Evaluating the Engine’s Condition; Noise Diagnosis

EVALUATING THE ENGINE'S CONDITION

Once the compression, cylinder leakage, vacuum, and power balance tests are performed, a technician is ready to evaluate the engine's condition. For example, an engine with good relative compression but high cylinder leakage past the rings is typical of a high mileage worn engine. This engine would have these symptoms: excessive blowby, lack of power, poor performance, and reduced fuel economy.

If these same compression and leakage test results are found on an engine with comparatively low mile age, the problem is probably stuck piston rings that are not expanding properly. If this is the case, try treating the engine with a combustion chamber cleaner, oil treatment, or engine flush. If this fails to correct the problem, an engine overhaul is required.

A cylinder that has poor compression but minimal leakage indicates a valve train problem. Under these circumstances, a valve might not be opening at the right time, might not be opening enough, or might not be opening at all. This condition can be confirmed on engines with a pushrod-type valve train by pulling the rocker covers and watching the valves operate while the engine is cycled. If one or more valves fail to move, either the lifters are collapsed or the cam lobes are worn. If all of the cylinders have low compression with minimal leakage, the most likely cause is incorrect valve timing.

If compression and leakage are both good, but the power balance test reveals weak cylinders, the cause of the problem is outside the combustion chamber.

Assuming there are no ignition or fuel problems, check for broken, bent, or worn valve train components, collapsed lifters, leaking intake manifold, or excessively leaking valve guides. If the latter is suspected, squirt some oil on the guides. If they are leaking, blue smoke will be seen in the exhaust.

Fluid Leaks:

When inspecting the engine, check it for leaks. There are many different fluids under the hood of an automobile so care must be taken to identify the type of fluid that is leaking. Carefully look at the top and sides of the engine, and note any wet residue that may be present. Sometimes road dirt will mix with the leaking fluid and create a heavy coating. Also look under the vehicle for signs of leaks or drips; make sure you have good lighting. Note the areas around the leaks and identify the possible causes. Methods for positively identifying the source of leaks from various components are covered later in this section. All leaks should be corrected because they can result in more serious problems.

Sometimes smell will identify the fluid. Gasoline evaporates when it leaks out and may not leave any residue, but it’s easy to identify by its smell.

Exhaust Smoke Diagnosis:

Examining and interpreting the vehicle's exhaust can give clues of potential engine problems. Basically there should be no visible smoke coming out of the tailpipe. There is an exception to this rule, however, on a cold day after the vehicle has been idling for awhile, it’s normal for white smoke to come out of the tailpipe. This is nothing else but the water that has condensed in the exhaust system becoming steam.

However, the steam should stop once the engine reaches normal operating temperature. If it does not, a problem is indicated. The color of the exhaust is used to diagnose engine concerns ( +++46).

NOISE DIAGNOSIS

More often than not, malfunction in the engine will reveal itself first as an unusual noise. This can happen before the problem affects the driveability of the vehicle. Problems such as loose pistons, badly worn rings or ring lands, loose piston pins, worn main bearings and connecting rod bearings, loose vibration damper or flywheel, and worn or loose valve train components all produce telltale sounds. Unless the technician has experience in listening to and interpreting engine noises, it can be very hard to distinguish one from the other.

When correctly interpreted, engine noise can be a very valuable diagnostic aid. For one thing, a costly and time-consuming engine teardown might be avoided. Always make a noise analysis before doing any repair work. This way, there is a much greater likelihood that only the necessary repair procedures will be done.

-- Customer Care: When attempting to diagnose the cause of abnormal engine noise, it may be necessary to temper the enthusiasm of a customer who thinks they have pinpointed the exact cause of the noise using nothing more than their own two ears. While the owner's description may be helpful (and should always be asked for), it must be stressed that one person's "rattle" can be another person's "thump." You are the professional. The final diagnosis is up to you. If customers have been proved correct in their diagnosis, make it a point to tell them so. Everyone feels better about dealing with an automotive technician who listens to them.

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Using a Stethoscope

Some engine sounds can be easily heard without using a listening device, but others are impossible to hear unless amplified. A stethoscope ( +++47) is very helpful in locating engine noise by amplifying the sound waves. It can also distinguish between normal and abnormal noise. The procedure for using a stethoscope is simple. Use the metal prod to trace the sound until it reaches its maximum intensity.

Once the precise location has been discovered, the sound can be better evaluated. A sounding stick, which is nothing more than a long, hollow tube, works on the same principle, though a stethoscope gives much clearer results.

The best results, however, are obtained with an electronic listening device. With this tool you can tune into the noise. Doing this allows you to eliminate all other noises that might distract or mislead you.

WARNING! Be very careful when listening for noises around moving belts and pulleys at the front of the engine. Keep the end of the hose or stethoscope probe away from the moving parts. Physical injury can result if the hose or stethoscope is pulled inward or flung outward by moving parts.

+++47 Using a stethoscope helps to identify the source of an abnormal noise.

Common Noises:

+++48 gives examples of abnormal engine noises, including a description of the sound, and their likely causes. An important point to keep in mind is that insufficient lubrication is the most common cause of engine noise. For this reason, always check the fluid levels first before moving on to other areas of the vehicle. Some noises are more pronounced on a cold engine because clearances are greater when parts are not expanded by heat. Remember that aluminum and iron expand at different rates as temperatures rise. For example, a knock that disappears as the engine warms up probably is piston slap or knock.

