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?