Turbo Actuator Repair
From Fozzee.net
To make a long story shorter, I was in Lincoln last weekend helping my brother out and threw a check engine light while running around town. The car seemed to lose power dramatically at higher RPM and was shifting funny. I assumed that there was some kind of vacuum leak going on, so I brought it back to Omaha (albeit very carefully, as freeway driving sucks with virtually zero turbo function). It got so bad in the hills climbing out of the river valley that I got off the freeway at Gretna and took US-6 home.
When I got it home, I put the laptop on it and found a code set for P0234 which translates to "Overboost Condition". Basically, this means that the engine is getting more boost than it's asking for. The forums over at TDIClub have an extensive thread on the topic.
Basically, I tried/did the following with no results:
- Clear the code and retry
- Replace Air filter
- Clean snow screen
- Inspect all vacuum lines
The issue simply recreated itself every time you got on the throttle... I'd taken it to work the last few days, and that was a challenge, driving on the freeway in morning rush. This issue /had/ to be resolved, and quick.
The next step was to inspect the turbo linkage, to rule that out (I was looking for a bad N75 vacuum valve). I had to pick up a vacuum pump (a MityVac to be specific, $40 for the complete set at HF) to allow me to manually test the vacuum actuator for the turbo vanes (which replace the wastegate and its actuator, in a variable geometry turbo). Basically, I disconnected the vacuum line up top at the N75 and plumbed in the pump. As I started making vacuum, I got no movement on the actuator.
I cycled it a few times and could tell that the actuator and its plumbing were holding pressure. A few more cycles, and I noticed that the linkage was moving just a tiny bit. I'd read on TDIClub that I should expect movement to start around 3"Hg and continue up to around 20"Hg, with a mechanical travel around 3/4". I definitely wasn't seeing that. I wasn't sure where the holdup was, but it was certain now that the problem was in this area... Either a gummed up turbo (I felt that this was unlikely, given the sudden onset of the issue) or an actuator that had failed, but not ruptured, somehow. A bit more work and some manual manipulation pressing on the linkage got me additional travel, with a nasty shelf around 12"Hg, where the linkage would "spring" into position.
To resolve, I ended up running to the hardware store (literally, as my car was up on ramps) for some penetrating lube and soaked down the linkage (it looks like a linear vacuum actuator, and a cam of some sort that rotates the insides of the turbo) as best I could, given that I was working clear down in the engine through a very small mirror. I continued to exercise it with the hand pump, and eventually got nice smooth, long action back.
A quick test drive with some major throttle mashing convinces me that I addressed the cause of the problem. How this particular linkage got so stuck so fast, I'll never know, but I think we're in the clear.
In retrospect, I wish I'd taken more pictures of this, even though it is pretty well documented online. To get photos of what I was doing, though, one would really need a long, flexible bore-scope.
