Ok, to recap, my car won't idle right. It sits at 2200rpm and will not go lower. The IAC valve is fully open all the time. The throttle switch works fine and AMM is fine too.

So what I tested is the Ohms resistance on the #2 terminal ecu imput for the thermostat (this is LH 2.2). I've never done ohms tests before, so I'm not quite sure how to read it. I set the meter at 2K and all I would get would be a short burst of 1.5XX to 1.9XX the it goes back to 1 unless I take the ground off and put it back on. Bentley says I should be getting about 2000 ohms resistance at 70 degrees farenheit, which it is right now. I'm figuring something is wrong here. If the ECU thinks that the engine is really cold all the time, then it will flood the engine with gas and leave the IAC valve open all the time, right? Though I'm confused how this would explain why the car runs for the first 20 seconds or so REALLY rough at about 1Krpm then runs smooth at 2200rpm.

I'm also a wee confused on which is the ECU temp sensor. There are two temp sensor-looking things on the block, one above the other. The bottom one has brown wire going to it, and the top one has a skanky piece of deteriorated wire going to it. I know the engine harness has some bad spots, and this is one of them. When I have the cash I'm going to put a new harness in. Anyways, is the top sensor the ECU temp sensor? If it is and that skanky wire is there, could I run a new wire into the car and splice it to the blue wire right before the ecu to rule out the wiring problem?

That's enough writing and questions. I really hope someone can steer me in the right direction here, as I want the car to be running nice again as soon as possible. Thanks for any and all help as usual,

Nate Gundy
--
'86 240DL sedan, 260K miles, M46, K cam, 25/23mm sways, 260 front and wagon rear springs; http://valvespringcompressor.weblogs.us/

"So what I tested is the Ohms resistance on the #2 terminal ecu "imput"".
When you're testing the temperature sensor, connect the ohm meter to the #2 pin in the harness plug, not pin 2 of the ECU. If you're using a digital ohm meter, it should read .1 or .2 when the meter leads are shorted together. Not sure what is met by "I set the meter to 2k". The symbol for ohms looks like a horse shoe on the meter face. Test the sensor with the engine cold and then hot to determine if it is working correctly. If you remove the idle air control, it can be tested by applying 12 volts to the motor. When looking at the three pin connector, the center pin is connected to the negative battery post with a jumper wire. A second jumper is run from the positive post to either of the outside terminals. The valve should either open or close (look in the end to see). Then switch the positive jumper to the other outside pin and the valve should go to the opposite position. jp

Hi Nate -

Temp sensors. The one you found with the "shanky" insulation is the one for the temp gauge. Non-shanky insulation will be yellow. It's one of the wires that goes through the gray firewall connectors, and all those are the ones that deteriorate.

The other sensor nearby is the knock sensor, single brown wire.

The ECT sensor is located below intake rail #3, right in there with the PCV hoses. Two wires, Blue and Black. I changed mine while also changing the oil separator box and doing a throttle body cleaning. PITA to reach otherrwise.

High idle. Has to be too much air, and often that's the IAC not working. First, the throttle position switch has to trip when the throttle plate is closed. See if it clicks, or use a meter.

Second, the ECU has to tell the IAC to go to work. Harder to test this.

Third, the IAC has to work. Test this quickly by doing as another post said - with the engine "idling" close one of the IAC hoses slowly with large pair of pliers. Drop in rpms means IAC not working.

Another test is, engine off, test continuity between all pairs of terminals on the IAC. If any pair shows open, IAC is toast. If they show resistance, then maybe it's just gummed up. Carb cleaner in the air hose end, and swabbing it out. Use care to avoid having the carb cleaner running up into the electrical workings. Look in the FAQs for more info.

If all but the ECU are OK, well.... swap in a known good one.

Good Luck,

Bob

:>)

I didn't think the IAC was capable of getting the engine to idle at 2500.

Do you have a tach or are you just assuming?

The IAC vents air around the throttle plate to control the idle speed. There is one hose that comes off of the fresh air tube before the throttle plate (near throttle body) and goes to the IAC and then another from the IAC to after the throttle plate. The IAC controls how much air is vented around the throttle plate. The more air....the higher the idle.

