How to: Leakdown and Compression Tests By: Brian Cummiskey There is some confusion between the leakdown and compression test. They are two different tests that test two different things. In this article, we will explain the basics of how to do each test, and what the results mean. Compression Test: To do a compression test, you remove the spark plug and put in the guage. Remove all the spark plug wires from the dist or just undo the coil if so equipped. Now turn the key with the throttle wide open and keep cranking the car until the guage reading rises as high as it seems it is going to go. Repeat those steps for all the cylinders. You want all the numbers to be somewhat close of each other. If some are way low, you know you're losing compresion through the rings or the valve seals, or possibly the head gasket. Usually if its the head gasket 2 cylinders will be normal and 2 will be low, the 2 that are low will be right next to each other, for example, cylinder 1 will read 60, cylinder 2 will read 60 cylinder 3 will read 150 and 4 will read 145. Since the 1 and 2 cylinders are right next to each other, chances are the gasket went bad between those 2 cylinders. Thats not always the case but usually is. It is also possible to have only one cylinder low and it can still be the head gasket though. Leakdown Test: To perform a leakdown, start by taking off the oil cap off. Then you bring the cylinder to top dead center (TDC). Hook up the leakdown attachment to an air compressor and screw it in the spark plug hole. Turn the compressor on and bring the cylinder pressure to the factory spec. (Consult your Helms manual for this number.) The attachment allows air into the cylinder and brings it to the factory pressure. You then listen for air coming out of the manifolds. If you hear air hissing out of either manifold or the muffler you have bad valve seals. If you hear air hissing out of the oil cap or breather valve (if the oil cap is still on the motor), then chance are your rings are bad.