My '88 300e has an economy gauge, FWIW.

Rgds,
Chris W.

My 91 300TE didn't have an economy gauge. Mark

My 1990 190e has a economy gauge.

Back to the OP's problem.....


I would check the operation of the idle switch as well as the idle air valve. Intermittency in either of those devices could plausibly cause your symptom.

Verify that the switch clicks when you take your foot off the gas every time. Verify that your accelerator linkage is not sticking or binding. Once you are sure that the switch is clicking every time, hook a meter up to it and make sure that it is functioning electrically every time.

If so, then put a fresh 9v battery to the terminals of your idle air valve. It should go clunk. If not, then the idle air valve is suspect.

Thanx tiny panzer.... will check... sorry I haven't gotten on I've been driving my sisters vehicle and ignored my benz

Replace the 2nd temp sensor at the rear of the cylinder head. it causes the symptoms you describe.

I had this exact problem when i got my 300CE. The indy replaced the entire air-flow meter/POT assembly with a good used one and the car's been running perfectly ever since.

I should also mention that as an experiment, i tried putting another POT onto the good AFM a few months after. I followed a few posts on here about calibration etc, and i got exactly the same high idle problems. By moving the POT around on the side of the air-flow meter, the idle speed is changed dramatically. I just put the original POT back on and problem went away, normal idle.

Since this high idle was a problem the previous owner never sorted out, i suspect it's why his transmission also gave up and had to be replaced - overheats when you're in heavy traffic and the engine is trying to do 1500 RPM. Previous owner spent a fortune trying to sort it out according to the receipts. I'd start there.