It seems that everyone will eventually have an idle issue. 1990 190E 2.6L
 

My car seems to get a high idle after the car warms up or after I've driven it for a while. If I turn the car off and turn it back on it goes to normal idle for a little while then it jumps back up. I checked/cleaned the IACV and tested it (works fine), both hoses going to the IACV aren't cracked and are connected, microswitch was cleaned and lubed, throttle cable was cleaned and lubed. If its not vacuum or IACV what could it be that causes the car to idle only after it has warmed up??

I was diagnosing a bad idle on my car and found:

1. Vacuum hose from inner side of throttle body was disconnected. This solved the hunting issue.

2. The high idle in my case was at cold start. This was a bad EHA valve. Replaced with brand new ($$$) and no more high idle.

3. If it's temperature dependent I would check the CIS temp sensor (4-pin in my 2.6, back of engine head). 22mm socket is what you need but one side has to be ground down to be able to remove (and install the new) sensor. Did this on mine too.

How high is your car idling when you say high idle? On my #2 situation above it went up to 2000RPM.

However right now I'm battling an odd non- or hesitant first crank start.

Well I took a shot in the dark and against what everyone says not to do, I went ahead and did it: I went out and bought a new O2 sensor (the 1990 Ford Mustang 5.0). Well it wasn't a complete shot in the dark. I had the car running and disconnected the O2 sensor and the car was running the same as when the O2 sensor was connected, only difference was that I got an CEL code 22. I looked at my records and saw that almost everything was relatively new and the O2 sensor was what stood out at last being replaced almost 35k miles previously. I know that I think the O2 sensors are recommended for every 60k miles, but since I can do the work myself, it doesn't bother me to do it sooner. Changed it in the parking lot at autozone in 15 mins right after I bought it. Drove the car for about an hour in stop/go traffic and the idle was a steady 500-600 rpm. When it was idling high I was at 1200-1500 rpm. I can feel slight roughness/vibrations with the car even though the idle is steady, and my spark plugs were done about 15k miles ago, so I will replace those and hope that the vibration ceases. My coolant temp sensor was replaced on the engine block was replaced sometime in mid 2013.

check the airflow potentiometer voltage. IIRC, should be around 0.70v +/- 0.10 volts.

Problem had been resolved. It was the O2 sensor.

My car's problem was also related to the O2 sensor, but on a more 1-2 basis.

Primary problem was the duty cycle could not be adjusted (needle was fixed, no fluctuation). Looked into what controls duty cycle and eyed the O2 sensor as being the culprit since it's not known when this was last done (long story).

Also did a test to see response to unplugged O2 sensor - caused idle to hunt worse than before, to the point of stalling. Plugged back in and went back to "normal" (with current ongoing problems).

Fast forward to day of installation of correct O2 sensor and duty cycle could now be adjusted. It was way out of range (too rich). Now we got it back to how it should be.

So it seems to me that the duty cycle was incorrectly adjusted based on a defective or incorrect O2 sensor. Installed new correct O2 sensor and ability to adjust back returned.

No more dips of idle RPM and so far, it's starting like normal already.

Thank you for the inspiration! :D

Oh good. I never figured out how to adjust mine to read it with a multimeter w/o duty cycle. I thought there was some mathematical way to convert the voltage to get a duty percentage.

BOSCH KE3-JETRONIC MIXTURE ADJUSTMENT

I think this has the mathematical approach of using just voltage. Lots of good info on there too.