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Losing boost/limp mode

4140 Views 8 Replies 4 Participants Last post by  pdq
I am new to the forum but not TDI's. I have a 2001 TDI. Frequently but not always, when calling for boost I'll get a few seconds of satisfactory power then cut out of boost. The car can hold its own but not accelerate on hills for example. Now, I have gone through the excellent section in the FAQ's on "limp mode" and its causes and everything listed in there has been done or tried. Where I am now is this: Completely rebuilt engine (very competent shop), new VNT turbo, new actuator, new N75 valve, all vacuum hoses replaced, new MAF, new fuel filter.
The car will run perfectly for days some times, then with no warning the turbo boost cuts out (always when on full boost - hills, etc). The CEL now comes on after each event though it didn't always. Also after restarting the problem will sometimes go away for a while, sometimes not. Often the CEL will still be on and the car has returned to normal operation (the next morning for instance). Then for no reason the CEL will go out and everything is fine for minutes, hours, days even....until its next episode. My mechanic is great but he is at a loss. We did have a similar problem years ago but much worse (hard smoky starts, engine cutting out completely) and the computer was replaced. That fixed it. I am betting lightning doesn't strike twice but we have tried everything we can think of and the problem persists.
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Still hoping for some input! Has nobody else seen and/or solved this problem?
I am thinking the turbo may be set just at the edge of overboost and during a long uphill run it senses too much boost and goes into limp mode. Dialing the actuator back a turn might help? Any thoughts out there?
Any advice is very gratefully accepted.
This is a tough one if you've already done everything in the article. A lot was changed but it's assumed that everything is good. I suggest checking the actuator position. It's possible it was misadjusted from the start. When you install it, the rod position should be the same as the old one. Maybe ther eis something funny about it. It's not an e-m-i-a-t-a/p-r-o-t-h-e turbo is it? Because those have been known to fail.

Logging the MAF and MAP values normally helps isolate the problem. Make sure all vacuum lines are in place and do a boost leak test. Maybe you have a tiny leak and when you have max boost it leaks, causing the sensor to realize something is wrong.

And of course, what is the error code associated with the CEL?
Thanks for the input. I have been fortunate to have a pretty good local shop with a certain love for TDI's. He has done all the work and I have been the guinea pig trying out each step we take. In short I haven't been there when the codes were being recorded and of course it never acts up when he has it. The last itme I was in the VAG reader was "low boost preesure", didn't get the exact code, but by then the car was OK again. He has had the reader hooked up while driving hoping to catch it in a bad moment and get some useful info...but no.
Now I am almost afraid to tell this for fear people will think I am crazy but at least 1/2 dozen times I have seen the problem occur in exactly the same spot on the highway during my daily commute. It is on a short but very steep uphill section. My foot is probably well down so I have chocked it up as a problem that occurs during full boost but it may be some else. What I can't imagine but we have found numerous wiring issues when the car has been apart (which is frequently) so maybe theres an intermittent short.
I will take your suggestions and let you know if we see any success! thanks .
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I should have added to the above that the turbo is a rebuild from his favourite supplier. Don't know who that is. But the low power problem was there before I had the engine rebuilt, still there after the rebuild and he thought the turbo was the issue. We replaced the actuator and the turbo and the problem went away for a few days then returned. The rest I have already recounted above.
You're not insane, that spot on the highway is putting a certain load on the engine which trips limp mode. Try downshifting at that spot or going up the hill at a different speed and engine rpm and see what difference it makes.

It's possible it could be a wiring problem to the n75 solenoid. It's also possible the turbo rebuild could be bad. If it got a new CHRA center housing rotating assembly it's good as new but the VNT turbo could be tricky to assemble. It's possible there's a vane problem that was there before. The ring that rotates the vanes can wear down and cause sticking. The lever that moves the ring can wear down and stick. The plate under the vanes can warp and cause the rings to stick. A new turbo *might* fix it but do more diagnosis. The lever that moves the turbo vanes should move with literally, no effort when it's disconnected from the actuator. With the actuator on, it should be hard to press down but come up smoothly from the spring resistance.
Some good thoughts. We did find a bad wire to the n75 early on and repaired it. I will try to see if it only happens under full load but I am pretty safe in saying it does. I will also try adjusting the actuator later. I have less than 2000km (I'm Canadian eh) on all the new parts engine included. So unless something was bad out of the box I am hoping its an adjustment or something we just haven't thought of yet.
thanks again.
me too

:panic:Same symptoms here, waiting with baited breath for replies
I just went through that same issue last week. By the symtoms it sounds like overboost induced limp mode. I was having the same problems and my newby post was for information on this very subject.

Confirm with a vacuum tester that the actuator moves as per spec and there is no leakage from N75 or the actuator diaphragm.

You can review my posting for all the details so I won't reiterate them here. The next step I would do is to map measuring block 11 in vag com and observe actual map vs requested map. If the actual rises too high it will trigger overboost limp mode.

The cure for my dilemma was twofold, a sticky actuator arm, then a misadjusted actuator rod. If you determine overboost, lengthen the actuator rod and try it again and again till mapped values are close.

I found it easier to remove, or at least loosen the 2 nuts holding the actuator on to relieve pressure on the actator rod thumbwheel (after loosening the locknut) to allow movement. Paint your starting position to be able to see how far you go in case you have to get back for some reason.

If you don't have vag-com and the code is for overboost you can still trial and error it, it will just take longer to confirm adjustment. Your never fail hill test can be the determining factor there.
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