter. Oil on the opposite side indi- cates front transmission seal leakage. Oil that has been sprayed in a circular pattern is also indicative of a crankshaft seal leak. Most crank- shaft seal leaks are caused by excessive crankcase pressure. Oil leaks streaking down the block can be due to a leaking oil gallery plug, cam plug, or seal retainer block. The block could also be porous or cracked.
Black Light Testing. When black light testing , a 1-ounce bottle of fluorescent dye is added to engine oil to help locate leaks. When a black light is used, the source of the leak will be highlighted in bright yellow-green streaks. A mirror can be used to bounce the black light into hard-to-see areas. Wash- ing the engine first is helpful but not necessary. There are two types of ultraviolet (UV)-fluores- cent lights available. The traditional black light type uses a vapor bulb powered by 110 volts. It requires a warm-up period of about 10 minutes. With the 110-volt lamp, yellow-green dye is easily visible in the leaking oil; oil without dye appears purple. Use care when handling the lamp. The bulb is fragile and is expensive to replace. A different type of light system uses a high out- put 12-volt UV/blue lamp that comes on instantly and is used with yellow glasses ( Figure 3.9 ). UV light can be damaging to your eyes, so use caution. With the 12-volt lamp and glasses, oil is yellow and oil with dye in it is a brighter yellow-green. When a leak is minor it might not show up after just a short time, so the car might need to be driven
SAFETY NOTE If you are working on a vehicle and your clothing catches fire, fall on the ground and roll around to smother the flames. Do not run.
OIL LEAKS
NOTE A loss of one drop of oil every 30 feet results in a loss of about 3 quarts of oil every 1000 miles.
Oil that leaks through gaskets and seals is a common cause of oil consumption.
SHOP TIP To test an engine for leaks:
- Clean the engine.
- Slide a piece of white newsprint or autobody masking paper under the vehicle while the engine idles.
- Fresh drops of oil on the newsprint indicate a leak.
Rear Crankshaft Seal Leaks
A rear main bearing seal leak can be identified when oil is found on the engine side of the flywheel
VINTAGE ENGINES
Most older engines with carburetors had mechanical fuel pumps. A faulty mechanical fuel pump can be responsible for oil consumption and other problems. A break in the pump dia- phragm will allow fuel to dilute the oil in the crankcase. The thinner oil that results leaks more easily past the piston rings, into the combustion chamber. Low fuel pump pressure and an oil dipstick that smells of gasoline are symptoms of the problem. Low fuel pump pressure can cause the vehicle to run out of fuel when partway up a grade. It can also cause vapor lock, which is when fuel boils in the line, temporarily depriving a carbureted engine of gas. Another problem from diluted crankcase oil is hydraulic valve lifters that become noisy because they leak down rather than opening the valves. A mechanical fuel pump has an oil seal above its diaphragm spring. If oil leaks past this seal, it can result in an external leak as it comes out of the vent hole in the dry chamber above the diaphragm. The problem will be worse if crankcase pressure is excessive.
CHAPTER 3 Diagnosing Engine Problems • 63
with that of the outside air. After a short warm-up period, it gives the computer information to control the air-fuel mixture. If the engine runs rough when cold, but the problem goes away after a short warm-up, the oxygen sensor could be masking an air leak. Check to see that the sensor is not dirty. Dirt or undercoating can plug the sensor’s outside air intake port, affecting the signal from the sensor. This can result in a richer than normal air-fuel mixture.
COMPRESSION LOSS
Compression loss, another reason for an engine overhaul, can be traced to two causes: compression leaks and engine breathing problems.
Compression Leaks
Compression can leak due to several causes: a blown head gasket ( Figure 3.10 ), burned valves ( Figure 3.11 ), worn or broken piston rings, a dam- aged piston ( Figure 3.12 ), or a broken valve spring ( Figure 3.13 ).
NOTE A broken valve spring is nearly always accompanied by water in the crankcase. Acid is a by-product of combustion that accumulates in engine oil. When acid combines with water, corrosion accelerates and the resulting rust raises stress in the surface of the spring, causing it to break.
When valve clearances are too tight the valves cannot seal against their valve seats, resulting in a compres- sion leak. Tight valves can result from wear to the valve faces ( Figure 3.14 ) or valve seats, either of which allows the valve stem tips to move deeper into the cylinder heads. Closer valve clearance can also result when valves have been adjusted incorrectly.
