Sudden bad idle, not surge

[dhermann]

My 90 TSi AWD just started having weird idle problems. The car starts fine and idles fine while it is cold, the cold and warm idle speeds are correct. As the car warms up the idle drops to 750 rpms or so like it should. When it is completely warm the car doesn't like to idle at all. It will idle fine but then I drive it and when I come to a stop the idle will drop below 500 rpms and the car tries to die. Sometimes it will kind of catch itself and bring the idle back up where it belongs. Usually when it does that it will catch itself twice at the below 500 and than idle normal. The most common seems to be it just dies, and occassionally I can rev it up to normal idle and let it go and it will catch itself and idle correctly. Like I said this only happens at operating temperature. I replaced the ECT sensor and am about to replace the O2 sensor and the fuel filter. Does this sound like I am on the right track or might it be something else? I am kind of getting tired of heel toe and e-brake stops at lights to keep it from happening.

 

[AlexRogan]

I have the same problem that seems to be getting better. I replaced the ISC, the coolant temp. sensor, ecu capacitors and none of those seemed to help. I've notice the o2 sensor reads rich when its in the process of stalling. I havn't run into anyone that had the right advice yet. Hopefully someone will see this post who's run into this. Also if the o2 sensor helps post it.

Good Luck

 

[Thomas91169]

i have this problem after leaving the battery off for more than 5 minutes and the ecu resets itself. the car will idle fine, but as soon as i give it any sort of load(like back out of the driveway) the car will idle at 250rpm and try to die.

my fix? drive it around for 2-3 hours. the more i drive it and come to stops the more it learns to fix itself. after the 1st hour it will only drop to 450. after it gets to that range i like to shut it down and let it cool for about a half hour, then start it up and take it out for another hour, and repeat this till its fine.

that being said, i try not to disconnect the battery as much as possible. i replaced the ISC thinking it would cure it but no effect. i just learn to live with it.

 

[nazthug]

mine is doing it too, it drives me crazy, i cant come off the rpms and out of gear, as the rpms drop, it doesnt stop at 800rpm, it goes to like 200 and tries very hard to die, and barely saves itself


it does this at full operating temps, worse at night with lights on, is it maybe battery related, like a bad alternator?

I jus replaced the battery

I hvae dsmlink and all that stuff, but i dont think it did it before its realy hard to drive, like backing out as well, give it gas, back up, put it in nuetral, rpms drop and then it wants to die badly, gotta keep giving it gas, its lame

wtf is it???

 

[G-UNIT]

CHECK your idle speed control wire & make sure it has a good connection on the terminal.

 

[dhermann]

I know my alternator and battery are good because I have had them tested. I am replacing the O2 sensor and fuel filter tomorrow and I will also check for the bad connection. I did just recently fix a loose wire that made the interior lights come on when hitting random bumps. The previous owner just took out the fuse that ran them and installed a switch to turn everything on when needed(or to use the stereo-- same fuse controls power on switching for it). I never realized that the same fuse also runs the backup for the ecu memory untill I read someone's post here. So maybe the ecu re-learning could also be part of the problem, but it did fine when it had to learn again everytime I wanted to use my stereo or have working interior lights and then shut them off to avoid battery drain when the car was parked. So I will see how things go tomorrow and then post my results.

 

[CurtTSI]

I had the same problem, caused by a bad ISC. Check the ISC coil resistances after warm up. Mine checked good, when cold, but when hot tested open circuit.

 

[dhermann]

So after ending up with rod knock and letting the car sit untill I had the heart to rebuild the motor, I finally know that my idle problem was caused by the O2 sensor going bad. Once I looked back on the things my car had been doing it made perfect sense.

I figured that using 10 gallons of gas in 2 weeks worth of driving 1 mile to and from work was just normal because I let it warm up fully and then drove it kind of hard for the one mile. Apparently when the O2 sensor is bad and it is running pig rich this also can be a problem ha ha. Anyway, I don't know if anyone else will get their magic fix from replacing the O2 sensor but it did work for me.