A leak down Test can be performed on a warm engine. The appropriate leak down testing apparatus will be required, along with a good air compressor. You should not have a cylinder leakage of more than 10% in any cylinder for a fresh/healthy motor. You should also not have more than 5% variance between any two cylinders. If you do, then you will need to further investigate the cause of the leak. More than 20% leak down is an unhealthy motor and you should investigate further and find the appropriate course of action for repairing/replacing.
You will need to make sure you are exactly at TDC for the tested cylinder, and on the end of the compression stroke/start of the power stroke. If you are 360 degrees out, you can have a leak from the intake or exhaust valves which will make your results useless.
If you have a leak down of more than 10%, you should be able to track down the source of the leak, which will help determine the best course of action for repairs.
One of the benefits of a leak down test is that it can help isolate the problem areas. You should be able to hear the leak, or use a plastic bag and some tape to help locate leaks. An audible leak out of the intake indicates a problem at the intake valves (either valve seat issues, bent valves, etc). An audible leak from the exhaust (and you will still be able to hear the leak at the exhaust tip) will indicate an issue with an exhaust valve. If you can hear the leak in the expansion tank (with the cap removed) then you have a blown head gasket, cracked head, or an issue with the block. You will generally hear some leakage in the crankcase. A little is normal, but a lot is bad. This is why the leak down tester has a gauge.
Be sure to record your readings for each cylinder, and we recommend testing each cylinder more than once as a sanity check.