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.

There is a vacuum pump in the right side trunk for door locks, are you hearing that? ( Remove the right trunk liner )

This engine should have 2 ignition coils, one end is on the spark plug and the other a wire. The coil can fail taking out one or two cylinders. If the coil boot fails it can take out a cylinder as well.

I know on the 97 cars with ME 2.1, if the computer detects a miss fire it shuts off the injector for that cylinder, I'm not sure if the 96 ( Pre ME 2.1 ) is that advanced.

Are you able to get any codes in order to verify you readers operation?

Thanks 97, I did have some codes at first, they were P0300 General misfire, P0304 Cylinder #4 misfire, and a couple of body codes. Since starting the process I have not had the misfire cause a code to be set...

Reader seems to work, it's just when I try to read other modules (car side modules) there are none other than the standard $64. O2 sensor can't be reached because the car side doesn't have it to read by my reader.

The vacuum pump cycling couldn't cause the miss that I am seeing, but annoying as heck and I'm not in the back seat. Does the vacuum system extend into the engine operation? Does it use the engine and the pump as a vacuum source?

Please show me what is it?
Have you simply VAG COM 409.1 or any 12V KKL adapter?

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Mercedes W124 diagnostic scanner (code reader and live data) with laptop

The trunk mounted vac pump is for body only and does not connect to the engine.

I'd have a close look at cyl 4 ignition system. Misfire codes are triggered by watching engine speed and if it falls too much between the prior cylinder and the one that is firing, a miss fire code is set. However the technology was still evolving and limits were sometimes set high to prevent a false code over a bumpy road.

You might consider the coolant temp sensor, since it semes to happen with temp. However, I just resolved a VW misfire due to plug cables that also only happened at warmer engine temps.

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