Compression and Leakdown Tests

Compression and Leakdown Tests

>Thanks to all who responded, both directly and on the list, about my
>question regarding "Oil in the air filter housing". Common comment was to
>perform a compression test and a leak down test. I am afraid to admit it
>but I don't know how whats involved in doing a compression test or what a
>leak down test even is. Would anybody shed some light for this newbie?

A compression test is a somewhat over-rated diagnostic procedure that measures the amount (in pounds per square inch of pressure above ambient on a gauge -"psig") that the air in a cylinder is compressed when the engine is turned over by the starter. A simplified list of the steps involved:
- Bring the engine to normal operating temperature.
- Remove all spark plugs.
- Kill the ignition system.
- Operate the starter until the cylinder being tested has pumped at least five times, watching the pattern of movement of the gauge.
- Write down the results, and proceed to the next cylinder.

The compression achieved is dependent upon many things. Among them:
- The compression ratio of the engine. Typically varies from about 8:1 for low power engines to about 12:1 for race engines. Higher ratios give higher compression readings.
- The valve timing. Low power engines have more conservative cams, so the valves open later and close earlier, high power engines keep the valves open longer. Wilder cams give lower compression readings.
- The condition of the engine. This is what you are trying to determine with the compression test. Bent valves typically result in near zero readings, burned valves in very low readings, bad rings may give anywhere from moderately low readings to higher than normal readings.

General guidelines for compression readings:
- The consistency of readings is as important as the actual readings. The lowest cylinder should be 90% of the highest, and if the lowest is less than 80% of the highest, there is a significant problem.
- The pattern of readings seen on the gauge for each cylinder is important. The pattern in pressure readings should be big jump, smaller jump, still smaller jump, still smaller jump, no jump, no jump. If the pattern skips a step or steps, the most common causes are sticking intake valves from crud build-up on the valve stems, or pieces of carbon sticking under a valve. These pieces of carbon typically come from removing the spark plug, and are not serious, except that they indicate a high rate of carbon build-up.
- A reading of less than 90 psig will cause hard cranking.
- Typical readings are from 120 to 150 psig.

Some problems with using compression readings to evaluate the condition of an engine:
- Slightly low compression may be due to sticking piston rings, which may be due to short trip, low speed driving, or poor quality oil, or not changing the oil. This can be cured with good oil and spirited driving. Sticking intake valves are caused by the same things, and have the same cure.
- An engine can give years of good service with compression readings that are not perfect.

In summary, compression readings can give some valuable clues to the condition of an engine, but must be taken with a grain of salt.

A leak-down test is a check of the amount and location of the leakage from a cylinder. A special gauge is used, together with a source of compressed air. The air is vented into the cylinder, and the gauge measures the percentage of air that leaks out of the cylinder. A simplified procedure:
- Bring the engine to operating temperature.
- Remove the spark plug and attach the gauge.
- Bring the piston in the cylinder to exactly top dead center on the compression stroke.
- Introduce the air and read the gauge.
- Record the readings and move to the next cylinder.

The leak-down test is better than the compression test for determining condition. In addition to the numeric readings, the test will show where the leakage is. If the rings are bad, you will hear a hiss at the oil filler. If the intake valve is bad, the hiss will be at the intake manifold, if the exhaust valve is bad, the hiss will be at the tailpipe. If the headgasket is blown, or a head is cracked, the radiator will be

Remember that earlier 928s often have oil in the intake, partially due to the design of the oil filler neck. This can be helped by improving the baffling of the crankcase vent into the filler neck.

Hope this helps.

Wally Plumley
928 Specialists

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