An aluminum piston expands more than the iron block, allowing the piston to fit more closely as engine temperature rises.

Also keep in mind that loose accessories, cracked flexplates, loose bolts, bad belts, broken mechanical fuel pump springs, and other noninternal engine problems can be mistaken for more serious internal engine problems. Always attempt to identify the exact source before completing your diagnosis. In most cases, the source of internal engine noises is best identified by tearing down the engine and inspecting all parts.

------- Type Sound Mostly Heard During Possible Causes

Ring noise High-pitched rattle -- Acceleration

¦ Worn piston rings or clicking

¦ Worn cylinder walls

¦ Broken piston ring lands

¦ Insufficient ring tension Piston slap Hollow, bell-like Cold engine operation and is

¦ Worn piston rings louder during acceleration

¦ Worn cylinder walls

¦ Collapsed piston skirts

¦ Misaligned connecting rods

¦ Worn bearings

¦ Excessive piston to wall clearance

¦ Poor lubrication Piston pin Sharp, metallic rap Hot engine operation at idle

¦ Worn piston pin knock

¦ Worn piston pin boss

¦ Worn piston pin bushing

¦ Lack of lubrication Main bearing Dull, steady knock Louder during acceleration

¦ Worn bearings noise

¦ Worn crankshaft Rod bearing Light tap to heavy Idle speeds and low load higher

¦ Worn bearings noise knocking or pounding speeds

¦ Worn crankshaft

¦ Misaligned connecting rod

¦ Lack of lubrication Thrust bearing Heavy thumping Irregular sound, may be heard

¦ Worn thrust bearing noise only during acceleration

¦ Worn crankshaft

¦ Worn engine saddles Tappet noise Light regular clicking Mostly heard during idle

¦ Improper valve adjustment

¦ Worn or damaged valve train

¦ Dirty hydraulic lifters

¦ Lack of lubrication Timing chain Severe knocking Increases with increase in

¦ Loose timing chain noise engine speed

+++48 Common engine noises.

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1. Spark occurs 2. Combustion begins 3. Continues 4. Detonation

+++49 Detonation.

1. Ignited by hot deposit 2. Regular ignition spark 3. Flame fronts collide 4. Ignites remaining fuel +++50 Pre-ignition.

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Abnormal Combustion Noises:

Detonation and pre-ignition noises are caused by abnormal engine combustion. Detonation knock or ping is a noise most noticeable during acceleration with the engine under load and running at normal temperature. Detonation occurs when part of the air-fuel mixture begins to ignite on its own.

This results in the collision of two flame fronts. One flame front is the normal front moving from the spark plug tip. The other front begins at another point in the combustion chamber. The air-fuel mixture at that point is ignited by heat, not by the spark. The colliding flame fronts cause high frequency shock waves (heard as a knocking or pinging sound) that could cause physical damage to the pistons, valves, bearings, and spark plugs.

Excessive detonation can be very harmful to the engine.

Detonation is usually caused by excessively advanced ignition timing, engine overheating, excessively lean mixtures, or the use of gasoline with too low of an octane rating. A malfunctioning EGR valve can also cause detonation and even rod knock.

Another condition that also causes pinging or spark knocking is called pre-ignition, which occurs when combustion begins before the spark plug fires. Any hot spot within the combustion chamber can cause pre-ignition. Common causes of pre-ignition are incandescent carbon deposits in the combustion chamber, a faulty cooling system, too hot of a spark plug, poor engine lubrication, and cross firing. Pre-ignition can lead to detonation; however, preignition and detonation are two separate events.

Preignition normally does not cause engine damage; detonation does.

Sometimes abnormal combustion causes engine parts to make an abnormal noise. For example, rumble is a term that is used to describe the knock or noise resulting from abnormal ignition. Rumble is a vibration of the crankshaft and connecting rods that is caused by multisurface ignition. This is a form of preignition in which several flame fronts occur simultaneously from overheated deposit particles.

Multisurface ignition causes a tremendous sudden pressure rise near TDC. It has been reported that the rate of pressure rise during rumble is five times the rate of normal combustion.

Cleaning Carbon Deposits--A buildup of carbon on the top of the piston, intake valve, or in the combustion chamber can cause a number of driveability concerns, including preignition. There are a number of techniques used to remove or reduce the amount of carbon inside the engine. One way, of course, is to disassemble the engine and remove the carbon with a scraper or wire wheel. Two other methods are more commonly used. One is simply adding chemicals to the fuel. These chemicals work slowly so don’t expect quick results.

The other method requires more labor but is more immediately effective. This uses a carbon blaster, which is a machine that uses compressed air to force crushed walnut shells into the cylinders.

The shells beat on the piston top and combustion chamber walls to loosen and remove the carbon.

Basically to use a carbon blaster, the intake manifold and spark plugs are removed. The output hose of the blaster is attached to a cylinder's intake port or inserted into the bore for the fuel injector. A hose is inserted into the spark plug bore; this is where the shells and carbon exit the cylinder. Once connected to the cylinder, the blaster forces a small amount of shells in and out of the cylinder. Hopefully, the car bon deposits leave with the shells. To help remove any remaining bits of shells, compressed air is applied to the cylinder. This operation is done at each cylinder. It’s important to note that any remaining shell bits will be burned once the engine is run again.

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