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Cylinder Leak Down Testing

How to Use a Cylinder Leakdown Tester

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Horsepower and torque are all about cylinder pressure. Compression ratio, cam timing, cylinder head flow, and all the rest of the goodies that go into an engine aren't worth much if what enters the cylinder doesn't remain there to be turned into cylinder pressure that will push those pistons around.

In the dark days of engine-building, car crafters had to rely on a compression tester to tell them if they had a weak cylinder by comparing the cranking pressures of the cylinders against each other. But this was crude and a less-than-accurate way to evaluate how well the engine was sealed up.

Today, it's common practice to use a cylinder leakdown gauge to evaluate how well each cylinder performs in retaining pressure. The process is simple enough--pump a measured amount of air pressure into the cylinder, and the gauge will tell you the percentage of leakage. But there's a whole bunch more to this story than just pumping air into a cylinder. Let's take a look at how to perform a cylinder leakdown test properly and what you can learn from the results.

Let's start with the leakdown tester itself. There are basically two different styles: single- and dual-gauge units. The single-gauge units would seem to be a little easier to operate, but they require you to pay particular attention to the inlet pressure, as each gauge will have its own requirement. If the inlet line pressure is too high or too low, the result will probably be in error.

Some twin-gauge models also require a specific range of inlet pressure, which you read on the first gauge. The second gauge will read a percentage of cylinder leakage. The Matco tool that we use in this story works in this manner.

Regardless of which tool you use, the point is to use your leakdown tester as a comparative tool. For example, your gauge might read 10 percent while your buddy's reads only 8 percent on the same cylinder. It's really not critical which gauge is correct as long as it operates consistently every time.

The first step is to ensure the engine is warm. Because we will be pumping air into the engine, leave all the spark plugs in except for the cylinder you're testing. As an example, let's start with a small-block Chevy, cylinder Number One. Yank the plug and turn the engine over until the piston is at top dead center (TDC). If you have balancer marks every 90 degrees, this will help. Now install the air-fitting adapter into the spark plug hole. You'll also need a breaker bar or ratchet and socket for the crank nut. Remember to double-check that your gauge reads zero before you start

The idea is to put air to the cylinder and then gently rock the piston around TDC to ensure the rings are seated. Keep in mind that cylinder pressure is used to help seal the rings to the cylinder bore, so you want to give them every opportunity to do so. Now record your leakdown percentage. It's really just that simple. Test the remaining cylinders in the same fashion and you're done.

Often, the cylinder pressure applied to the piston may turn the engine over. The key is to have the piston as close to TDC as you can so the crank, rod, and piston are all as vertical as possible. Once the rod journal has leverage on the crank on either side of TDC, the cylinder pressure combined with that leverage may force the piston down the bore. This is the reason for leaving the spark plugs in the engine.

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Then what? What about diagnosing the results. Air will get out if it leaks and that is the purpose of the test. Not just that it leaks, but from where and how to fix it.

Car Craft