So I found some threads similar to my issue but nothing exactly the same. Anyway, my idle on my 1990 420sel was perfect up until I changed my thermostat and flushed my coolant. While changing my coolant and thermostat I disconnected te idle air control valve connector, the coolant temperature sensor and the other sensor directly next to the coolant temperature sensor. Also while flushing the coolant I got water all over the front of the engine. After I was done with my job and everything was reconnected I started it up. When it started it refer to about 1500 as usual then the idle quickly went down to about 650 or so then as the engine warmed up the rpms slowly crept up until about 1400 rpm. For a few days it was an intermittent problem but now it's constant. Everytime I start it the rpms slowly creep up until about 1400 rpm. Sometimes it will vary while the engine is running between 900-1400. Since then I've cleaned the idle air control valve, I've shaken it and it rattles but I'm not sure if rattling means it is still in good shape 100 percent of the time and lastly I've cleaned the connection for the idle air control valve. None of this has made a difference at all. I'm almost positive it's the idle air control valve but I would like to make sure before I just go out and spend money. If anyone has any ideas or other possibilities it could be and ways to test what's wrong please let me know. Thanks you.

Model year? And if 1986, the production date from the data plate on the left front door pillar.

I edited my post

Three things to start with:

1) The temperature sensor should be a two pole type, unless the car was originally a California car, in which case the sensor is a four pole type. Proceeding as with a two pole type, each of the pins should show a resistance to ground of between 250 - 5000 ohms, depending on temperature.

2) With the throttle closed, pin #1 on the connector to the throttle valve switch should show 0 ohms resistance to ground.

3) Check the Overvoltage Protection relay (OVP) for a blown fuse. The OVP usually has a red top with a clear cover; fuse under cover.

Sooooo. The over voltage protection relay was all good. I went to check the throttle valve but I really am not sure what the throttle valve is. If it has another name I might know what it is or if you could explain it or post a picture i'd greatly appreciate it. Lastly I checked the two pole temperature sensor. The pin closer to the engine was reading within tolerance(I can't remember the exact reading) but the pin closer to the radiator only gave me a reading of 4 ohms which is out of tolerance.

The throttle valve is the throttle that controls the flow of air into the engine; Mother Benz uses a four-bit name for a two-bit part. The throttle and the switch are located at the front of the intake plenum, and buried where they are just barely visible. However, the harness from the switch comes up to an accessible junction block located just behind the cold start valve, and hiding under an air hose from the left cam cover. The center pin is a ground for the switch; pin 1 should have continuity to ground with the throttle closed, and pin 3 should have continuity to ground with the throttle wide open.

In the event that the good temperature sensor is presently connected to the ignition box (EZL), reversing the connector would connect the good sensor to the ECU, and thereby to the idle speed controller. Useful as a quick check.

A couple of tests...

1. Measure the voltage at the two wires going to the idle control valve. Is the voltage slowly changing along with the idle? Or voltage staying the same/no correlation to changing idle, yet idle increases anyway? That can rule out the electronic Idle Control Unit and the various inputs going into it.

2. I made an idle control valve bypass hose out of heater hose and garden sprinkler fittings found at a hardware store. Then connected that in place of the Idle Control Valve. Use vice grips to squish the hose in the center to only allow a little air through. Start the car then adjust to what the idle should be. Let car warm up and see if idle remains the same and/or if you can adjust it after warm to be the correct idle. This would eliminate air leaks elsewhere - if maintains idle, then it is possible for the Idle Control Valve also to keep the idle at what it should be.

What about how the pin closer to the radiator on the temperature sensor is only reading 4 ohms which is way out of tolerance according to what it was said it should be. Could the idle control module think the engine is cold the entire time it is running and just be keeping the idle that high? Oh and by the way I did another check. I checked how much voltage the idle control valve is getting when the engine is at 1500 rpm and it is receiving about 4 volts.

The temp sensor can be a problem or another problem - do check that.

Then the actual test with factory specs for the ICV is amperage or mA, but that is difficult to do. Easier to pull the plug out a little and read voltage - see if that is changing or not.

But I don't think the Factory Service Manual has any specification for voltage? So don't know if 4 volts indicates anything.

Anyway in this case, a change in voltage/current might indicate the electronic Idle Control Unit is making the idle go up. No or little change might mean the valve itself is faulty.

I had a similar problem and wound up replacing the ICV. That fixed my idle.

I've heard of testing the idle air control valve by applying 12 volts to the pins. But how exactly do you do this test? If you know could you please explain this. Thank you.

Well you would just disconnect the electrical connector, then apply 12 volts DC to the pins on the ICV and you would hear it "click".

If you did that with the engine running, first the engine would race real fast as you disconnected the electrical connector (no power to ICV), then the engine would die as you applied 12 volts as this would cause the ICV to totally close off air to the engine.

That would just be an extreme test for totally open / totally closed - might even wreck it if 12 volts was left applied instead of just briefly applied. And would not test to see if the ICV could accurately keep the engine at a certain idle speed.