1998 B5 - Hunting Idle after Throttle Body Cleaning

[ectwo]

I just started a tune up on my (new to me) 1998 A4 B5 1.8T. After changing plugs, oil change, I decided to clean the throttle body. I took it for a spin after the spark plugs, worked fine. Did the oil change and the throttle body, fired it up, and erratic idle, hunting from 1400-1750RPM (rhythmic) ~ 1 Hz.

Initially at start-up it idles fine at just below 1000RPM, stab the throttle and it starts an endless hunt. The only way to stop the hunting is to load up the alternator with electrical load (both heated seats, lights, fogs, fan motor on high) and it chokes the hunting back to 1000RPM. Shut off the load, the RPM still holds, any throttle action causes it to hunt again.

I cleaned the IAT sensor, it made no difference. There are no engine lights lit up, everything except for the idle is normal.

When I watch the throttle body or feel the pedal, you can feel and see the throttle body hunt. As the RPM's rise, the throttle is at rest (foot off position), when the rev's get too high, the throttle body actually increases throttle position, and the rev's drop, almost like the injectors shut off.

On the road it drives with authority like a good tune-up as long as I'm on the throttle (1400RPM to redline). If I lift off the throttle, the RPM's hang and stick before dropping back into the 1400-1750RPM hunt. If I drive with RPM's < 2000RPM, near the hunting RPM, and lift off the throttle, the motor starts bucking (exactly like when you throttle up in 1st gear and lift off, drive train buck), pretty sure it's not a mis-fire, and you can feel the throttle moving like it's hunting in gear. The hunt has no power loss because I can ease into 1st without any gas, RPM's are high enough. Aside from the erratic idle, which is rhythmic, the mileage still seems okay, except all the self rev'ing at a stop

Reading other posts on the net, I don't believe I have bad vacuum lines, and I inspected the hoses for a few hours. I tried the multiple rumors of doing a 'Throttle Body Adaptation' without VAG-COM, every way, motor off, key to on, door open, door closed, wait 5 minutes, and every other combination of hood open, and throttle positions. Don't think that works at all on my 98.

I'm thinking that since the TB was caked with carbon, maybe it wasn't fully closed when the last owner got a throttle body adaptation done, so I'm actually getting more airflow at a lower throttle body, therefore the RPM's are raising in comparison to the ECU idle throttle position. Anywho, I got VAG-COM from Ross-tech coming in the mail, and I'll try doing a TBA like the website says. I'm a pretty good ecu tuner with HPTuners for LS1 engines, and this throttle business is odd. I even cleaned the throttle body of a newer 2002 1.8T GTi, and never had this problem, I've cleaned lot's of intakes/TB's on Toyota's, GM's, Dodge, VW, Honda, never had this problem before.

[mcleodbz]

I had a similar problem in my old 96 1.8T Sport (AEL engine), after replacing turbo, CAT and cleaning throttle body. Disconnecting the battery, and leaving for 30 mins stopped the hunting, and after that the car idled perfectly. Maybe worth a try.

[Crasher]

There is no way around it, you need to have it re-adapted.

[ectwo]

Just got my Ross-Tech VAG-COM cable, TBA did the trick. Actually, after 400km of driving before I got the cable, the hunting was getting less and less, but the RPM's would still lazily float down.

Now to enjoy my new shorty shifter I made tonight, 1.5hours with a M12x1.5 die and a hacksaw, and my shifter is now 1" shorter....Think I'll hack off a bit more tommorow.....