Repost from link:

As some regular readers here may remember, I've been having problems with my throttle body for over a year now. The car drove without problems for a while, then the tach needle would start jumping up and down erratically, finally the EPC light came on and throttle control disappeared almost entirely. The first time around, the VAG program told me that the throttle body was malfunctioning (sensor readings out of range, motor non-functional, etc.). I replaced the TB last year in January, the car drove fine for about 2-3 weeks, then the same thing happened again. Last year I sent my ECU to APR and asked them to resolder their chip on the board (thinking it might be a loose solder joint). When I got it back from them, the car drove without any major problems for nine months. Then the problems started appearing again at the beginning of December. This time I bought a new ECU (thinking that perhaps my ECU was malfunctioning due to a defect in the main board - perhaps because it'd been worked on by APR and Autothority before them), sent it to APR, and had them modify it again. When I installed the new ECU, the car drove for one day and the problem appeared again.

So I was trying to troubleshoot the issue this weekend (with lots of useful advice from Mat Reid), mainly by following the factory manual procedure (test voltages, test continuity, etc.). All tests checked out OK and the manual's final advice was to replace the TB. But I refused to beleive that it was the TB, since I'd replaced it last year and it still failed.

So finally I decided to take out the current TB, put in the other one, and see whether it would work with both ECUs (the TB that was on there was failing with both ECUs). When I took off the TB I noticed rusty water dripping from the bolts/bolt holes. I put on the other TB and tried it with both ECUs - it worked fine.

So here's my current theory: I have an Aquamist water injection system built into the car that sprays water into the intake, right after the intercooler and I suspect that it's the water which was causing the TBs to fail. In the summer it was warm enough that the water evaporated before it could accumulate. In the winter, however, water mist probaly collected in the TB housing and caused a short in the electronics. This is most likely why the TBs would work for a while if they were left outside the car for days/weeks, but then failed after a few days of use.

Of course this theory could be false as well, but there's a good chance that I've finally found the culprit. If so, I advise anyone who has water injection built in to make sure that they're spraying the least amount of water into the intake; otherwise you might well face a problem similar to mine (if you have a car with an electronic throttle valve) unless you live in a region where it's always warm (but even in that case, water in the TB housing cannot possibly ever be good for it). The TB was of course bolted tightly onto the intake manifold, yet the water still managed to escape past the two metal plates.

I haven't decided yet whether I'm going to reactivate the water injection system again with a smaller nozzle (I think my current one is .7mm which is activated around 5 lbs of boost). But apparently there is an upper limit to how much water you can inject in a DBW car.


**NOTE** After I deactivated my water injection system over two months ago (1/08 - it's now 3/30), I did not have any further problems with the throttle in cold weather. I am going to put in a smaller nozzle (.6mm) and run that for a while. In the mean time, I would advise against using a .7mm (or larger) at low boost (6 PSI).

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