Pull the fresh air hose off and check that the throttle plate is completely closed and not sticking open.

Then, pinch the hose between the fresh air hose and the IAC (engine running).
The idle speed should drop to base idle (no IAC control).

If it does drop to base idle then there IS something wrong with the IAC solenoid or circuit.
If the idle doesn't drop then you have something else wrong.
I would verify that the hose after the IAC is not leaking and sucking large amounts of air into the intake causing the high idle.

Well, I took off the IAC and cleaned it really good, taking it apart and inspecting the contacts and such as well. It looks fine. The hoses are not leaking, so I know that's not the issue

Yeah, I have a tach. Just below 2500rpm....more like 2100-2200

I know the throttle is completely closed...I have actually used 2 different ones with no change. If I close off the IAC intake hose, the car dies. I guess I should adjust the base idle a bit there, but that's not the core issue I'm dealing with.

So at this point, I know that SOMETHING is wrong with the IAC valve, be it in the circuitry or the valve itself. I still think it could be the engine temp sensor or some of the wiring there, but I'm not sure. I'll report more back when I discover anything in a few days. thaks for the info,

Nate Gundy
--
'86 240DL sedan, 260K miles, M46, K cam, 25/23mm sways, 260 front and wagon rear springs; http://valvespringcompressor.weblogs.us/

I only have experience with my engine with LH2.0, so I do not
know exactly if it applies. But if you are having a wiring
harness rot then that is most likely the cause of the idle
problem.

Problem 1 - Which temperature sensor

I do not know your engine, but you should have two sensors.
One is used for the ECU. This is the one you are trying
to measure. The other one is for the instrument gauge
in your dash. I cannot tell you which is which, even
in my car I have forgotten.

Problem 2 - Using a ohmmeter.

You did not say what type of meter, but I will assume this
is a digital multi-meter on the Ohm scale. When you use this
you have two things; An amp-meter and a battery in series.
The battery provides a known voltage, and the amp-meter measures
the current flow. This is then converted to "Ohms" on the
digital readout.

Your reading goes from 1.5K or 1.9K then to 1.0K. What I
think is happening is you have a short in the wires, and
as things move (even when everything seems still), the 2K
Ohm resistor from the temperature sensor wire is shorted
to some other resistances to give a lower measured value.
Two resistors in parallel are added by reciprocals

1 1 1
---------- = ----- + -----
R-parallel R-1 R-2

Therfore if you are measuring more than just the temp sensor
you will always have a resistance that is lower that 2K.
This is because some of your current is going into a second
path. More current, same voltage = less resistance.

To test this out, unplug the temperature sensor, and measure
directly from the sensor to ground. This removes the wire
and any possible short from the measurement. Because you
do not know which sensor, you may have to do both. If this
is near 2K then your wires are shot, and the idea of a newly
sliced wire is a partial solution. (Replacing the wiring
harness is the only real answer)

Donald E Booth
Full time electrical engineer, part-time shade-tree mechanic
1984 245DL, B23F engine, M46 Transmission, 239K miles

Nevermind, just read that it idles at 1K for the first 20 sec. That rules out the throttle cable.

Did you check the throttle cable, where it connects to the throttle itself? Mine stuck at around 2000 a few weeks ago and the cable was in a bind. I know it sounds pretty simplistic but it might save some time if this is all it is.

Oh if only it were that, I'd be a happy man. Yeah, I've checked that three or four times now, to no avail. it's something else, unfortunately.....
--
'86 240DL sedan, 260K miles, M46, K cam, 25/23mm sways, 260 front and wagon rear springs; http://valvespringcompressor.weblogs.us/

Nate,
I did a quick look into the archives on 'high idle' and some possiblities could be a fault with your ECU, switched throttle and IAC connectors, and constant voltage to your IAC from a degrading wire harness. Lots of posts and discussion, worth a look.
Good Luck. Let us know how you make out.

Erich
Ottawa, Canada