Blown head gasket
FIGURE 3.10 A blown head gasket. (Courtesy of Tim Gilles)
FIGURE 3.11 (a) A burned exhaust valve like this one causes lower or no cylinder compression pressure. (b) Severely burned valves. (Courtesy of Tim Gilles)
(a)
(b)
NOTE A burned valve can result when an engine with an exhaust leak has been driven for a period of time without repair ( Figure 3.15 ). The valve typically runs at temperatures in excess of about 1300°. When the relatively cold air in the engine compartment is sucked in through the exhaust leak, the valve suffers thermal shock.
Exhaust leaks make noise at ½ crankshaft rpm.
64 • SECTION I Engine Construction, Diagnosis, Disassembly, and Inspection
Breathing Problems An engine that cannot breathe properly is suf- focating and will not be able to develop sufficient compression. Engine vacuum will drop off, further lowering compression. Breathing problems can be traced to such things as worn camshaft lobes that do not open the valve far enough ( Figure 3.16 ), or late valve timing (see Chapter 10). Valve timing can become retarded (late) when a timing chain becomes so worn that it skips a tooth.
NOTE Late valve timing will cause poor low rpm performance although engine performance might be acceptable at higher rpm.
FIGURE 3.12 A damaged piston resulting from detonation. (Courtesy of Tim Gilles)
FIGURE 3.13 A broken valve spring. (Reprinted Courtesy of Caterpillar Inc.)
SHOP TIP Hard valve seats that are starting to burn can turn a spark plug blue because of cobalt deposits that flake off the valve seat.
Wear FIGURE 3.14 Valve face wear will cause the valve stems to move into the head, eliminating valve adjustment clearance. (Courtesy of Tim Gilles)
FIGURE 3.15 This broken exhaust manifold bolt resulted in a burned exhaust valve due to thermal shock. (Courtesy of Tim Gilles)
Broken bolt
66 • SECTION I Engine Construction, Diagnosis, Disassembly, and Inspection
shown in Figure 3.19b has adapters to accommodate both thread sizes and different thread lengths. It has a Schrader valve, very similar to a tire valve, for sav- ing the pressure in the gauge so that the technician can read it. The advantage of the screw-in tester is that it can be threaded into the plug hole, leaving the tech- nician free to crank the engine.
NOTE Ideally, the test is done with the engine at normal operating temperature. The battery must be fully charged for a com- pression test to be accurate.
Compression Test Procedure
- Use caution when removing secondary (spark plug) wires. First, twist the rubber boots to loosen them from the spark plugs.
smog pump lines into the exhaust manifold, or an adapter can be substituted for the EGR valve. According to TRW, removing the oxygen sensor to perform the test can give an inaccurate reading due to a venturi effect in the exhaust system. Specifica- tions vary among manufacturers. As a general rule, pressure should not exceed 1.75 psi at wide-open throttle (WOT) under full load. Breathing problems can also be found using a vacuum gauge (covered later in this chapter).
Testing for Compression Loss
For an engine to run, it must have three things:
- Sufficient compression
- Fuel (in a flammable ratio to air)
- Ignition (timed at the appropriate instant)
A quick way to check for uneven compression between cylinders is to disable the ignition system and crank the engine. An uneven cranking rhythm indicates unequal compression.
NOTE If any of the following tests indicates a possible valve seal- ing problem, be sure to check the valves for correct clear- ance adjustment before proceeding with a repair.
SHOP TIP A quick judgment of engine compression can often be made simply by cranking the engine with the ignition sys- tem disabled and listening for an uneven rhythm.
Compression Test
One of the most common and least expensive pieces of test equipment is the compression tester. A compression tester is simply a pressure gauge that is inserted into a spark plug hole. There are two styles of compression testers. One is held in place while cranking the engine ( Fig- ure 3.19a ). It is handy on in-line engines because it is fast and easy. The other type is the screw-in tester. Spark plugs use one of two thread sizes. The tester
FIGURE 3.19 Compression testers. (a) This compression tester can be used when there is easy access to the spark plug holes. (b) A screw-in compression tester with adapters. The adapter shown on top is for larger-diameter spark plug threads. The two adapters beneath it are for different lengths (reaches) of the smaller spark plug thread size. (Courtesy of Tim Gilles)
(a)
Larger diameter
(b)
CHAPTER 3 Diagnosing Engine Problems • 67
- Insert the compression gauge into a spark plug hole.
