EJC, based on my experience I'd be looking at your Idle Control Valve. On mine, I was losing power to the unit due to a problem with the ECU (i think). Check the voltage across the terminals and see what's going on. Should be about 6v in normal operation, if the voltage drops, the unit will revert to limp home mode, which will give you the lumpy 500 rpm idle and stalling issues you describe. Most likely in your case will be a defective OPV, which delivers power to the Idle Control Valve. In my case it appears to be the the KE control unit that's going bad.

I understand that actual control of the ICV depends on current, not voltage. I tried to hook it up to a meter to measure the current in the same way I rigged the EHA but for some reason it wouldn't have it. Refused to idle, even though there was current and power to the ICV. Maybe the meter is restricting the current flow and affecting operation of the ICV, however it seems to work fine with the same harness monitoring the EHA. Considering we are dealing with 600 - 900ma with the ICV current and a range between + and - 60ma with the EHA i find this puzzling to say the least. Any Bosch wizards out there have an explanation for this?

Tip about the Lambda tower makes perfect sense, mctwin2kman. Will try to find one, hopefully I can get it as a separate part. Anyone have the part number to hand?

Thanks all, on on.

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1987 300e manual 250,000 km (sold)