[wayne strutt]

I don't know if I have the correct area of this site to ask this question If not please redirect me.
I have a 2005 Jetta TDI. It has 100000 km and has started losing power if accelerated hard . This happens whether the engine is hot or cold. It will not accelerate fast enough for the automatic to shift into 2nd. Using the tiptronic will eventually get you to your destination. If I stop and shut it off for a minute and restart, it is back to full turbo boost until the next time it has a mind of its own. Does anyone have an idea where to start ripping it apart as I have no hair left.
Thank you
Wayne Strutt

 

[chittychittybangbang]

The first thing you should do is get the codes. Autozone will do it for free. It will let you know if the ecu has logged any problems.

Two main things here : it is fine until you accelerate hard and it's okay if you shut off and restart.

This leads me to suspect that it is a bad turbo regulating solenoid or bad vacuum line ( air leak ). These cars also have grounding issues.

A clogged fuel filter is possible.

It doesn't sound like a transmmission issue although it's possible. It's not a defective MAF because turning the car off resets it, although a bad ground on the MAF might be suspect although not likely.

Again, because you say it's okay after you restart the car, it sounds like there's an overboost or underboost, something that trips the computer into safety mode. Right now it's minor but it's not going to fix it self.

Lastly, you say that it's not even enough to accelerate to 2nd gear?

 

[diesel guy]

The fuel filter in the fuel tank pump might be somehow clogged but that doesn't sound like that is the problem. I disagree about the clogged fuel filter tho because shutting off teh car won't solve the problem. The fuel filter in the fuel tank pump could be sucking and holding a large piece of debris to it when you request more fuel, but this doesn't sound like the problems either.

My bet is on some component that controles the boost, either the solenoid or a line.

Look at teh drivers side of the engine. Find the metal tandem pump, the pump on the top side of the engine. Look to the rear, behind the intake piping there is a solenoid mounted on teh firewall, the N75 valve. This controls the boost. You can try cleaning it with electronic cleaner spray or heck for a worn hose nearby.

Another place to look is the air sensor. Look at the pass side above the alternator. There are intake pipes. There is an air sensor going into the pipes, this is where the ecu reads air pressure. It is called the g31 valve. Check the wiring, etc around it as well.

Another issue could be a hoose intake hose. What happens is the air escapes out and it sends the turbo into overboost and limp mode. Check for any vacuum or boost leaks as well. If you go to the FAQ section, there is a link for how to do a boost leak.

 

[blupearl]

the fact that you say it doesn't occur until you give it hard acceleration say to me that it's a n75 valve or vacuum line problem. All generations of vw diesels have this problem where a leaky vacuum line or bad n75 valve will put you into limp mode and restarting the car will reset it. What happens is that the ecu sees that it's entering a bad condition so it puts the car into limp mode. It's not that bad yet so it doesn't happen all the time but it will soon if you don't fix it.

I don't think it has anything to do with the transmission since you say you can still shift it tiptronic. I don't think it has anything to do with fuel either, although like CCBB said, it's possible but not likely.

 

[wayne strutt]

Thanks for the suggestions everyone. I think that probably the n75 valve must be the culprit as you have suggested. I don't have an Autozone in Ontario that I know of but will get the codes checked. I drove it tonight engine was hot air temp cool and accelerated hard with no loss of power . I could not force the symtom. The engine pulled hard right up to the red line in 5th gear with no hesitation but did belch black smoke which it does not usually do.
Wayne Strutt