Replaced the coolant temp sensor with no change...
Hey all, looking for some help with my stock 92 GTI 8v. Car runs great and pulls strong and cold starts very well. The only problem is after it warms up a bit and you let it idle, just coming off the throttle it finds a perfect spot to idle at for a second or two (850-890RPM) then it bogs down to a somewhat rough idle around 800RPM and develops a slight hesitation or missing that seems to be in all cylinders when it bogs down but it runs very smooth in all other areas of the rev band. Its not too bad and Ive never had it stall out but its bugging me. Ive gone through and checked the timing and set the idle by the Bentley manual, replaced the idle screw O-ring, replaced the exhaust which was leaking. I cant find a vacuum leak anywhere. Also tried cleaning the ISV and put some injector cleaner through it with no luck. Anyone else have this problem?
I know that you have checked for vacuum leaks but the issue still sounds like a minor vacuum leak that sucks enough air at idle to make a difference in the idle. There are a bunch of places to check. The dipstick funnel, the ISV elbow into the intake manifold, the gasket for the round PCV Valve on the cylinder head cover, the vacuum system check valves to include the vacuum switch for climate control, the injector sealing "O" rings, intake manifold gaskets, etc.
Having said that I also think that I would check the intake air meter, ring out the wiring to the ECU and finally the ECU itself by replacing it. The air meter is easy to check with a multimeter. There is also a temp sensor in the air meter. A decent manual will tell you how to check the wiring and the air flow meter.
One other quick check that you have not mentioned is the idle switch on the throttle body. Move the throttle and listen for a faint click. You can also check the switch with a multimeter.
One time it took me a year to find a warm idle issue on a car I owned. Turned out to be a corroded wire for the O2 sensor -- there was a tiny hole in the insulation on the O2 sensor wire which allowed water into the wire and the wire had corroded internal to the insulation. The corosion gave a less than pristine connection. FR