Cylinder Leakage Test (CLT)

The cylinder leakage test can accurately pinpoint 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
• 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 cylinder 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

• 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


The leakage tester offers three advantages over a compression test:
1. The test can be performed on an engine that is removed from a car (such as an engine purchased at a salvage yard).
2. The exact source of leakage can be pinpointed before engine disassembly.
3. 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.


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.


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.

Electronic Cylinder Power Balance Testing

Vehicles produced since the mid-1990s have sophisticated on-board diagnostics called OBD II. Figure 3.21 shows a screen shot of a scan tool during an electronic power balance test. The scan tool disables fuel injectors rather than the ignition system. This eliminates the possibility of catalytic converter damage and oil dilution from raw fuel entering the crankcase during the test.

FIGURE 3.21 Electronic cylinder power balance. When an injector is

disabled, engine rpm should drop

 A cylinder that causes less drop than the others is not pulling its full load. Variations in rpm drop between cylinders should be less than 5%. A problem could be caused by the ignition system or fuel system, or the engine could have vacuum leaks or compression problems. Occasionally, rpm will rise as a cylinder is shorted out due to exhaust gas entering the intake manifold. This can be caused by an EGR valve that is open. (EGR should be closed at idle.) Like an air leak (also called a vacuum leak), EGR leaks cause a drop in manifold vacuum. But EGR leaks do not respond when you richen the mixture 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.

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 significant 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 pronounced as speed is increased.

Compression Test Procedure

• Use caution when removing secondary (spark plug) wires. First, twist the rubber boots to loosen them from the spark plugs.

• Clean around all the spark plugs with compressed 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

• Insert the compression gauge into a spark plug hole.
• Crank the engine through at least four compression 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.

• Connect a remote starter between the S terminal on the starter solenoid and the ungrounded battery 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-todistributor wire on cars with electronic ignition. Follow the manufacturer’s instructions.


Interpreting Compression Test Results. If all cylinders are performing equally and engine performance
is acceptable, the engine passes the test. When compression test specifications are available,
they are only an estimate. If specifications are not available, locate the compression ratio in the specification manual and use the following
formula:

Compression Ratio × Atmospheric Pressure +

Atmospheric Pressure + 5 (Volumetric Effi ciency)

For example, to figure out the approximate compression on an 8:1 engine at sea level (14.7 psi atmospheric
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

next to each other have low compression, a blown head gasket is usually indicated. One or several cylinders with low compression and no apparent pattern of loss often indicates burned exhaust valves with rough idling as a symptom. At higher rpm, the rough running from the burned valve may
disappear.


Wet Compression Test. When cylinders show poor results, perform a wet compression test.
• Squirt about a tablespoon of oil into each low cylinder. The oil makes a seal around worn rings, boosting the compression reading.
• When cylinder readings are low but increase to normal during a wet test, a piston ring problem
is indicated.


Running Compression Test
The running compression test, or volumetric efficiency test, is used when the cause of a cylinder misfire
cannot be pinpointed or when an engine lacks power. First, perform a regular cranking compression
test and record all of the results. Then install all of the spark plugs except for one and install a compression
tester in that hole. Start the engine, and depress the release pin on the Schrader valve to allow the reading to stabilize.


Snap Compression Test. The next step is to snap the throttle wide open and let the engine return to idle. When it is first snapped, the throttle plate is wide open while the piston speed is relatively low. This will result in a higher reading. The compression gauge will hold this reading until the Schrader valve is manually released.
Record your snap compression test readings. Typical readings will be around 80% of cranking compression
readings. Lower snap readings mean that the intake system is restricted. Higher readings indicate an exhaust restriction. Problems in only one cylinder point to a worn cam lobe, broken valve spring, etc. Problems in all cylinders can be traced to a restriction in the air intake system or a plugged exhaust pipe or catalytic converter.

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.


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 (Figure 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

a

b
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.

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 saving 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 technician free to crank the engine.

Breathing Problems For Engine

An engine that cannot breathe properly is suffocating 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 . Valve timing can
become retarded (late) when a timing chain becomes so worn that it skips a tooth.

