Consistent Overboost

[panda-52]

I have a 2002 TDI with 310,000 miles with an overboost problem. Using vag-com I see a pretty consistent pattern where the actual boost is greater than requested. In the higher gears heavy acceleration above 3000 rpm causes it to go into limp mode and occasionally throw an intermittent overboost code. I've swapped the N-75's but the same pattern remains. The intake was cleaned 30,000 miles ago at the last TB change. I've replaced the vacuum lines, EGR and MAF recently. I've checked the accuator operation and it seems ok, moving with very little vacuum and snapping back without obvious sticking. I'm the only one to work on the car since new so the turbo linkage has never been adjusted. The thing I'm having a hard time understanding is the fact that the overboost so cleanly tracks the requested but the trace is just higher. If the vanes were sticking it seems like there would be a noticeable delay where the actual remains high while the requested falls off. But this is not the pattern that I'm seeing. Anyone have any theories about this and how to fix it? I don't want to buy a new turbo and then find the issue remains. BTW it doesn't seem to effect the mileage or driveability, until it goes into limp mode.

[chittychittybangbang]

You checked the actuator but make sure to check the pressures at which it moves, not just that it moves. Sometimes the spring wears out.

[panda-52]

I believe it started to move with 2 maybe 3 inches and was against the stop at around 16 or 17 inches, something in that range. The return was very snappy and I couldn't detect any hanging up on the return. The spring tension made it pretty hard to move by hand.

[Frank M]

Does it go into over boost at 3,000 rpm or above?
is it the same all the time?
do you have some restriction at the EGR anti shudder plate?
how was the intake cleaned 30,000 miles ago?
can you bring the rpm's above 3k if you slowly accelerate without giving more than 1/3 throttle?

[panda-52]

The EGR is new, the problem predates this. The intake was cleaned by being hot dipped, it was in like new condition when it was reinstalled. Yes it can accelerate to over 3K in higher gears if not done too quickly. I believe it can reach red line in the lower gears (1st and 2nd) too with close to full throttle. Honestly I been able to trip limp mode with no codes for awhile probably 30K miles or more. It just seems easier now. If you don't get on it too quickly the car drives fine.

[Frank M]

Originally Posted by panda-52

The EGR is new, the problem predates this. The intake was cleaned by being hot dipped, it was in like new condition when it was reinstalled. Yes it can accelerate to over 3K in higher gears if not done too quickly. I believe it can reach red line in the lower gears (1st and 2nd) too with close to full throttle. Honestly I been able to trip limp mode with no codes for awhile probably 30K miles or more. It just seems easier now. If you don't get on it too quickly the car drives fine.

You may have a restricted exhaust.

[panda-52]

I could believe that at 310,000 miles. Thanks for the tip. Is there an easy (or not so easy) to test?

[Frank M]

Originally Posted by panda-52

I could believe that at 310,000 miles. Thanks for the tip. Is there an easy (or not so easy) to test?

Other than disconnecting the front pipe there is no way to tell other than the typical symptons you have.
If it had an OČ sensor you could remove it and put a pressure gauge there. Then give full throttle for a moment and watch the reading.

As you know we do not have an OČ.
An option would be to make a hole in the front pipe and weld a threaded flange in the hole, as an example, one used for an OČ. Then after the test put a threaded plug in the hole or an old OČ sensor. This would be good if one had the means to do this.

[panda-52]

Would I need to replace both the muffler and CAT to solve a restricted exhaust?

[LWT]

In the carb days, we tested for exhaust restriction or pluged converter with a simple vaccuum gauge. with engine idle reading should be smoth and above 15". watch gauge while slowly open the throttle and watch vaccuum gauge. at 2500 rpm trans in neutral the vaccuum should be equal to or higher than idle. if a restriction is present the vaccuum will go down. at 3000 gauge down to 10" The vaccuum gauge was connected to manufold vac port or tee into the power brake vac line or any manufold vac source. Also have a helper listen to the end of exhaust pipe. at high rpm you will hear a whistel , labor , scram, noise. Listen to the tail pipe of a known good car. I hope this helps.