Weird Idle Problem

[TurboStutr]

My car recently started surging a little bit when i am coasting in neutral to a stop, for example up to a traffic light. It has been idling high too, around 1,100rpms...and it will surge up to around 1,500 rpms, then back down. BUT, it doesnt do this when I'm stopped completely, it just idles around 1,100rpms. I know surges can be caused by vacuum leaks, but wouldn't it happen all the time then? Any help would be great. Oh, its a 94 FWD Turbo Eclipse 5-speed with about 113k miles. Thanks.

 

[jott5555]

mine is wacked out too.. when i start it up it idles at 2000 freakin rpm unless im in gear. then after its warmed up coasting to a stop it fluxuates between 1000 and 1500 then at a complete stop it hits 800ish and feels like its going to stall... pos dsm!!!!

 

[91Bomb]

mine is wacked out too.. when i start it up it idles at 2000 freakin rpm unless im in gear. then after its warmed up coasting to a stop it fluxuates between 1000 and 1500 then at a complete stop it hits 800ish and feels like its going to stall... pos dsm!!!!

Sounds like a cold idle problem. Check the wire connection to your isc motor (controls cold idle) and if contact is good you might have a bad isc motor. If this is the case you could try taking it out and cleaning it 1st. Also, check your fast idle switch wire connection while you're at it.
My fast idle problem was easily solved when I discovered the wire to the isc motor had broken off and I soldered it back on.

Sometimes I coast to a stop in neutral with my auto trans and the rpms will be like what you mentioned but I figured the vehicle speed sensor (or something like that) was telling the ecu to raise the rpms because the car was still in motion -- I'm not too worried about it because when the car comes to a stop rpms are at normal idle speed but in my case the idle is not rough.
However, my car didn't always do this either so I'd also be interested if there is a quick fix for this problem.

 

[Wheels-a-turnin]

It also could be a small vaccum leak. Ex: Pull a vaccum line and watch your car struggle to idle.

Also the vaccum leak would have to be after the ISC to cause this idle surge.

 

[TurboStutr]

Thanks for the feedback, I'll check on the ISC as soon as i can. If its gotta be replaced, any idea on how much they cost? Also, my buddy had a 91 Galant VR-4 and had a problem with what he called the stepper motor(keeps ilde up when the A/C is turned on). He said it sounds like the same problem with my car, so i'm not sure if the ISC and the stepper motor are the same thing or not. But I'll get on it and see what happens. Thanks again.

 

[jott5555]

im going to use a mustang t-bodie and manually set the throttle.. screw the isc. screw the fiav..

 

[91Bomb]

I believe the isc motor is a stepper motor and that it steps down the rpms gradually during cold engine warm-up. Once the engine reaches a certain temp the idle is swithced over to its warm/normal idle mode and the isc's function is complete. One of the ways I know this is because when the wire to my isc motor was disconnected my rpms would not drop at all at cold start-up. Rpms would just stay at about 1500 and not gradually drop down -- Of course I woudn't just sit there while engine is continuously running that high so after a breif time I would put the car in gear which of course brought down the rpms. Once I reached normal operating temp the car idled perfect and right where it should -- 750rpm +/- 100rpm. (I believe the thermostat opens and coolant flows to the throttle body area and that's PART of what makes the idle transition posssible -- not 100% sure though)
Check that wire and conection to the isc motor and see if its on solid and/or carefully disconnect it to see if the contact is corrosive. I think you can check that motor with a voltmeter too (see service manual) Cost for a new one is like $75 but I've read these motors get gunked up so remove it 1st and try cleaning it. There's also an idle speed switch near the isc that controls the switching from cold to normal/warm idle speed -- check that too. If you have to replace an electrical part and aren't 100% sure about it order from someplace like Tallahassee, Fla. mitsubishi because unlike most places they'll except returned electrical parts. Good luck.