Compression test
A good thing to do now and then is to do a compression test. This is a quick, easy test that requires few tools. You will need to get a compression gauge, about $20 at Advance Auto. You will also need a torque wrench and the proper extensions and socket to pull out the spark plugs. Step one is to warm the car up to normal operating temperature. Next, shut the car off and let it cool for a minute or two. Get some gloves if the engine is too hot to touch. Turn off ALL accesories and lights. Put the car in neutral if MT, or park if AT. Now disconnect the spark plug cables from the spark plugs, leave the other end attached to the ignition coil. Carefully remove all of the spark plugs and attach them to a piece of cardboard with the cylinder number marked like the pic below. I call the left-most cylinder #1. You can write the readings directly on the cardboard next to the proper cylinder.
Now you need to disconnect some relays. They should be close to the fire-wall, near driver's side. You may have 2 or 4 of them, I pulled all of them. This will disable the fuel injection and spark. Now you need to screw the compression gauge fitting into one of the spark plug holes. It should be tight enough to get the rubber gasket to fully seal. Now crank the engine with the throttle open for several turns. Get someone to look at the gauge and see when it stops moving [thats when you stop].
The gauge should read 170 - 225 psi. Once you measure all of the cylinders, compare them. They should not be more than 25% off. On my most recent test, I got 205-205-210-210. They should all be pretty close to each other. If one of the cylinders is below the limit, you should check some things out. Check if the gauge was inserted properly first. Try pouring a little bit of engine oil in the low cylinder's spark plug hole. Check its compression again. If it goes up, you may have a worn or damaged ring or cylinder wall. If the compression does not rise, you may have a leaky valve seat or leaky head gasket.
thx Team 2GNT
DIY: Compression Test for the 420a
