View Full Version : My DSM is idling too high. Can you help?

12-23-2005, 09:41 AM
Hey, guys. Ever since I put my car together after the theft, it's been idling too high. I had to put a different throttle body on her, so I figured it was just the BISS. I tried spinning that in, and it didn't help. I bottomed the screw out, and it still idled at around 1900 rpm's. I woulda thought that screwing it all the way in woulda killed it.

Anyway, before I jump into trying to diagnose this problem, I want to get as much background as possible. If I'm going to start searching, I want to search it all. So, I was wondering if you guys could tell me what to check. Can you guys list as many things as possible that control idle speed? That way, when I jump into it, I'll be able to check everything at the same time. I'd rather do it that way than check one thing, quit till later, check another thing, quit till later, and on and on. Ya catch my drift?

Anyway, if you guys can help me create a list of things to check to back my idle down, I'd appreciate it. By the way, I drive a 1991 Talon TSi FWD 2.0L Turbo.


12-23-2005, 09:45 AM
Make sure your idle switch is working properly, which is easily checked with a logged.

Also sheck for boost leaks, anything leaking after the throttlebody will cause a higher idle.


50Trim Boy
12-23-2005, 09:45 AM
intake/boost leaks? Bad idle stepper motor?

12-24-2005, 08:30 PM
Is the throttle body gasket leaking?

01-04-2006, 02:35 PM
Hey, guys. Just thought I would give you an update of my idling problem. I haven't been able to spend as much time on it as I would like with the holidays and all, but I've been able to do a little bit. Just thought I'd post what I've done. If you wanna give some feedback, cool. I'd appreciate it. If not, that's cool, too. I can understand wanting to wait until I get back with more.

I took my car over to my buddy's mechanic shop a few days ago, and we did a little bit of looking into the problem. The first thing we did was to eliminate two possibilities. We bottomed out the BISS and that didn't fix it. Then, we adjusted the depth of the sensor that is responsible for the hard stop on the throttle cable. We actually went so far as to completely remove it so that there was no more hard stop. Even with it removed and the BISS bottomed out, the idle was still almost 2000 rpm. Too much air is still getting past the throttle plate.

Oh, almost forgot. We pretty much eliminated the possibility of either a vacuum or boost leak. We hooked a smoke machine up to the hose that leads between the turbo and the intercooler to put check for boost leaks. Got nothing there. The only smoke escaping was passing all the way through the system and coming back out the turbo. By extention, I don't think that there are any vacuum leaks that would lead to this problem. We would've seen smoke coming out of them, since the majority of them hook to the throttle body one way or another, correct?

So, our next suspicion was the ISC motor. We hooked a scanner to the plug near the fuse box and checked some of the readings. One of the readings said that the number of steps from the ISC was 27. So, my buddy got this bright idea and he told me to disconnect the MAF sensor and to take off the snorkel that the boost hose hooks up to at the throttle body inlet. He wanted to run it with that off of it so that he could play around with it. I did what he said, then we started it again. Now, the scanner said that the number of steps was 58. He played around for a while and didn't like what was happening, so he told me to put it back together and take the ISC out, but leave it plugged in. Again, I did what he said. This is the point at which we're getting closer to pinpointing the problem.

My buddy held on to the ISC motor and told me to turn on the key. I didn't start it, just powered it up. Nothing happened. The ISC didn't move at all. He thinks it should've done something. So, it sounds like a bad ISC motor, right? Well, maybe not. I happened to have another ISC motor, and we replaced the one that was on it with this one. Then we went through the whole process again that we'd gone through with the other one. Same results, almost identically. So, unless both ISC motors happen to be bad, it sounds like it's not getting power.

I have yet to check the plug yet to see if it's getting power. That's the next step. I've also got two more ISC motors that I can plug into it if I find that I have power. There's almost no way that all four could be bad. Anyway, I'll keep you guys posted as to what I find out. If you have any suggestions for me, I welcome them. For instance, what in the hell could cause there to be no power at the ISC?

Thanks for reading and thanks for all of your help.

01-04-2006, 02:41 PM
Upon power off of your vehicle the main relay stays on for a determined amount of time to "park" the stepper motor of your ISC so it is in a known location. I can't quite remember if it initializes. Normally a bad ISC will cause surge or an extremely low idle/stalling condition. Have you measured how far your throttle plate closes at throttle stop? Also what is the condition of your air fuel mixture at idle? If you've modded your car and your idle fuel table has changed it will mess with your idle like that. Just a few things to check, it probably could be your ISC but I figured I'd give you something to go play with next.

01-04-2006, 06:17 PM
I wouldn't trust a smoke test (non-running engine).

The intake can leak under pressure or vacumn conditions and not show up with a smoke test.

A full intake pressure test is the best way to find these types of leaks.


01-05-2006, 06:30 PM
Didn't get a chance to check any of the other things yet, but I had a few minutes yesterday, so I played around with the ISC motors that I have. I've got access to another 1g DSM. It's a '92 Non-turbo Talon. I unplugged the ISC motor that is on that car, and I used the plug to see if the ones that I had would do anything when powered up. Sure enough, all three ISC motors started vibrating as soon as the key was turned on. Didn't start the car. Just turned the key on. I did the same test on my car and nothing. Seems that my car isn't getting any power to the ISC motor.

I seem to be hearing a lot of chatter about the ISC motor not causing high idle. Seems that the popular response is that a bad ISC motor would cause it to idle too low. If that's the case, is it possible that the ISC motor is not responding because the idle is too high from something else? This doesn't make sense to me as it should vibrate when the key is on before the car is started, shouldn't it? I'm going to follow the suggestions I've got from you guys plus the vfaq very soon. I'll let you know what I find out.


01-05-2006, 10:32 PM
I checked my isc once as you did. I pulled it out and checked it by turning on the ignition (no start) and it would move in and out as I turned the ignition off and on. That was not my problem and my Talon idles fine. Looks like you've lost voltage as you suspect.

Ign 5
01-06-2006, 09:33 PM
pvc valve!!! lol my car did the same thing..smoked a lil bit thou...so maybe not...but while the car is running try takin off your oil fill capp. if it is suckin hard...and like hard to get ure capp off then thats what it is...

01-12-2006, 03:09 PM
I'm convinced that the ISC is the problem. Another guy who contacted me told me that, just for the hell of it, he went out to his car and unplugged the ISC. When he did that, his idle shot up to around 2 grand and sat there. That sounds like exactly what's going on with my car. No power to the ISC. Now I just have to figure out why. The ISC itself is okay. I don't have a voltmeter, so I'll have to find one in order to work on this problem. I'll keep you guys posted.

Thanks for the help.

01-12-2006, 03:18 PM
The ISC is a stepper motor, all "power" and "grounds" run to/from the ecu.

So, it's either bad wiring or a bad ISC driver circuits in the ECU.