compression/leakdown tests

[Bryan]

Quick question; is it possible to have a compression test that checks out good but a leakdown test that does not? How bout vice-versa?

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[Tom]

Could happen. A compression test will catch any major leak. It is looking just for a snap shot, what pressure can the cylinder develop when the piston rises.

It does not measure any leakage of that pressure, just measures that the pressure is there. Ever had a tire with a leak in it? You can put the air hose to it, and fill it up, 32PSI, no problem, but wait 5 minutes and it is flat again.

If the leak is relatively small (such as a small crack in the head gasket) it is fully possible to get a good solid reading from your compression test, but then perform a leak down test, and you may discover that over the course of a few seconds to a few minutes, that pressure would drop dramatically.

Now, will that necessarily have an adverse impact on the way the car runs? Maybe not. A small crack in the head gasket may not have any real effect on engine performance. The engine may run nearly perfectly, and the only real symptom may be the need to constant top off the coolant.

I suppose it might be possible to have a bad compression test, and a good leakdown, though I am uncertain of the exact conditions that could cause those results, I am sure someone else here will chime in with more info.

[Brett]

I would suggest making sure your doing the leak down test correctly. Take a look at Erics videos on compression and leak down testing. On the leak down test you need that specific cylinder to be at TDC on the compression stroke.

[EricTheCarGuy]

I agree, if you get strange readings, check your methods of testing. One thing that’s often overlooked with compression testing is blocking the throttle plate open. I wish I would have made that more clear in the video that I made.

[Author]

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