Cylinder Leak-down Test

One of the simplest but most useful pieces of tune-up equipment is the cylinder-leakage tester. It can tell you if your engine has damaged valves, worn rings, a blown head gasket or a cracked block, thereby pinpointing compression leakage.

The cylinder-leakage, or leak-down, tester operates on a simple principle: A cylinder with its piston at top dead center (TDC) and both valves closed, should be reasonably airtight. By injecting a measured amount of air into each cylinder and observing the rate of leakage, you’re able to see if the cylinder-piston-valve assembly is good.

If the cylinder doesn’t hold the air, it has to be escaping – either past the rings, through one or both of the valves, through a crack in the head or block, or past a leaky head gasket. By simply listening at various points of possible escape, it’s relatively easy to determine exactly where the problem is. Worn rings will allow the air to seep into the crankcase and out the oil filler tube; a burned exhaust valve will allow air to exit through the exhaust system; a burned intake valve will allow air to exit through the carubretor; a cracked cylinder head or block will allow air to escape through the radiator, as will a defective head gasket, which also may let the escape from under the cylinder head.

Okay, that’s what a leak-down tester can do for you. If you’d like one, you can buy one from any of several reputable manufacturers for a few hundred dollars. Or you can make your own for about $35.

First, you need an air regulator. It must be fairly small and easy to handle, and it must be the self-relieving type. That is, capable of providing 0 to 100 psi without bleeding the line it’s connected to. You’ll need a regulator with two outlet ports, threaded for pipe thread.

The next item on your list is a 100 psi pressure gauge available from plumbing supply houses. Be sure to get one with a threaded fitting to mate with your regulator.

Next stop is you neighborhood auto-parts store for a length of high-pressure hose, an adapter to screw into your engine’s spark-plug holes and some air-line quick couplers to tie it all together. Either a length air-line hose 12 to 18 inches long or a high-pressure grease gun hose will work. Depending on the size of the hose’s threaded end you may need bushings. At any rate, it’s likely to be male thread, so buy an air chuck to fit it.

You’ll also need a second male air chuck to screw into the inlet port of the regulator, and a quick coupler with a 1/4 inch male threaded end. These items are available in most auto-supply stores or through large chain stores and should cost less than $5 total.

One of the male air chucks screws into the regulator’s inlet side. Use sealant on all threads to make sure the connections are air-tight. The pressure gauge screws into one outlet port and the female quick coupler goes in the other. Plug any other open ports in the regulator.

Screw the remaining air chuck onto one end of the hose and the spark-plug hole adapter on the other. Make sure you use the adapter that’s sized for your car’s spark plug holes; keep the other one in a safe place. There, you have a cylinder-leakage tester!
Now, the next step is to learn how to use it. Start by removing all the spark plugs from your engine. (Make sure to mark all the wires first.) Next, you’ve got to bring piston No. 1 to TDC on the compression stroke – both valves closed. Screw the hose with the spark-plug adapter into the No. 1 cylinder’s spark-plug hole. Connect the other piece of your tester to the air supply by using the male chuck that’s screwed into the regulator. This charges the tester with pressure. Adjust the regulator until the gauge reads 100 psi. Now connect the two pieces of the tester together.

Since you’re no longer injecting air into the cylinder, your gauge reading will drop off as some air seeps past the rings. A 20-percent drop in pressure is the generally accepted limit for a healthy cylinder. In other words, if it drops below 80 psi you’ve got a problem. Before condemining your engine, however, be sure the test are done on an engine warmed to operating temperature. (A cold engine will not hold air as well as one that’s warmed up.) Also, be sure the piston is at TDC on the compression stroke. Remember – all cylinders leak, but at different rates. The less leakage, the better the cylinder.
by John Conway