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[Dmaggs]

My car runs great, but idles too fast. I initially assumed it was the IAC. I took that off and cleaned it thoroughly. I have ensured that coolant and air each can flow freely where they are supposed to. I then put the IAC in boiling water and the valve inside fully closed off the air flow. Assuming this meant my IAC is good, I went ahead and reinstalled it.

I next made sure the coolant lines through the throttle body were also clear. I then bled the system. And yet, for some reason, my IAC is still not closing when hot coolant runs through it.

I know this is the issue because if I manually pinch of an IAC air hose the idle immediately drops down to where it should be. So my question: how could the IAC work in a pot of hot water but not work with hot coolant flowing through it? It just seems like there isn't enough heat from the hot coolant even though flow is unobstructed.

I suppose the a test could be to somehow flow hot water through the IAC off the car but I don't currently have a setup to do that. The old put it in a pot of hot water trick seemed a good enough test and yet here I am. Any ideas??

[Rocketman]

Sounds like the wax motor portion of the IAC is working properly.

There's an electric portion to the valve too, that works with the mechanical wax motor. There are a few things that will cause an idle up command to be sent - the AC system, electrical load control, and power steering pump have control authority over the IAC/BSC. There may be other systems that have influence as well

Have you disconnected the connector while it is running to see if there is a change in idle? (i am not sure if that is how these idle valves work, worth a shot though?)

There should be a section in the FSM for troubleshooting the IAC, but I am not sure how in-depth it is.

[Dmaggs]

Excellent advice, thanks! I unplugged the IAC electrical connector while running it the engine immediately idled down to right where it should be. So clearly all part of my IAC (electrical and wax) are functioning correctly.

Something is signaling the IAC electrical components to add extra air. My leading suspect is a faulty engine coolant temperature sensor. That is cheap and fairly easy to attempt to replace. I do wonder though, is there any way to tell what system might be giving the IAC the idle high command? I have the A/C turned off so I don't think it would be that. But power steering and other systems can also control it you say? If it's one of those systems I'm trying to figure out how I might be able to tell.

As another clue, the engine temperature gauge on the dash reads just a bit above cold even on a fully warmed up engine--another symptom of a bad ECT? That's where my hunch is stemming from anyhow for the moment.

[Dmaggs]

Ok I found in the manual the chart of everything that could be affecting the IAC solenoid. So the high idle command is coming from one (or more than one) of these:

1. ignition coil
2. idle switch
3. engine coolant temperature sensor
4. volume air temperature sensor
5. barometric pressure sensor
6. oxygen sensor
7. A/C switch
8. P/S pressure PSP switch
9. on-board diagnostic

 I think it will prove to be the ECT sensor--that's also an easy fix if I'm right!

[Rocketman]

Check the ECT yes.

Also have you verified the thermostat is operating correctly, if it's stuck open the engine won't reach operating temps, may cause the idle and gauge on dash reading low. It should be a 195° tstat

[Dmaggs]

Worth a check for sure--I am able to get heat out of the events pretty quickly after starting the car up--I suspect that a stuck open thermostat would not be allowing the engine to heat up quickly enough to get heat quickly. But I will check the thermostat and maybe grab a spare just to have it handy.

[Dmaggs]

Installed new ECT and the temperature gauge is now much more responsive--so it seems like the old one was faulty or at least lazier than it should have been. But something is still constantly giving the idle solenoid the command for more air. I guess something else from my list of 9 items above. I don't know what to try unplugging/replacing next but will have to go somewhere next. Maybe O2 sensor?

[EShepherd]

Installed new ECT and the temperature gauge is now much more responsive--so it seems like the old one was faulty or at least lazier than it should have been. But something is still constantly giving the idle solenoid the command for more air. I guess something else from my list of 9 items above. I don't know what to try unplugging/replacing next but will have to go somewhere next. Maybe O2 sensor?

If you take the time to really sit down and explore the FSM, it goes into detail on how to test pretty much every one of those sensors. With such a big list, it honestly might be better to do a bunch of testing in this case. It'd be a shame to replace a half dozen perfectly functional (and rare AF) OEM parts before finding the problem.

[Dmaggs]

Yes agreed--I got this car very cheap because it was not running. I don't want to now turn it into an expensive car on the basis of many unnecessary spare parts. The good news is that it is highly drivable and enjoyable even as I sort through this problem. The car runs fine on a fast idle if the solenoid is plugged in and it also runs fine with the solenoid unplugged, albeit without useful throttle adjustments to compensate for added load (I would not, for example, try to use the A/C with the solenoid unplugged).

I did some bench testing on the the old ECT and the resistance readings at different temperatures are clearly out of spec. So that was absolutely a good replacement to make. I am working through the FSM but have not found test for each and every sensor I need to test just yet. To some extent that's just because sorting through the order of the manual takes some time--but I am not certain if all of these sensors can be bench tested. Will have to keep searching the manual a bit at time as I work down my list.