- Crank the engine through at least four compres- sion strokes.
- Check and record each pressure reading. The gauge will move four times, or with all the plugs removed you can hear each compression stroke as the compression in the cylinder being tested slows the engine.
CASE HISTORY
A group of students was performing a compres- sion test on a teacher’s car. During the compres- sion test, the compression was checked repeatedly by several students (the throttle was not blocked open). After completing the compression test, they tried repeatedly to get the engine started. Finally a spark happened, fuel in the muffler of the flooded engine was ignited and a deafening explosion ripped the muffler apart.
Interpreting Compression Test Results. If all cylinders are performing equally and engine per- formance is acceptable, the engine passes the test. When compression test specifications are avail- able, they are only an estimate. If specifications are not available, locate the compression ratio in the specification manual and use the following formula:
Compression (^) × Atmospheric (^) + Ratio Pressure
Atmospheric (^) + 5 (Volumetric Pressure Efficiency)
For example, to figure out the approximate com- pression on an 8:1 engine at sea level (14.7 psi atmo- spheric pressure):
8.0 × 14.7 + 14.7 + 5 = 137.3 psi
Variations in compression among cylinders should be no more than 20%. When two cylinders
- Clean around all the spark plugs with com- pressed air. Then remove all spark plugs, so that the starter can crank the engine easily.
- Block the throttle in the wide-open position. This can be done with a throttle depressor ( Figure 3.20 ).
SAFETY NOTE Blocking the throttle open during a compression test pre- vents an engine with a carburetor from sucking fuel into the cylinders.
SHOP TIP On some vehicles, when the starter is not easily accessible, a wire in an easy-to-reach location under the hood can often provide access to the starter circuit. Study the wiring diagram for the vehicle.
- Connect a remote starter between the S terminal on the starter solenoid and the ungrounded bat- tery post.
- The compression test is done with the ignition switch in the “on” position to prevent damage to the electrical system on some vehicles. Disable the ignition system by pulling the battery-to- distributor wire on cars with electronic ignition. Follow the manufacturer’s instructions.
CAUTION If you crank the engine without disabling the ignition sys- tem and you have a spark plug wire disconnected, you can damage the ignition module or computer.
FIGURE 3.20 A throttle depressor is used to hold the throttle open during the compression test. (Courtesy of Tim Gilles)
CHAPTER 3 Diagnosing Engine Problems • 69
overheat. Converter overheating can be minimized by dis- connecting the smog pump during the test.
SHOP TIP Using an oscilloscope when performing a power balance test allows the height of the firing lines to be observed. A poorly performing cylinder with a high firing line can indi- cate a lean air-fuel mixture; a low firing line can be indica- tive of low compression.
The cylinder power balance test can also be done with the engine running at higher speeds than idle. Compare results at low and higher speed. An engine with a burned valve will perform poorly at low engine rpm but would improve at higher rpm. A leaking valve does not have as sig- nificant an effect at higher speed as it does at low speed because the air coming into the engine and leaving it is moving too fast and has a much higher volume. A restriction in the intake, like that caused by a worn cam lobe, with hydraulic adjustment will result in little change in engine operation at idle. The problem will become gradually more pro- nounced as speed is increased.
EGR leaks do not respond when you richen the mix- ture like air leaks do. The cylinder causing the rise in engine rpm is the one from which the exhaust gas for the EGR valve was picked up. Retest at cruise rpm and the problem will disappear.
CAUTION Cylinders should be shorted for only a few seconds at most. Raw fuel entering the catalytic converter can cause it to
FIGURE 3.21 Electronic cylinder power balance. When an injector is disabled, engine rpm should drop. (Courtesy of Tim Gilles)
VINTAGE ENGINES
Prior to OBD II, manual power balance testing was a routine procedure. On engines with contact point distributors, it was a common practice to disconnect each spark plug wire on an idling engine and note the rpm drop. You can perform a power balance test on cars equipped with electronic ignition electronically. Allowing the coil to produce ignition sparks while spark plug cables are disconnected can increase current flow in the coil to the point where the coil and igni- tion module can be ruined. Engine analyzers and hand-held scan tools have a feature that auto- matically shorts out cylinders, one at a time, without removing spark plug cables. It is important that each cylinder be shorted for the same length of time and that the speed of the engine returns to normal before shorting the next cylinder. The test is most effective when done at the lowest engine speed possible (500–600 rpm). Vehicles equipped with computer-controlled fuel systems will automatically raise the engine idle to compensate as each cylinder is grounded out. Disabling the computer’s spark advance is required to perform this test. Consult service literature for the correct procedure.
