Any good mechanic or DIYer will tell, "The best way to see what shape you engine is in is to do a compression test."
1: Start your engine and get it up to running temperature. Once at running temperature, turn off the car.
2: Remove the two fuses in the number 1 and 2 spots in the fuse box.
3: Remove the black cover covering the spark plug wires. This is held in with 6 Torx screws. The size is T-27
4: If the spark plug wires are not marked, make sure you mark which one is which so that you can put them back in the correct place.
5: Remove the spark plug wires. They are very snug so be careful not to break them.
6: Remove the coil wire from the ignition coil.
7: Remove the spark plugs. Screw the compression tester in cylinder number 1.
8: Get in the car and run the starter. Make sure the engine turns over 6 -7 times while holding the gas pedal to the floor. Check reading on gauge. Make sure you do this at least twice for each cylinder. If you get different numbers each time on the same cylinder, average the numbers together.
Once you have done the compression test on all 5 cylinders, you can start putting everything back together. Make sure you check the spark plug gap on all the plugs before you put them back in the car. The gap should be .028 inches. Mine were way off, which explains the slight miss I was getting at full throttle.
These were my numbers on my 850 T5. The car has 190K miles so I am happy with them.
Cylinder 1: 150
Cylinder 2: 150
Cylinder 3: 150
Cylinder 4: 145
Cylinder 5: 145
