Had the exact same problem as you with my 1990 300SE, with the only difference it solved it self after I replaced the head gasket. And here is the weird part: after the first start up with the new head gasket the idle immediately went up to 1100rpm and after being warmed continued to 1800rpm like it always did, stayed at 1800 rpm for a few seconds and I heard a bang, idle immediately dropped to 700rpm. And that's where it stayed, the idle was perfect for almost 1.5 years, unfortunately the high idle has returned on occasion after I washed the engine bay a month ago. But only for the first 10/15 minutes after the first start up in the morning.

Changed or checked every part that could have caused the high idle, nothing fixed it or would even change the idle speed. Just as in your case if I disconnected the icv the idle would drop to 1100 and to 800/700 in gear...

Good luck.

Yeah, I am struggling.

I am going to try and do a smoke test tonight just to be sure of the vacuum leak problem.

To me it seams like some sort of electrical malfunction based on a sensor reading or something. If it was purely a vacuum leak I would expect high idle even on a cold engine. In this case it’s like the ECU is opening up the icv for no reason. Or... there is a leak and the icv is opening to make it what would be a normal idle without one.

I love these engines but there are so many places for a vacuum leak on CIS injection. I can’t complain too much though, it is a 28yr old car and I have had it since 2005.

Ok, I finally found something.

I did the smoke test, and I found one leaking fuel injector port. But then I noticed more leaking from under the air flow sensor housing. I decided to dig deeper. I removed it (getting easy at this point), re-seated the fuel injector and did my smoke test again.

I found a leak on both sides of the throttle body where the shaft comes out and attaches to the throttle linkage and where the position sensor is fixed.

Questions:

Is this normal?

Can this be rebuilt or do I need to find a whole new throttle body?

During my smoke test I had smoke coming out of the valve cover. Is this normal or should there be a check valve in between the intake and valve cover?

I think that you need a new valve cover gasket. As for the throttle body, it sounds as though that is worn as well.

There is a PCV valve between the intake manifold and the valve cover, but it only closes when the engine is running and the throttle plate closed. The valve cover and oil filler cap should have a good enough seal to prevent smoke rolling out. The valve cover will fill with smoke during a smoke test due to the PCV breather connecting to the intake manifold.

Check the plastic underneath K Jet fuel injection, it does crack over time with age and there is a large hose clamp that connects it to K Jet unit that can get loose.

Sorry, I wasn’t very specific. Smoke was coming out the hole where the breather line attaches. The line that goes from the valve cover to the air filter housing This

It sounds like this is normal.

Also, I have replaced all the hoses and lines as well as the valve cover gasket.

I was able to find a throttle body at a junk yard about an hour away. I friend and I booked it and got everything pulled just before a big storm rolled in. I was able to pull the entire intake manifold that was completely intact a new valve cover, air filter housing and various other things for $100.

Got home and spent a good amount of time cleaning the new throttle body. I just got it installed and my idle problems are solved. Idle has a tiny stumble stumble every once in awhile, but I had adjusted the air fuel mixture while I had that air leak.

Thanks for everyone’s help!

That’s awesome! $100 well spent!