70 • SECTION I Engine Construction, Diagnosis, Disassembly, and Inspection
- Bubbles in the radiator = blown head gasket, or a crack in the head or block, which allows the regulated air to enter the cooling system
SHOP TIP If leakage is past the piston rings, the PCV valve could allow air to travel into the intake manifold where it can cause the technician to mistakenly suspect a leaking intake valve. To avoid this situation, remove the oil filler cap, or disconnect the vacuum line to the PCV valve or pinch it with pinch pliers.
The leakage tester offers three advantages over a compression test:
- The test can be performed on an engine that is removed from a car (such as an engine purchased at a salvage yard).
- The exact source of leakage can be pinpointed before engine disassembly.
- A racing camshaft will not affect the results of the test. It would cause lower readings on the compression test, however, because engine vacuum is lower at cranking speeds with a racing cam.
NOTES
- If a high mileage engine tests OK on the power balance test and has compression within expectations but has excessive cylinder leakage test results, carbon deposits in the combustion chamber could account for the relatively good compression, although the engine will probably have excessive blowby and may lack power.
- When a cylinder bore has considerable taper wear on the top, the reading can be improved if the piston is moved slightly past TDC into a less worn area.
Engine Vacuum Intake manifold vacuum can be very useful in determining engine problems. Vacuum readings compare pressure in the intake manifold to atmo- spheric pressure. Vacuum is measured in either inches of mercury (in. Hg) or millimeters of mer- cury (mm Hg) or kilopascals (kPa). The vacuum gauge ( Figure 3.23a ) is connected to a manifold vac- uum source ( Figure 3.23b ).
Cylinder Leakage Test (CLT)
The cylinder leakage test can accurately pin- point causes of leakage. Regulated compressed air is introduced into the cylinder through its spark plug hole ( Figure 3.22 ). The piston is positioned at top dead center (TDC) on the compression stroke, ensuring that both valves are completely closed.
- It is better to perform a cylinder leakage test when the engine is warm and the rings are sealed with oil. Otherwise, a small amount of movement at TDC can allow the piston ring to move off its ring land, allowing leakage (see Chapter 11).
- An acceptable leakage reading on the tester’s gauge is usually less than 10% to 15%, although vehicles with up to 30% leakage might still be performing to the owner’s satisfaction. The owner might not notice the power difference until it is restored after an overhaul, because the loss in power has happened gradually. If a cylinder shows high leakage, listen to locate the sound of the leaking air. The following lists cyl- inder leakage test results. Here are some possible locations for leaking air and the likely causes:
- Oil filler = leaking rings or piston
- Manifold intake = leaking intake valve
- Exhaust pipe = leaking exhaust valve
FIGURE 3.22 A cylinder leakage tester is connected to the cylinder through a hose to the spark plug hole. (Courtesy of Tim Gilles)
Compressed air input
Regulator
Adjusting knob
72 • SECTION I Engine Construction, Diagnosis, Disassembly, and Inspection
intake manifold diminish because the size of the leak is proportionally less as the engine breathes more air. A leaking intake manifold gasket can result from sloppy cleanup of gasket surfaces during manifold installation, from failing to clean out bolt holes, or from bottoming out bolts that are too long. A car with an oxygen sensor feedback fuel sys- tem can run rough when cold but run fine after warm-up when the computer responds to the oxy- gen sensor signals by compensating with a richer air-fuel mixture. One vacuum test for a manifold gasket leak is to pinch off the two PCV valve hoses—the one to the intake manifold and the breather hose to the air cleaner—then run the engine. If there is vacuum at the oil filler opening, an intake manifold-to- crankcase vacuum leak is indicated.
NOTE Low vacuum at idle is normal for an engine with an aftermarket high-performance camshaft. Vacuum will increase as rpm is raised. This is caused by valve overlap, which is discussed in Chapter 10.
To test for weak piston rings (Figure 3.26) :
- Raise the engine speed to about 2000 rpm.
- Snap the throttle closed and watch for an increase of 2 to 6 inches of vacuum above normal.
- Worn rings will not increase vacuum sufficiently during deceleration. Generally, the higher the rise, the better the condition of the rings.
