My 88' 300E has had a rough idle off and on for some time. I have replaced the plugs and used a fuel injector cleaner in the tank. It comes and goes. Now it also idles erratically at times. The tach normally idles at around 600 rpm, when it lopes it drops to around 400 and goes up to about 1500. At times it will drop too low to recover and will stall. Any idea what might be the problem?

I think i have a solution for you, I hope, for your sake. The other day I was bored, (this is going to sound really stupid) and I went under the hood of my C280 and I unplugged the wire that hooks onto the tube that comes from the air filter housing. I then started my car and it did the same thing as you've explained. It dropped real low, then popped up to 1500 RMP, never stalled though. So maybe your connection there is faulty? I am not sure if that's the Oxygen sensor, but it could be? HOpe this helps. Try and clean the connection and see if it helps.

Early 300Es don't have the sensor that you are talking about.

I would check the overvoltage protection relay behind the battery. It has a 10amp fuse on top of it. Check the fuse and it would be great if you had another relay to plug in. Also, check the air flow bypass valve. It's right under the air filter housing.

Try disconnecting the two pole wire on the valve, your idle should go up.

Vin