Most common cause of misfire?

[StereoInstaller1]

At idle, the M20 in my E34 has a bit of "puh puh" noise common of a misfire.

All new sensors, nice condition hoses and vacuum lines, intake boot is new, all carefully assembled.

What would you do first to diagnose?

Luke

[MR 325]

Check the plugs, cap and rotor.

[StereoInstaller1]

Check the plugs, cap and rotor.

Yeah, I haven't pulled them yet. Something tells me the hack who did this motor has no clue on gap.

[jlevie]

A smoke test and fuel system tests would be my first diagnostics. Next I'd replace the ignition parts if they are over 5 years old and the plugs need to be NGK ZGR5A. The O2 sensor needs to have less than 100k on it. If not done in the 10k, a valve adjustment is indicated.

If the misfire persists, I'd run compression and leak down tests to make sure there aren't weak cylinders. If that is okay I'd have the injectors cleaned and flow tested.