I have been working on my sister in laws C220. This is the one I replaced the engine harness, ignition low voltage and high voltage harnesses, and the transmission harness last year.

Now the check engine light has come on without setting any codes that I can see. I am using a high low range OBDII diagnostic device from Harbor Freight. Fuel pressure hold 50 psi while running and doesn't drop while the engine miss is present. Front O2 sensor test low on the bench and rather erratic. O2 sensor should not make it miss individual cylinders. I do hear a pump in the back cycle most of the time. It's coming from the fuel pump corner of the car. What other pumps are in the right rear quarter of the car?

It seems to happen while transitioning thru 80-90 Celsius on the water. Seems like it has lost but two cylinders when it happens. Turn it off wait 5 minutes and it's ok, just the MIL showing lit.

A year ago I went thru the car from front to back. Harnesses, spark plugs, belts, water pump, coolant hoses, brakes & rotors, tires.

Being a 1996, I think first year MB had OBDII, and the codes were pretty weak.

Help would be appreciated. If you have seen similar problems, please chime in thanks.