Performing a compression test on a running motor can give you a wealth of knowledge about the engine. For instance, numbers too low may indicate a bad head gasket, likewise a large variance in numbers can help you isolate problems down to the cylinder. Most importantly, compression testing is perhaps the single most valuable tool when purchasing a used car.
Note: This tutorial highlights a compression test on a 240sx with a KA24DE motor. However, compression testing procedures are similar on virtually all cars.
The only tools required to perform a compression test is a compression tester and a spark plug wrench. Optionally, a battery charger can be used to ensure that the car is cranking at a constant voltage.
This is a simple compression tester that can be picked up at the automotive section of your local Walmart.
Park your car on level ground and engage the parking brake. Open the hood and locate the ignitor by following the single cable from the distributor to the driver’s side strut tower. Disconnect the cable from the ignitor. Disconnecting this cable removes the spark from your distributor which will prevent your car from starting during the test.
Note: If you have a motor other than the KA24DE you simply want to follow the spark plug wires back to where they terminate. For instance, with the CA18DET motor, I would follow the coil packs back to the single plug located next to the intake manifold.
The ignitor is the small black unit with the blue ignition wire to the right of the power steering reservoir.
Remove the 4 spark plug wires from the cylinder head by pulling them up. Set them aside while keeping in mind their order. Using the spark plug wrench, remove the spark plug from cylinder #1 and thread the compression tester into the cylinder head. Next, have a partner ensure that the car is in neutral and then crank the car. Keep cranking the car until the gauge does not rise anymore. At this point, stop cranking the car as you now have the compression on cylinder #1. Press the release valve on the tester and then remove the tester from the head. Reinstall the spark plug and then move on to the next cylinder. Repeat these steps for each cylinder.
The compression tester threads into the cylinder head in place of the spark plug.
Once you have the compression numbers for all cylinders, verify that the spark plugs are reinstalled and reinsert the plug wires in the correct order. Now connect the distributor back to the ignitor by reconnecting the cable previously removed. The car should now crank right up and you should now know the compression of your motor.
Note: Using an external battery charger while running compression tests will provide more accurate results as the car will always draw the same amps.
Note: Although not absolutely necessary, some users may want to depressurize their fuel rail before running a compression test. To depressurize the rail, simply start your car and then pull the fuel pump fuse while it is idling. The car will die once the fuel rail is emptied. Depressurizing the system will prevent the cylinders from flooding during the test.