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I'm thinking of doing a leakdown test on an engine before I install it in my car. Of course the problem is that the engine will be cold and things won't seal the way they do when they are hot. So how does the leak down testing procedure change when the engine is cold?

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    Try dropping some ATF in the cylinder before you try to do the leak down. I'd expect the reading to be higher than normal. As long as each cylinder has a roughly equal reading and cranking compression is OK, you can probably say that the engine is in good condition. – Ben Jul 13 '16 at 22:55
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    @Ben But isn't that kind of artificially sealing in the cylinders? It's like when you do the wet part of a compression test in order to test just the valves. – Robert S. Barnes Jul 14 '16 at 4:21
  • My concern with doing a leak-down test on a hot engine is removing the spark plugs. It is never a good idea to remove the spark plugs from a hot engine (with aluminum heads) in fear of damaging the threads. – Scott Oct 28 '18 at 1:56
  • I feel like a cold leak down test is good enough knowing the numbers will be possibly higher loss than in a warmed engine. But key is are the cylinders nearly equal and am I talking 5-10, 10-20, or a 30%+ numbers. If I got a 30 cold I’m not going to warm it up to see if that cylinder is really a 28 or even a 25. I’m about to test a 4 cylinder w 153k miles with warm compression test results of 192-198 psi for all cylinders. Compression is very good but I want to know the leak down too. – Jeremy Jul 7 at 11:25
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So searching the Internet, I've found some conflicting opinions, with an occasional person saying either that cold leak down tests are completely invalid, or that cold numbers will be two to three times warm numbers.

Some people relate the numbers to how big your ring gap is:

I just did a cold test on my fresh off the dyno engine, over the past 20 yrs I've gotten use to zero gap second rings that read 0-4% cold most of the time and 0% when warm.

Mainly though, I saw a number of individuals claiming that there is little difference:

doing the test on a hot motor will only yield slightly better results.

or

Cold leakdown tests on a good motor will still be under 5%. The rings bridge the gap between the piston and bores and it wont matter if the engine is cold. The only time I've seen a dramatic difference on cold motors are when the valve clearances are approaching zero.

I've also ran across professional sites which seem credible saying the same thing as above:

The first step is to warm up the engine to normal operating temperature. This is done so that the rings have expanded and hence will seal better, and should give more meaningful results. This is not absolutely necessary, however, and the test can certainly be done with the engine cold. The test results will not vary (in my experience anyway!) all that much.

I even ran across one site, Duncan Racing International, which claims leak down tests should only be done cold:

All leak down tests must be performed on a cold engine. *NOTE: Performing this test on a hot engine can cause internal damage to the engine.

So my conclusion from all this, even though I've never done a leak down test, is that the results should be very close to those of a warm test.

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