Hot idle speed oscillating

My 1990 Mitsubishi Eclipse (DSM 1G 2L non-turbo) is starts and idles cold fine, and runs fine warmed up. When warm and I put it in neutral (AT) the idle oscillates between 1800 and 2200 with about 1/2 second cycle time.
MAS was the first guess -- so I put a rebuilt mass airflow sensor assy in and same problem. Checked all the connections and vacuum - all OK.
Also took the throttle assy apart and cleaned all - helped the running but still oscillating. Cleaned Idle air motor, checked voltages and resistance for all sensors and also voltages coming from the computer, and all OK.
About out of ideas -- anyone have some thoughts on how to start troubleshooting this?
Thanks in advance.
Bob
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Guv Bob wrote:

fine, and runs fine warmed up. When warm and I put it in neutral (AT) the idle oscillates between 1800 and 2200 with about 1/2 second cycle time.

still oscillating. Cleaned Idle air motor, checked voltages and resistance for all sensors and also voltages coming from the computer, and all OK.

Hi, Checked fuel pressure, filter, pump?
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None of the above would be likely to cause the reported symptoms - and most would affect normal and high speed operation.
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Only have a couple of seconds... but try Googling:
"idle speed hunting 90 Eclipse" (sans quotes)
Good Luck!
Erik
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On 6/2/2012 4:48 PM, Tony Hwang wrote:

i missed the original post, but i'd be looking for a vacuum leak. Probably on a gas deteriorated pcv line.
--
Steve Barker
remove the "not" from my address to email
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Thanks Steve. Will check this out.
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Hey Tony, yes - both OK.
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125K
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I don't know the car. Is it possible to read computer, if it has one. I'm thinking of idle motors, oxygen sensor, manifold leak, ....
Greg
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Yes, computer codes shows all sensors being monitored are OK. Also checked O2 and others with meters and so far all OK.
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Another question, aside from the varying idle speed, is why it's idling between 1800 and 2200 to begin with. That is 1000 RPM too high. Perhaps going down that diagnosis path will be more fruitfull. Reading the computer out would be an excellent idea. Some of the auto parts stores, like Advance, AutoZone etc will do that for free. But I figured out a long time ago it pays to have an OBD II reader if you intend to do work on the car yourself and to keep stealership/ repair shops honest even if you're not.
Also, with some things, paying a mechanic $90 for an hour's labor to diagnose it may be worth it.
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Thanks folks for the help. It's running much better but still checking more sensors, etc.
Any idea what this part is? It has 4 leads that feed into the wiring harness and can't trace it any farther. Looks like a solenoid but can't find it in Haynes or other manuals I have. Not sure if the first photo took, so uploaded it again...
http://imageshack.us/photo/my-images/833/dsm1g20.jpg/
http://img593.imageshack.us/img593/8655/dsm1g20.jpg
Sorry for posting in the alt.home. repair group, but it's the about the best repair newsgroup I've found - alt.autos, and mitsubishi are dead.
Thanks again.
Bob

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I'm glad to say this is my last post on this topic. Made the last few adjustments and it's idling and running stable and like new again. Here's an excellent procedure on how to set the idle. Much easier to follow and better illustrations than most shop manuals.... http://www.vfaq.com/mods/BISS-1G.html

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I'd post in auto repair group, unless you live in your Mitsubishi.
Christopher A. Young Learn more about Jesus www.lds.org .
My 1990 Mitsubishi Eclipse (DSM 1G 2L non-turbo)
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From sci.electronics.repair
EXCELLENT info. J understood the problem right off the bat and that's exactly what it was. The guy's a genius!
Hold on, fuck all this. If you can't find it in the codes, it can only be a few things.
Dig up, the first check is to keep it running until it does it. If it won't do it out of gear get some really good wheel chocks. When the idle hits a high point, unplug the IAC (or it's equivalent). If the hunting stops there are one of two things possible, but one solution might do it.
More than likely the IA path is partly clogged, especially where the pintle goes down. This skews the actual IACS from the true reading, indicating to the computer the amount of IA is higher than it is.
The IAC responds based on presets in the computer so therefore it will cut the IAC postiton to where it does not give enough IA. Then the computer goes into "stumble" mode, thinking (rightly) that it is about to stall because the idle speed has dropped too much. It will then open up the IAC to allow more air. The computer will always look for the lowest idle attainable. This sustains the oscillation. (hunting)
Now, if you rebuild the throttle body, it will be cleaned and hopefully be alright but then there is a possibility that the seat for the pintle valve has been eroded too much. You usually cannot machine this out because you cannot usually readjust the IACS to compensate.
But if you see a bunch of black shit in there of course clean it somehow. Throttle bodies really never need much of a rebuild other than that, if it's worn you might have to machione down a bore so the IAC drops lower into the hole. (same shit happened to carbs that were run a long time without an air filter, but the solutions were different)
However, if it is actually worn like that there is a workaround. There is a tang on the throttle linkage right there on GMs, it may be hard to find. On an American car it is a tang and you just bend it, on other cars you might have to shim it, or even find the actual adjustment. This will keep the throttle from closing too much. You MAY have to do something about the TPS if it is not adjustable.
The drawback to not replacing the throttle body for this problem is that the IAC pintle might get thrown down into the seat so hard the it will get stuck there, and then you will have a stalling ocndition and a constant check engine light.
So if you fuck with the idle position of the throttle, do it only as much as you need. (both of these solutions are against "code" but legal if it pases emissions, and it should)
If you don't do something though, it will eventually start stalling at red lights. Alot of people told me "well it was doing that idle thing for years and now it stalls". What, did you think it was going to fix itself ?
But it is not sensors. It is probably not the IAC itself, it is that things have changed for the IAC. The parameters of the intake cuircuit.
J
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