2009 mini

Just a quick question if a car has low compression on when someone will say a bad pissing ring could that cost courage rich or would it cause a lean condition.

A bad piston ring would be a remote possibility and it would depend on if it was a maf or map style engine. Since cylinder pressures would bleed into crank case and be sucked into intake through pcv valve. If it was a maf system it would cause a lean condition. Map system depending on leakage you may not see it in fuel trims but you would see lower vacuum if it was a high enough leak. Only was to tell if it was a piston ring would be a cylinder leakdown test with both valves to cylinder closed and listening through oil fill cap or dipstick tube.

There is 18% difference between cylinder 4 being the low one and the rest is that too high of a difference

10 % is the general rule for compression tests. Do you have a incylinder pressure transducer to see the cylinder pressure waveform and a ocillascope? If yes test with that and sync it with the ignition to that cylinder.

If you dont have a in pressure transducer cylinder you can make on from a high side a/c pressure sensor and use some lenghs of wire to tap into your ac pressure sensor on car for sensor power and ground amd probe into the signal wire with your scope.

This will tell you if its a valve, cam, problem, timing belt problem. Let me know if you have those tools

No I don't have those tools

Relative compression waveform