FIGURE 3.16 A severely worn cam lobe will not properly open a

valve.

If the timing chain has skipped and valve timing is retarded, suction will be felt at the exhaust
pipe. This happens because the exhaust valve is still open during the piston’s intake stroke.

Breathing problems can also be traced to carbon buildup around the neck of the valve (Figure 3.17)
or to restrictions such as a dirty air cleaner or a blocked exhaust.

FIGURE 3.17 A lack of carbon formation on the top of the piston,
beneath the intake valve, indicates oil leakage through the valve guide.
The valve in the photo was found when the head was disassembled

Catalytic converters in the exhaust system can become plugged after running for a prolonged period with an ignition system defect. A rich air-fuel mixture can also cause a converter to plug when it overheats and melts internally. OBD II vehicles (later than 1996) will go into limp-in mode when the computer senses a catalyst damaging misfire.


An exhaust restriction can also cause an automatic transmission to shift harshly or late due to the resulting faulty vacuum signal or increased throttle pressure in the transmission. Exhaust backpressure can be tested using a fuel pump vacuum/pressure tester connected to thesmog 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. Specifications
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


SHOP TIP
A blocked exhaust will be evident when the engine rpm is raised quickly. A roar will be heard on the engine side of the air cleaner.





SAFETY NOTE
A catalytic converter on a pre-OBDII vehicle can become so hot that it can start a fire if the vehicle is parked above dry grass.

FUEL MIXTURE PROBLEMS FOR ENGINE

Emission control and fuel system malfunctions sometimes mimic engine problems. Occasionally, important items are neglected during an engine job. Larger engine shops often employ specialists capable of diagnosing these complicated problems.

An air-fuel mixture that is too lean (too much air/too little fuel) can cause burned internal engine parts.


Oil/Fuel Wash
An overly rich mixture (too much fuel/too little air) can cause fuel wash or oil wash (when oil is washed from cylinder walls, resulting in cylinder wall wear). Leaking fuel injectors can result in cylinder wall oil wash, too. They can also cause intake valve deposits that will affect engine idle and emissions. A bad vapor canister purge valve can also cause oil wash. When only one side of a piston is worn, this is often due to oil wash. The worn side will be the major thrust side. An explanation of piston thrust surfaces is provided in Chapter 13.

(a)

(b)

(a) A burned exhaust valve like this one causes lower or no cylinder compression pressure.

(b) Severely burned valves

 Many areas use reformulated gasoline (RFG) to lower air pollution. RFG does not cause problems with
engine wear under normal conditions. But a richer than normal air-fuel mixture can result in accelerated
engine wear because the alcohol or ether in RFG dilutes the oil on the bearings, cylinder walls, and piston
rings. When mixed with the oil, it does not evaporate as easily and the diluted oil loses much of its lubricity. When a low mileage engine fails and there is no evidence of excessive dirt, abrasives, or machined material, diluted engine oil is a prime suspect.


Oxygen Sensor Problems
Modern engines use oxygen sensors in the exhaust to compare the oxygen content of the exhaust

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 , burned valves , worn or broken piston rings, a damaged piston , or a broken valve spring

When valve clearances are too tight the valves cannot seal against their valve seats, resulting in a compression leak. Tight valves can result from wear to the valve faces 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.
Exhaust leaks make noise at ½ crankshaft rpm

FIGURE 3.12 A damaged piston resulting from detonation

FIGURE 3.13 A broken valve spring.

FIGURE 3.14 Valve face wear will cause the valve stems to move
into the head, eliminating valve adjustment clearance

FIGURE 3.15 This broken exhaust manifold bolt resulted in a
burned exhaust valve due to thermal shock

DIAGNOSING ENGINE PROBLEMS BEFORE A REPAIR

how to troubleshoot problems on a running engine. Also discussed are some of the external causes of engine problems that 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. Sometimes a problem with a system causes an engine to fail. If the problem is not taken care of, the failure will recur. This chapter should serve as a reference for future problems. The descriptions of various problems 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 determined
that an overhaul is really necessary. The damaged starter motor drive frame shown in Figure 3.1
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 determined
while the engine is running. A thorough discussion of the problem with the owner of the
vehicle is also helpful. Sometimes an owner’s driving 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 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 dilution 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 dilution in the oil. Before leaving on a long vacation trip, the customer checks the oil and the dipstick registers “full.” But when the diluted oil becomes thoroughly 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 deposits on the necks of the intake valves are another indication . Look for carbon deposits when disassembling the cylinder head.


