Any fault codes?
Start by cleaning the throttle body. Perform throttle body alignment with VCDS.
Hello there, recently I just replace the radiator, water pump and o-ring on the oil cooler, so the battery got disconected from the negative and the only sensor that I unplug was the MAF next to the air box. So when I put everything back together, run the car, everything seems to be fine until I run the ac and the engine is running at normal temp, the engine start to go up and down, if the engine is cold with the ac on runs fine. If I dont use the ac idle is perfect. So please any advice is welcome and thank you. GOLF 1997 2.0.
I've had the same issue for a while now. Haven't been able to resolve it. just so you know what i've tried:
replaced all van lines, MAF with another good maf. tb and intakes all cleaned. replaced pcv. capped off evap to see if it was leaking. i changed to a BFI chip and the idle w/ AC on went from being about 1k rpm at idle, then faltering down to normal to hitting steady at 2k PRM and normalizing.
If you don't have a scanner that can read a datastream, it'd be a good idea to get one. VCDS is highly recommended, but any generic OBDII scanner with datastream capability should give you those values (if using generic, it helps to also log the voltage values of both O2 sensors).
That aside, check for vacuum leaks, especially in the EVAP-related vacuum circuits.
Last edited by Anony00GT; 04-26-2012 at 09:23 PM.
Go back over engine, and make sure that you don't have any disconnected or cracked/leaking vacuum hoses.
You disconnected the battery, so the ECU engine map was erased. The engine is going to run kinda funny until the map is regenerated. Just drive the car, and in a week or two (depending on how you drive the car, how long, and how far), the engine should start running more normally.