An engine compression test is used to investigate the condition of the engine. The basic theory is that by using the starter motor to crank over the engine, you remove the spark and fuel and see how much pressure each cylinder will build.





The compression test is often confused with a leak-down test. We always start with a compression test because if the compression across all the cylinders is above the minimum required spec, and each cylinder is within 10% deviation of all the other cylinders, then it's fair to assume the engine is in good shape.

Engine Compression Test Guide and Results

The engine leak-down test comes after you perform a compression test, if one or more of the cylinders showed bad compression. By adding a few drops of oil through the spark plugs holes at the top of the cylinder you can effectively temporarily seal the piston rings. Then you re-do the compression test and see if the numbers improved. If the compression holding did go up, then you know the problem is with the piston rings. If the numbers didn't get any better, then you can be sure it's not the piston rings, but instead could be a head-gasket, or bad valves. This will mean the difference the between pulling the whole engine out, versus just removing the head.





Procedure...

If your car battery is weak or not powerful enough, there's a very good chance this test will kill your battery. It's a good idea to have a charging kit near-by just in case.Check out this Article on How To Relocate your Car Battery.

Start by removing the strut tower bar (if you have one). Then, using an allen key, remove the cap screws from the coil-pack cover. Next, you'll need to unplug the ignition amplifier situated at the rear, bolted to the coil-pack cover. There's one large plug, and a ground wire that need to be disconnected, then you should be able to lift up the cover just enough to reach your hand under, and unplug the ignition harness from the under-side of the amplifier.

Ignition Cover Removed for Engine Compression Test

Now you should unplug each coil-pack from the ignition harness, and remove the harness completely. Next there are a couple of screws off-set in between each coil-pack, remove them and lift the forward three coils vertically upward, using caution to not tear any of the boots. Then you can lift up the rear set, and you should now see the heads of the spark plugs. Worn coil-packs are often the cause of miss-fire problems, the procedure for testing coil-pack condition is outlined in this tech article: How To: Coil-pack Resistance testing

Use a spark plug removal tool with a ratchet extension to carefully remove each spark plug. Then, screw the compression tester adapter into the spark plug hole #1, and connect the gauge to the hose. Now, you'll need to make sure no fuel or spark is triggered while doing the test, this is accomplished simply by disconnecting the Cam Angle Sensor, located on the front of the engine on the timing belt cover.

155.5 PSI after 3 cranks

167.5 PSI after 5 cranks

This next part is easier to do if you have the help of a friend but it's not absolutely necessary. If you have a friend, tell them to sit in the drivers' seat and press the throttle down to the floor and keep it there, while you hold the gauge in your hand and watch the pressure build. Turn the key and allow the engine to turn over between 3 to 5 times. If you're doing the test just as a quick check of the condition, then three cranks per cylinder may suffice. By the third crank you should see a pressure of at least 150 PSI to indicate proper sealing. If you're doing the test to see what the real maximum seal pressure is then it's recommended to do 5 cranks, where you should see a pressure of near 160 or above.

Analysis of Compression Test results...

A brand new fresh engine built to Nissan OEM spec will see just near 170 PSI on each cylinder. A new engine fitted with a thicker head gasket such as 1.1 mm or 1.2 mm will see less pressure due to the increased volume of the combustion chamber, and you'll see near 150 or 160 PSI. An older engine with roughly 100,000 km's on it may see near 160 or 155 PSI.

Healthy Engine Compression Results 150 PSI

If your Engine is blown or not holding compression you may need to consider removing the engine, here's a link to our Guide on How To Pull the Engine and Engine Removal.



Anything lower than 140 PSI, or seeing more than 10% deviation between cylinders indicates a potential problem with piston rings, or valves or the head-gasket. If you do find low numbers, then it's a good idea to drop some oil into each cylinder and re-do the test. This is the leak-down test, which will now tell you whether or not its the piston rings or something else causing the problem. A well maintained high-mileage engine can easily see above 160 PSI though, it all comes down to if the engine has been abused or not.