AEB 1.8t Idle Hunting
 

I recently fixed all of the VAC leaks on my 97 1.8t. I had the battery off for a while to do wiring for the boost gauge. When I put it all together, I did a throttle body alignment in VCDS and everything seemed fine. In theory, I have no more un-metered air getting in. The gauge reads -18inhg at idle and below -22inhg when slowing down so I'm confident that I don't have any VAC leaks.

When I first start the car, the idle sits steady. When I come off the gas from a load (driving to coming to a red light for example), The idle will bounce violently between 800 and 200 rpm, almost stalling. It hasn't stalled on me yet.

I unplugged the MAF and the idle bouncing problem went away. But the turbo also was disabled cause I didn't have the MAF plugged in so I don't know if it has anything to do with the turbo, but I don't think it does anyways.

I've read that this can be caused by an extremely dirty throttle body cutting off all of the air when it's closed. The ECU will then take a couple of seconds to compensate for the lack of air giving the engine time to bounce. I probably need to clean it.

When not touching the pedal and the car is off the throttle body angle is 3.9 degrees. Should be 3.8? When the car is running and off the pedal, the angle is around 8 degrees. Does this sound normal?

Ignition timing (BTDC on vcds) is 12.1. I think this might be too high as well, Should be around 8?

Although I don't think I have any boost leaks, I think I still have a boost leak as the DV diaphragm definitely has a hole in it. It's still hooked up to the TIP and the cross pipe, but not to the intake manifold. I stuck a screw in the end of the DV boost line just in case any boost was leaking out through there.

Everything is normal from 2000 to 6000rpm under WOT. Smooth power and you can hear (and see with the boost gause) the turbo spooling.

So, possibly bad MAF, or really dirty throttle body?

Thoughts?

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I guess a broken DV could also cause the problem if its running way too rich because of air not being returned. But this would only happen if my broken DV was somehow venting to atmosphere (cracked housing?).

Again, thoughts?

Cleaned the TB and upgraded the DV. Runs like a dream now.