I had pretty much tyhe same problem though not quite the same on m '88 300E. Rough idle, would drop down to 500 or below and the car would surge a bit at low speeds when coming to a stop. Turned out to be a leak at the power brake vacuum booster.

A very knowledgeable guy suggested that, because of the surging, I probably had a bad booster and that I should disconnect the vacuum line and plug it. If the idle smoothed out then it was the booster. All I did was tighten the fitting at the booster and the problem went away.

Smooth idle, no surging, smooth, even acceleration. It's worth checking out the vaccuum booster.

I've been through the same problem, changed ICV, hoses, relays, swapped out the potentiometers, etc etc. The problem kept getting better then coming back in a slightly different form in a week or so. It went away entirely when I changed the coil and plug wires.

Not saying that the other parts didn't need replacing - after 15/16 years if you start replacing some parts in the fuel system, sooner or later you'll end up replacing them all - but in my experience, the only way to deal with this problem conclusively is to give the entire system an overhaul.

Another major factor for me was the condition of the wiring harness and connectors. A lot of the connections were suspect, particularly at the ICV and EHA valves. Fixing these helped but, as I said, the problem didn't disappear entirely until I put in a new coil and plug leads.

A few of the guys in the forum suggested the coil as the culprit but I wasn't sure until I noticed the idle/surging problem in one of its final manifestations appeared while I was driving up into the mountains. Remembered from my bike days that coils when on their way out will break down under load so went ahead with the replacement. Haven't had a problem since.

Good luck

1988 300e 111k miles

Mine was fine. Then we had a 13 degree morning and the same surge problem started happening. The mornings have warmed up to the low 20's. It was 50 today.

Seems to go away....after driving a few miles and getting to operating speed... I'll be busy for a couple of weeks, but plan to clean the IAC or ICV and check for any vacuum issues.

I don't know if the cold weather and the problem are a coincidence or not.

I have an Expedition that had the same problem. Stealer wanted 550 dollars to fix it. I bought a part for 55 dollars... later learned that some brake cleaner really does the job, so I have a spare.

My approach with the 300e will be to pull the IAC and quirt it down real good.

Icv
 

Can someone point out where the Idle Control Valve is lccated on the 88 300e?