VINTAGE ENGINES
Ignition timing is adjustable on engines without computer controls. When low, steady vacuum is found, check the ignition timing. Late/retarded ignition timing can cause an engine to run hot. If the timing is retarded a great deal, check the camshaft timing (see Chapter 10). Excessive timing chain slack can allow the timing chain to skip a tooth on its sprocket. Because the distributor is usually driven by the camshaft, this shows up as late ignition timing during an ignition timing check.
(^20) VACUUMIN HG. 4
6
810
(^12 14 16 18 ) (^2426) 3028 (^20) VACUUMIN HG.
(^64)
810
(^12 14 16 18 ) (^2426) 3028
Compression (Rings)
Good Bad
Momentary low with a high return while decelerating
Momentary low with a low return while decelerating
FIGURE 3.26 Vacuum readings for weak piston rings.
Sticky valves are indicated when the needle drops quickly or drifts. This could be because valves are hanging up in their guides rather than closing freely ( Figure 3.27 ). The movement will have no apparent rhythm. Leaking valves ( Figure 3.28 ) are indicated when the needle drops at regular intervals. A power balance test will pinpoint the low cylinder.
Bad Valve Springs. Sometimes valve springs that are too short or too weak have been installed by accident. Otherwise, valve springs weak enough to cause this problem are rare. To test for weak valve springs with a vacuum gauge ( Figure 3.29 ):
- Raise engine rpm slowly to 2000.
- Look for rapid fluctuations of the needle as speed increases.
- An exhaust analyzer will have a good hydro- carbon (HC) reading at idle but bad under acceleration.
CHAPTER 3 Diagnosing Engine Problems • 73
Restricted Exhaust. To test for a restricted exhaust ( Figure 3.30 ):
- Raise the engine rpm quickly to 2000 to cause the vacuum reading to go momentarily low and then release the throttle quickly.
- Vacuum should return smoothly and quickly to higher than normal levels.
- A slow, hesitating return can indicate a breath- ing restriction.
Other Causes of Low Vacuum. Port fuel injected engines use O-rings to seal each individual fuel injector where it enters an intake port ( Figure 3.31 ). When an O-ring leaks, the resulting vacuum leak and lean air-fuel mixture in that cylinder cause a rough idle ( Figure 3.32 ). If an engine is equipped with an air pump for emission control, be sure that it does not have a
When an engine with weak valve springs is accelerated, hydraulic lifters can overfill (pump up). There is no noise, but the engine will run rough when it returns to idle. According to one Auto Engine Rebuilders Asso- ciation (AERA) bulletin, incorrect valve spring ten- sion can result in a rough idle that is only apparent on initial cold startup. OBD II engine computer systems sometimes sense this misfire and cause the malfunc- tion indicator lamp (MIL) on the dash to illuminate.
(^20) VACUUMIN HG.
(^64)
810
(^12 14 16 18 ) 24 26 3028 (^20) VACUUMIN HG.
(^64)
810
(^12 14 16 18 ) 24 (^2628) 30
Good Sticky valves Bad
Normal steady vacuum reading
Intermittent drop or rapid drifting
FIGURE 3.27 Vacuum reading for sticky valves.
(^20) VACUUMIN HG. 4
6
810
(^12 14 16 18 ) (^2426) 3028 (^20) VACUUMIN HG. 4
6
810
(^12 14 16 18 ) (^2426) 3028
Good Bad
Normal steady vacuum reading
Definite drop at regular intervals
Leaky valves
FIGURE 3.28 Vacuum reading for leaking valves.
(^20) VACUUMIN HG. 4
6
810
(^12 14 16 18 ) (^2426) 3028 (^20) VACUUMIN HG. 4
6
810
(^12 14 16 18 ) 24 26 3028
Good Bad
Normal steady vacuum reading
Increased rate of fluctuations with increased speed
Weak or broken valve springs
FIGURE 3.29 Vacuum reading for bad valve springs.
(^20) VACUUMIN HG.
(^64)
810
(^12 14 16 18 ) 24 26 302 8 (^20) VACUUMIN HG.
(^64)
810
(^12 14 16 18 ) 24 26 3028
Restricted exhaust
Momentary low with a smooth, rapid return
Momentary low with a hesitating, slow return
Good Bad
FIGURE 3.30 Vacuum reading for restricted exhaust.
FIGURE 3.31 O-rings seal the fuel injectors where they enter the intake ports. (Courtesy of Tim Gilles)
