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 senosors 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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On Fri, 1 Jun 2012 22:33:04 -0700, "Guv Bob"
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 senosors and also voltages coming from the computer, and all OK.

disconnect the idle speed control and see if it levels out. If so it is most likely idle speed control related.
If not something else is changing the idle speed and the system is chasing it's tail. I'd suspect an air leak in the intake .
You don't by chance have the AC or defrost turned on??????
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wrote:

Thanks a lot, Clare. Will check it out tomorrow and post an update. No AC or defrost on.
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Still looking for the problem.
Probably not related, but 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 Ford group, but it's the best auto newsgroup I've found - alt.autos, and mitsubishi are dead. When I post in electronics newsgroups I get "post in an auto group..."
Thanks again.
Bob
wrote:

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Welll... 4 leads? And it looks like a relay? Then I'd say its a relay. Test it.. You could ohm it out. You should get some sort of resistance across two of the pins. Maybe about 100 Ohms. That would be the coil. And the other two should read either open or closed, depending on what it was supposed to do. Then put 12V to the coil pins and the thing should click and if the other two pins read open before then now they should read about 0 Ohms. If you get no change or a high resistance after the relay switches then its no good.
Fred
Guv Bob wrote:

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.

found - alt.autos, and mitsubishi are dead. When I post in electronics newsgroups I get "post in an auto group..."

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 senosors and also voltages coming from the computer, and all OK.

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On Fri, 1 Jun 2012 22:33:04 -0700, "Guv Bob"
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 senosors and also voltages coming from the computer, and all OK.

When my 88 Ford did that it was caused by a faulty Idle Air Bypass Valve, which is used to control the idle speed. It's like a mini-throttle body just for idle speed control.
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wrote:

Doesn't the Mitsubishi engine use an idle pintle? Looks like an acorn on a stick. A solonoid pulses it to allow air to bypass the throttle body butterfly valve when it is fully closed. They say NEVER take that thing apart, but I've done it to my Intrepid. Just be VERY careful about loosing parts. I cleaned mine up with carb and brake cleaner (don't let the brake cleaner get on anything but the pure metal parts) and be careful when you reassemble it. Any mistakes and your engine could race at wide open throttle when you first start it back up.
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a lot of people tried disassembly and cleaning of the ford IABV. Results were hit and miss. I tried cleaning mine a couple times and it made it worse. Gave up and just bought a new one. There was also a very specific procedure for adjusting the base idle speed and screw position. Sometimes following that adjustment procedure helped and sometimes it didn't. There were also several un-official adjustment methods also which sometimes helped and sometimes didn't. Some vehicles just seen like they have ghosts in the idle circuit.
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Newsgroups: sci.electronics.repair Sent: Wednesday, August 01, 2012 9:11 PM Subject: Re: Hot idle speed oscillating
.... 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.
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Thanks for posting the end of the story.
On Mon, 13 Aug 2012 10:49:10 -0700, "Guv Bob"

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.

goes down. This skews the actual IACS from the true reading, indicating to the computer the amount of IA is higher than it is.

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)

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.

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)

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.

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.

you need. (both of these solutions are against "code" but legal if it pases emissions, and it should)

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 ?

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