Yet another m103 idle problem
 

1991 300E 4matic.

Car starts up cold and idles at 1k - 1.1k. As soon as it hits operating temp the idle shoots up to 1.9k - 2k. Disconnect icv and it hands out at 1100 when warm and ~500 - 600 idling in gear.

I think this started after I did a head gasket replacement, but I am not sure as the car sat for some time.
The air pump failed, so I removed it. I removed it and plugged the appropriate lines. While I was doing the head gasket I decided to delete the egr line, block the egr valve and plug the hole in the intake manifold.

Vacuum is at ~18 - 19 in-hg and is steady at idle (icv connected or disconnected)
Replaced idle control valve with a brand new one.
Fixed a few vac leaks around injectors.
4 prong temp sensor checks out against this
2 prong sensor does not read the same as 4 prong (not sure if it is supposed to)

Perhaps I was wrong, but I was pretty sure I could delete the egr stuff since I have seen many people block their egr valve. (We do not have inspections where I live)
What is left? The 2 prong temp sensor? The O2 sensor?

Other than the egr stuff I am at a loss.

sounds l I'm e a vacuum leak
 

Use some carb cleaner and spray around possible vacuum leak areas, one that comes to mind is around the intake manifold gasket possibly from the head gasket work. A change in idle as you're spraying should help isolate the leak. Anouth possibikity is an unplugged vac line somewhere less obvious.

Does the throttle close all the way and actuate the idle switch? It is back by No 5 and 6 intake runners.

Yes, the throttle does close. And I hear the switch. Even if I move the throttle linkage a tiny bit and hear it click there is no change.

As for looking for leaks, I found a number of minor ones and repaired them. Even they did not affect the idle much.

What is most odd to me is that the idle only goes to 1500 - 2000 as soon as the engine hits operating temp.

if not air, fuel?
 

The idle increase at temp is odd indeed.

When you did the head gasket did you have coolant in the combustion chamber? There is a posibility that burnt coolant has coated your primary O2 sensor, and it may be reading lean and thus over fueling. If this was the case, You might notice some oscillating in idle once at temp.

Still, leaning towards a vac leak, possibly the icv is plumbed backwards??

That's the "micro switch", which has to do with fuel shut off on overrun. There's also a switch on the rear side of the throttle valve shaft below the fuel distributor. It's connected to both the EZL and ECU via a three pin connector that is under the air cleaner on top of the inlet manifold and easy to access. One is ground and the other two are system voltage for the EZL and ECU.

I had a similar high idle problem and checking continuity at the connector showed that the switch was not working. It appears to be very difficult to remove due to its location, so my first "fix" was to thoroughly douse it with electrical cleaner. That got it working again and idle control has been normal for many years.

Duke

Try using the x11 diagnostic plug. Use a multimeter and set it to VDC and use pins 2 and 3 while engine is running. The voltage will oscillate between a narrow range of values ex., 2.3 - 2.9. Spray carb cleaner around the air filter housing boot near the fuel distributor and see if the values change. If so then there is a leak somewhere there. Also, try tapping on the throttle body air potentiometer sometimes it will get stuck. It happened to me yesterday and my idle shot up to 2k. I had a long extension and slid it down to the throttle body and gave it a few taps and it was back down to normal idle.

I will check the voltage on the x11 in a little bit.

I assume we are talking about the throttle position sensor located on the side of the throttle body?

I disconnected that and there was no change, but that could mean anything.

What are the proper resistance ranges for it? I assume if I move the throttle the resistance value between pins ? and ? will change (since it's a potentiometer)

Try this:
https://www.landiss.com/mixture.htm

This is for the air flow ratio right? I am looking for the potentiometer on the throttle.

That was for the values of the x11. Attached photo shows where to tap to see if idle is corrected. Have engine running, no need to remove anything if you have a long screwdriver or extension. When the idle revs up to the out of spec rpm give the area a tap (trying to unstick it if it is stuck, so enough force to cause vibration).

At idle (icv connected or disconnected) voltage between x11 pins 2 and 3 is 7.11v

From the throttle position sensor:
Measuring resistance pin 1 being closest to the front of the car. Pin 1 to 2 2.797kohm closed, 3.723kohm at wot, pin 3 to 2 2.917ohm closed, 3.902 at wot.

I did remove the throttle position sensor when I was cleaning the intake during the head gasket work. I wonder if I jacked something up.

Can anyone validate these readings?

Try this:
https://www.startekinfo.com/StarTek/outside/12264/Program/ETM/ETM.pdf

FSM for 91 300E electrical troubleshooting.

Man, that has a lot. I am not exactly sure what that potentiometer is called in the docs, but I didnt find anything that indicated a valid range.

Would any of you great people with an m103 be willing to take the same readings on the throttle position sensor?

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!