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Everyone loves having their engine run at peak performance. I mean, who goes out and brags about how their car runs like run over donkey ass. When your engine seems to be tired or is running out of steam and just doesn’t have that pull anymore, we want to know why, where, how. This is where compression tests and leak down tests come into play. They will show you the health of your engine and if/where a leak is occurring inside the engine.

Everyone, well most people know that the internal combustion engine used in today’s cars has 4 strokes, intake, compression, power and exhaust. When one of these strokes isn’t holding up its part of the agreement, this is where you feel that tired/lazy feel from your engine.

Now that we have the bare basics out of the way, where do the compression and leak down tests come into play?

The compression test is the easier of the two but it’s also the one that gives you the least amount of information. To perform a compression test you need to do a few simple steps. First, you need to make sure your battery has a full charge or have a battery charger/tender on it while doing this test. Next you need to eliminate the spark and fuel delivery to the car. It’s also important to make sure the car is at operating temperature.

On mustangs, the easiest way to do this is un-clip the injectors and coils. Now that you have the spark and fuel disabled it’s time to start the test.

One cylinder at a time, remove the coil and plug. You now screw in the adapter for the compression gauge down into the spark plug hole and be sure to only tighten the adapter hand tight. Now you can attach the compression gauge. With the throttle wide open try starting the car for 5-7 seconds and record your findings.

For this example let’s say the magic number is 180. Your first cylinder gives you 175, that cylinder has great compression. As you move from each cylinder you’re looking for consistency. There is going to be a slight variation from each cylinder, but as a general rule of thumb you are looking for a difference of 20-25psi max. If you start seeing a variation greater than that, then you are working with tired cylinders.

The other type of compression test is the wet test. You do everything the same as above, but this time you add a cap full of transmission fluid into the cylinder through the spark plug hole. If the readings go up, this means you have piston rings wearing out. If nothing changes you have a leak elsewhere.

Leak down test is just what the title says, looking for where the leaking cylinder pressure is going. To complete this test, you need an air compressor and a leak down gauge.

Like the compression test, you do one cylinder at a time. The only difference is you need to have the cylinder at TDC (top dead center). Easiest way is to get a long straw or screw driver and set it in the spark plug hole. Next turn the engine by hand until the object in the plug hole stops moving vertical.

Now, screw in the adapter into the spark plug hole, just like the compression tester gauge. Next you hook the adapter to the leak down gauge, and then hook the air compressor to the leak down gauge. When you look at the leak down gauge you see it has a knob on it. With the air compressor hooked to the gauge, turn the knob until the one gauge’s needle reaches 100. Now watch the other gauge and record the reading. 20% is max for leak down loss.

Now that the gauge is hooked up and pressurizing the cylinder you need to listen for where the air is escaping.

If the air is coming out of the throttle body, then you have an intake valve that is not sealing all the way. If the air is coming out of the exhaust, the exhaust valve is not sealing all the way. If you take the oil dip stick out and you feel/hear air escaping through the dip stick the rings are leaking. You can also take the oil cap off to listen/feel for the air, another way to see if the rings are leaking. If you pull the spark plug next to the cylinder you are testing and you hear/feel air out of the plug hole, the head gasket is leaking.

Now that you have the tools to diagnose the health of your engine, you can save money and guessing on what might be wrong with the engine in your car.

You can even rent these tools from some auto parts stores if you don’t want to buy them yourself.
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