Man this is a weird one....my 91n/a runs great during the day but at night when i turn the headlights on and roll up to a stop the idle starts hunting from 300 to 700 rpm....Ive tried everything....adjusted tps with leds and multimeter, no vacuum leaks, adjusted idle set screw(but it doesn't seem to move past 750 rpm no matter how far it is screwed out), tested alternator, tested battery, swaped bac valves resistance right on both that i tried and clicks with 12v applied.....the only thing that i don't understand is when i unplug the bac at idle the idle doesn't drop at all....this was with both bac's that i tried...also i unplugged the bac with headlights on and still my idle hunts..... i wouldn't think that there is a vacuum leak anywhere because it should be doing it all the time not just when there is a substantial load on the alternator....i also checked for codes...no codes in system...like i said the car runs great until you put a electrical load on it...Is ANYONE OUT THERE THAT THIS HAS HAPPENED TO OR KNOWS WHAT COULD BE CAUSING THIS PROBLEM?????

 

This problem only happens when the car is completely warmed up....and a load on the electrical system.....

 

a load on the electrical system means a load on the alternator which means a load on the motor itself. check the tps.

 

i've set the tps with the light method and using a multimeter....its set at 1.0v for short range and .8v for long....do u think i should just maybe replace it???

 

Not to jack the thread- but mine does the same only when it is cold, once it is warmed up the idle levels off-- seems the opposite of your problem.

 

the only thing that i can find wrong with the car is that the back up lights don't work....but would that have anything to do with it??? Is there any other solenoids or sensors that factor in the idle when placed on an electrical load other than the tps, bac and the idle set screw????? Maybe a bad ground could cause this??? Is anyone out there that knows what could cause this????

 

i have the same problem... when i step on the brakes.. it surges.. when i put the lights up.. it surges... when i .... bump my music it surges..
i have a s4... i am convinced that it is the shitty alternator on the s4's that is the culprit.

im gonna get a FD alt soon, and see if it solves the problem.

-cam

 

This MAY help you:

http://www.geocities.com/huguesdc1/faqidle.html

For online manuals,
look at my links page on my web page in sig.

hugues-

 

thanks for the link....im just wondering if my ecu could be bad.....i can unplug the bac and the idle stays the same iv'e tried two bac valves and both don't change the idle when unplugged or when test connector is grounded .....