1G Idle Issues and the ISC Driver

[Piece_By_Piece]

I felt the need to post this for the newer members out there because it seems that when people post about idle issues resulting from a shorted ISC, it is not emphasized enough that this condition may have already resulted in damage to the ECU.

I experienced the textbook ISC failure a while back: rapid revving between ~1000-1300 RPMs. It was easy to search the forums and figure out that the ISC was responsible. Thanks to Terry's Talon Troubleshooting Tips #3 (http://www.dsm.org/how-tos/tttt/tttt3.html), I was informed that it is a good idea to pop open my ECU to check for visible damage to the driver. Lo and behold, extra cripsy, burnt, obvious damage. I sent the ECU off the the ECUTuning guys for a repair, installed the new ISC, and everything was good... for a short while.

I was doing my first rebuild not too long after, and after putting everything back together, I had weird idle problems, These weren't the obvious ISC issues like before. The idle was high, would lope sometimes, would stumble a bit... I went down the list checking for leaks, sensor resistances/outputs, etc... and I found I had another (damn near brand-new) shorted coil in the ISC.

I didn't really drive the car with these idle issues like I did the first time when I was clueless of the potential damage this can cause. I though because I caught it quickly it didn't damage the ECU. Of course I popped the ECU open again to check for damage, and I didn't see any. I installed another ISC and still had idle issues. I thought for sure it was something else, being paranoid about the rebuild... but I couldn't figure it out.

I eventually decided that maybe if I got DSMLink, I could look at the logs to help diagnose the issue. I sent out the ECU, and surprise, surprise: they told me the driver output was abnormal, and they had replaced it for me, again...

I got lucky those guys at ECUTuning go above and beyond with their ECU service, but I'm sure someone out there is going to run into this same situation as I did.

I'm not sure how to go about actually testing the driver (hopefully someone could comment on that), but definitely don't rule out ECU damage if you have idle issues still after having your ISC go bad.

Hopefully this is helpful to someone chasing down mystery idle issues!

 

[steve]

This is worth repeating. Sorry if it's gotten lost in the 30+ years of data. I know I've mentioned it many times.

It's easy to find when it smokes the IC (you can smell it from 20 ft) but about 25% of the ECU's that came through my shop had one of more weak ISC drivers that couldn't pull their signal down low to cause the ISC to step.

When you replaced the ISC the first time, what type of ISC did you use? I've yet to see a black plastic unit short the coils but it's possible. The metal cased ones are time bombs.