I have the same code, and experienced the check engine solid and glow plug flashing combination, intermittently. In addition, the car will occasionally refuse to start (takes 5-10 tries to get it to start, after which it runs like crap for 5 seconds and then smooth). I ran across the following link, which told me to clean out the throttle body.
http://engine-codes.com/p0638_volkswagen.html
I removed the air hose between the throttle body and the intercooler pip, and cleaned the gunk out from around the round flap/valve just inside the throttle body (quite a PIA without removing the throttle body from the engine, which I will do the next time). Anyway, I just did this tonight so it's too soon to know if it will help.
Yesterday morning I had a CEL + erratic throttle response on my way to work. Incidently temps were ~23F, the coldest morning of the season. I kept the RPMs up above 2000 for the rest of the way to work (10 mi). Drove at lunch and home again in the evening, the light on the whole time, but engine seemed normal.
This morning I ran errands, driving normally. The CEL light stayed on the whole time (but no other lights). I had the code read at AutoZone and got P0638: Throttle actuator control range/performance- bank1
Explaination: Failed TPS
Probable cause:
Failed accelerator pedal position sensor
dirty or defective throttle valve
failed throttle valve control module.
As the engine was by now nicely (and gently) warmed up, I decided to see if some exercise might help the situation. I drove the car hard for about 10 miles of acceleration/high RPM driving in 2nd/5th gear, giving it lots of WOT, revving up to 4000, and cruising between 2000 and 3000 with coasting in gear back down to 2000. About 1 mile into the 10 mile drive, I noticed the CEL was no longer lit. Got home and looked up the code and found this thread.
My question...is, if this is a sticky drive-by-wire sensor/actuator, are there parts of the drive-by-wire system that can/should be lubricated? My plan is to lubricate all possible sticking points, if possible, and not do anything else "unless the CEL or hesitation/surging return. I plan to chalk it up to the "cold" temps in the 20s. Do I need to have the codes cleared if the light is off? Any other suggestions?
By the way, I had an episode of the iced-intercooler roughness my first year, and this is not the same. And I'm aware of the "hill-cresting" or "stumbling" and this throttle thing is not related.
Regards,
Joe