I have a 97 M with 146k miles. Bone stock right now. I'm wanting to go FI so I did a compression test today and the results were 218-223 across all cylinders. This is the process in how I did the test. Warmed the car up to operating temp, shut off and unplugged CAS and ignition coils, removed all spark plugs, tested each cylinder by cranking for 5 seconds with gas pedal to the floor. I know those numbers are way too high, so do I just chalk it up to the compression test gauge being wrong or should I be concerned about too much carbon buildup. The variation between cylinders was very small so that's a good thing.

took video of the test

 

Always test your test equipment!
I once tested 6 different compression testers on the same car in a large shop I worked at. Every one read differently!
You may be able to check accuracy of your tester by hooking it up to an air compressor that has a gauge on it.
I'd bet your tester reads high.
What I look for is readings close together which you have in spades.
You can see the tops of the pistons with a pen light. To have equal high readings due to carbon would be strange.
Cam timing and valve adjustment have more impact on a compression test than you might imagine.
Valves too loose or intake cam timing advanced would create high readings. Shimmed valves seldom loosen and not all would loosen the same amount.
incorrect valve adjustment after a valve job might cause this.
You are probably fine...

 

Your test procedures are almost perfect. Only thing I add is hooking car up to a battery charger during the process as I have seen a drop in cranking speed on the last cylinders tested.
It's not a big issue on a four cylinder but important on eight or more cylinders.
Not doing it at operating temps or with the throttle open is the normal mistake.
Your car may have hydraulic lash adjustment. If so disregard the bit about valve adjustment.

 

Specs for a 99 from the OEM workshop manual (ie: using a properly calibrated tester)

Standard: 209psi
Minimum: 146psi
Max diff b/w cyl: 28psi

Manual notes that it's a dry test, and if numbers are low to then re-check with a wet test.


Were the plugs wet with oil when removed?
if the rocker cover gasket is leaking into the plug wells it can give a falsely high reading, since any oil in there can dribble into the cylinder when the plugs are removed, essentially giving a wet test.

 

Thanks for the info. I'm going to say I'm good then!

 

Nope. All the spark plugs were dry and look good. I think its the gauge as it is a $20 one I bought on Amazon.

 

It's fine. The only numbers that matter are cylinder to cylinder. There's a lot of variation in gauges.