If you apply compressed air into a cylinder while it’s at TDC,. that compressed air allows you to determine if there’s a problem with an intake valve when air comes out the intake system, or if there’s a problem with an exhaust valve because air comes out the exhaust pipe. Or if there’s a problem with the rings because air comes out the dip stick tube. Or if bubbles start forming in the coolant because of a head gasket breach.

Leakdowntest vs compression test

@texases, Yes, there are reasons to go ahead and run the leakdown test even if the compression test is OK. @Tester had a good example although I’m scratching my head as to why a compression test didn’t catch the head gasket. The weakness in the compression test is the variability you get with it. The acceptable range is wide because atmospheric pressures affect the result, engine temp (hot when you do #1, warm when you get to #6) and battery condition (how fast the starter spins the engine. The leakdown eliminates most of these, except engine temp plus it gives you a nice defined % leakdown number that can be compared to an absolute. When you run a compression test and all is well but the problem persists… get out the leakdown tester!

It’s like if you thought your bike tire had a leak, but not sure. So you connected a bicycle pump (which has a built in pressure gage) to the bike tire, pumped up the tire, then left the pump connected. If the tire has a leak, the gauge will slowly move down. “Leak Down” in other words. No leak, the gauge will remain steady.

Leak down testKit

If you are trying to find a problem a compression test is quicker. If you want to find the condition of your engine, a leakdown check is best. The percent leakdown you get is indicative of the engine. A well broken in engine can leakdown as little as 3-5%. A well worn motor may be showing 10%. Over 20 on any cylinder, or all tells you a rebuild is in order.

Since a compression test is quicker, I would do that first, if the results are good I wouldn’t bother with a leakdown test.

Leak down testwithcompression tester

@Mustangman - if there’s not any apparent problem, and a compression test looks good (even pressures within spec) is there a reason to do a leakdown test?

I had a Jeep Wrangler with a 4.0 liter engine come in with misfire codes for cylinders 3 & 4. After checking the obvious things that can cause a misfire I did a compression test. All readings were within specs. So then I did a leak-down test. When compressed air was introduce into cylinder 3, air started coming out the spark plug hole for cylinder 4. And when air was introduced into cylinder 4, air came out of the spark plug hole for cylinder 3.

Leak down testwithouttester

Leak down Tester

A compression test is a dynamic test where the pistons move up and down quickly where it won’t indicate a problem in these areas. But a leak-down test is a static test. And will show the real problem over a compression test.

Do you only do a leakdown if the compression test is bad? Any basics on this? They are just words to me. When do you do on, not the other, or both?

Leakdown test is how well the cylander holds pressure. Pressure test is compression while the engine is working. Both can be used to evaluate head gasket and or cylandar performance. Outboard motors have been my experience with leakdown tests, cars usually seem to navigate towards compression tests.