Different types of valve guide seals are described . Valve guide seals should always be replaced during a valve job while the heads are disassembled. includes a procedure for replacing valve guide seals without
removing the heads from the engine. Oil Consumption from Piston Rings When oil is consumed past piston rings, one common cause on high mileage engines is lack of ring tension resulting from cylinder wall and ring wear. A record of poor engine oil maintenance contributes greatly to an increased wear factor. An engine that suffers from a lack of regular oil changes will often have plugged oil control rings (Figure 3.3).

FIGURE 3.3 A Plugged oil control ring.

The piston’s oil-control rings need to be able to scrape oil from the cylinder walls and return it to the crankcase
through the underside of the piston (Figure 3.4).

FIGURE 3.4 Oil ring drainback. Slots 

(a) or holes 

(b) in the oilgroove return oil to the crankcase as the piston moves down.

Spark plugs provide a window to conditions in the cylinder because they extend into the combustion chamber. Figure 3.5 shows abnormal spark plug conditions associated with oil consumption.

FIGURE 3.5 Examples of oil-fouled spark plugs.

consumption might be caused by worn or stuck piston rings


Excessive Rod Bearing Clearance
A high mileage engine will probably have worn crankshaft bearings. Excessive bearing-tocrankshaft 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, overwhelming the oil rings, which cannot return all of the oil to the crankcase (Figure 3.6).

FIGURE 3.6 Increased oil clearance causes more oil to be thrown onto cylinder walls

Whatever oil enters the combustion chamber will be burned with the airfuel mixture. Even with normal rod bearing clearance, 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 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

FIGURE 3.7 Plugged oil drainback holes will cause exhaust smoke

engine and major service will soon be needed. The oil remains in the valve cover area instead of returning
to the crankcase; it floods the valve guide, making 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 problem 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 covered 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 crankcase. One possible reason for excessive oil leakage is a positive crankcase ventilation (PCV) valve that

FIGURE 3.8 Oil can be drawn into the intake manifold past a faulty manifold gasket

To see if the PCV valve is working properly:
• Pinch the line that leads to it, or cover the end of the PCV valve with your thumb.
• With computer idle speed adjust disabled, if the PCV valve is good, idle speed should drop.
Blocking the flow of air to the PCV valve enriches the air-fuel mixture.

Be sure to check for a restricted filter or a kink in the breather line from the valve cover to the air cleaner. This can result in oil leakage caused by crankcase pressure.

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 failure 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.

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 rubbertipped blowgun (regulated to no more than 2–3 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 suction
from the crankcase vacuum of the PCV system.

Oil that leaks through gaskets and seals is a common cause of oil consumption.

Rear Crankshaft Seal Leaks
A rear main bearing seal leak can be identified when oil is found on the engine side of the flywheel

or torque converter. Oil on the opposite side indicates front transmission seal leakage. Oil that has been sprayed in a circular pattern is also indicative of a crankshaft seal leak. Most crankshaft 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. Washing the engine first is helpful but not necessary.
There are two types of ultraviolet (UV)-fluorescent 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 output 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.

FIGURE 3.9 A UV light powered by the vehicle battery
is used with yellow UV glasses, which prevent eye damage

When a leak is minor it might not show up after just a short time, so the car might need to be driven for a day or so. After the leak is repaired, the engine is cleaned and rechecked with the black light. The fluorescent dye stays in the oil. The dye is not harmful and the manufacturer says that it dissipates within 300 miles of driving powder. Refer to Chapter 16 for more information on gaskets and seals.