Compression test/Valves

JHP · 06-25-2021, 01:10 PM

If compression is good, does that definitively mean the valves are seating properly? Or can it still have a bad valve that only a leak down test would detect? thanks

Jim2 · Jim2's Garage

A leak down test (pressure differential test) is far more sensitive when detecting air leakage. In addition it allows you to put your ear to the intake, exhaust/muffler, breather, or cylinder head to listen for the location of the leak. Similar sound info could be had by simply routing compressed air into your plug hole if you concoct a suitable hose. Leak down testers really don't cost much and you only need a small air compressor.

Jim2 · Jim2's Garage

To expand on your question the answer is no, the compression test is not definitive. Though the leak down test may and will show some leakage, it could lay within acceptable, or, not great but deal with it later, or alarm bells. All engines have some leakage but when one cylinder is a blatant outlier it's then up to you to decide if you want to leave it or dive in.

kamaro · 06-26-2021, 04:44 AM

If compression test is good and the difference between all cylinders do not exceed 10%, then the chance that you have a healthy engine is 99%, and for me, that its fairly enough to make me sleep well at night!

JHP · 06-26-2021, 03:21 PM

thanks guys! I Just been trying to trouble shoot a hot cyl #6. After checking everything related to vacuum, fuel, and ignition multiple times, I have come down to it being a mechanical valve issue but the compression was good so was just wondering if it could still be a valve issue. I suppose Ill try to do a leak down test next.

Alan L · Alan L's Garage

Post some numbers and it will help.
And some symptoms. What is wrong with #6?
Alan