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Old 08-14-2002, 04:18 PM   #2
AZScoobie
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Default Re: Compression Test vs. Leak Down Test...

Quote:
Originally posted by dug-e-fresh
Which of these are more indicative of engine health? Should both be done as a cross functional check... or is one better than the other.

Just to note, I haven't had any problems, but just out of curiosity I would like to have my engine tested to see where I stand.

I know Pepboys sells a do-it-your-self tester for I beleive compression testing, is this thing worth the trouble? Should I just take the car to a professional shop and have one or both done?

Opinions, facts, ideas are welcome...

Thanks,
Doug

I like to do both and here is why...

During a compression test you are converting your cylinders into air pumps. You screw the tester into the spark plug hole and then crank the engine over (with ignition and fuel disabled and throttle wide open). The tester itself works like a tire pressure gauge. You end up with a max PSI rating of that cyl. This is is indicative of the compression ratio. A compression test is only useful when all cyl's are tested so you can compare PSI ratings. We know they should match, or at least be within 10-20% of each other or there is a problem.


Case in point: Years ago I had a watercooled VW I built for SCCA pro solo. After a season I performed a compression test. cyl 1-4: 110,120,90,40. Not good... the last cyl was way down on compression. Why? A compression tester will never tell you why. It will only tell you how much compression you have.

Next up, leak down test. A leak down test pumps compressed air into your cyl through your spark plug hole. You must rotate your engine until that cyl is top dead center (all the way UP with all valves closed). The tester has a readout and shows the % of air that is escaping out of that cyl. 10% is considered normal.. This is metal to metal and you can expect some leakage. My ratings where 8%, 5%, 12%, 40%. That last cyl that was down on compression was leaking 40% of the air I was pumping in. Not good. Where is the air going? First thing you do is run to the back of the car and listen at the tail pipe. If you hear a rush of air that means the air is escaping out the exhaust valves. If you hear air at the intake then you might have bent intake valves. Open the radiator cap and listen/look for air bubbles. If so you might have a blown head gasket. Next up, take off the oil cap and listed for air. If you do, then you probably have a ring to bore leak or a scuffed bore.

You start with a compression test. Keep in mind that a cold motor will have less compression then a warm motor. Dont pay attention to how high the reading is. All you care about is that all cyl's are close to one another. In the case of my old VW the exhaust valves where bent (ooops, over rev)and not sealing. I left it alone because the engine was burning no oil and it was making 76hp at the wheels which was good for a motor rated at 72hp at the crank.

HTH

CT
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