hey i have a 98 a4 1.8t q, and randomly the idling will start going all crazy. sometimes when i am driving and i put it in neutral at a stop the idle will start dropping really low. it usually idles around 800 and when this happens it drops down to like 400-700 and will go up and down but stay within that range. when this happens i cant feather the gas and the car wont go and the only way to get the car to go is if i STEP on the gas and rev it up to like 5000 the car will finally move forward. Everytime the idle gets wierd i have to turn of the car and turn it back on and everything will be ok again. i took it to the shop and the mechanic says he doesnt wrong with the car because it never has the idling problem when he drives it so he cant scan it when the problem is there to find out what it is. any of you guys have an input. my mechanic is pretty legit and even he is stumped.


side note- how much does it cost for a stock cat convert for a 98 a4 1.8tq for parts and labor? i found parts for like 250 but how much would labor typically cost?

 

almost sounds like the problem i had when one of my vacuum lines was off of my DV, but it only happend when i had the AC on and i would not have to step on it to go. Morale of story..... check your vac lines they can be fragile little things.

 

I looked on ecs tuning and they were like $279 for a OEM and $89 for a universal. I had messed my cat up and ended up just getting a test pipe, which only was about $100. It's not a bad job to do yourself if you feel up to devoting a Saturday to it. The only hard part to me was getting the to the bottom bolts and just undoing the bolts because of the rust. Nothin a lil PBBlaster didnt fix.

 

Likely possibilities:
-Vacuum leak
-Bad front O2 sensor
-Bad/dirty MAF
-Bad coolant temperature sensor (believe it)
-Ignition related, perhaps ICM

etc.