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2002 Jetta TDI. New engine installed after botched timing belt job (lawsuit pending), I get a turbo overboost limit w/loss of power & acceleration, only after extended driving & during heavy throttle condition. I've checked vac lines & boost actuator rod movement using MightyVac. N75 boost solenoid has been replaced, but problem persists and is difficult to troubleshoot because of the conditions under which it happens. Engine restart fixes problem until next time.

Could bad MAF sensor cause this? I don't want to buy one if it's not likely this is the cause. And why would a new engine cause this problem to start?

Any other ideas would be appreciated.
 

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Totally new engine or rebuilt or used engine put in?

Turbo overboost limit is limp mode. There's an article on limp mode in the FAQ: http://www.myturbodiesel.com/1000q_how_to/multi/limpmodetdi.htm , overboost limp mode is normally sticking turbo vanes. It could be some carbon buildup or on the edge of getting more stuck. Try some hard runs to cook the turbo and see if that helps. Don't buy a MAF if you just suspect it, confirm it with a Vag Com test as seen in the FAQ: http://www.myturbodiesel.com/1000q_how_to/a4/maf.htm
 

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Totally new engine, but the original turbo, pump, etc. Please define "hard runs", insofar as gear/RPM/speed/duration. What I really question is why it occurs only after extended driving? Is it feasible to remove the turbo to clean it?
 

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Not sure why it only occurs after extended driving. You have to do more tests to figure out why otherwise I'd just be throwing out guesses. An example of a wild guess: a wire or vac line heats up and that causes some crack to be exposed. MAF is possible but do more tests that were linked above.

Read the limp mode article that was linked above, more diagnostic steps there. It also answers your question on how to remove the turbo to clean it: http://www.myturbodiesel.com/1000q_how_to/a4/turboremovalTDI.htm

from the FAQ:

you should rev the engine high and hard on a regular basis, like when entering highway on ramps (always observe posted speed limits), to raise exhaust gas temperatures (EGT). Raising EGT will burn up and blow out carbon and soot build up out of the VNT vanes and help prevent them from getting stuck. Only do this once the engine is fully warmed up and let the car cool down with a period of normal driving afterwards. The key is both high rpm and high load. High load at low rpm or high rpm in neutral with no load will not raise EGT high enough and long enough to burn up the soot.
 

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If you've confirmed a good and free VNT lever, vac line, all the way back to the n75 valve then follow it upstream.

Check the vac reservoir and the other vacuum lines. If you still can't figure it out then start tracing and replacing vac lines - it's a lot cheaper than the MAF and it doesn't sound like the MAF is bad.

It's possible a vac line is collapsing or is pinched from the install. It must have something to do with the extended driving and something that changed before and after the new engine. Probably a vac line. Check the MAF anyways since these often go bad.
 

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Limp Mode

I use a diesel additive every other tank full. Sea Foam is the best. The suggestion of running up the RPMs is correct. Rev Limit is 5100. I did a complete teardown and cleaning of the turbo & EGR to clean the crap out and my local VW tech suggested the method of ever other tank full adding a diesel additive. I also run up the RPMs in 2nd & 3rd to 2.5 - 4k when getting on the highway. 1999 TDI, 165K and going strong. Occasional run to 110mph, but I run out west a lot.

Chuck
:eek:
 
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