Turbo Problem Or What?

[JohnR]

Was coming down a hill on the way home tonight. I floored it in 4th up to about 90 (momentarily) when it suddenly lost power. It maintained 70 ish for the rest of the way home, but the acceleration was not there. It felt like the turbo had dropped out. When I got home, I put vag-com on the engine module and it reported "positive pressure deviation" or something like that. I cleared down the fault code, and now it revs freely again. Anyone know what is the likely cause of this??

 

 

Thanks,

John

[Biscuit]

car probably thought micheal schumacher was behind the

wheel and said bu**er this!

lol

:16:

[Scorpiorefugee]

John,

 

This is probably not a lot of help but my S reg has done this 3 times now over several thousand miles. In each case simply switching off and restarting has cured the problem so I have to ask if you are sure that resetiing the fault code cured it or was it simply the fact that you stopped and restarted. If so, the fault code may mean little.

 

With mine, the first time it seemed to be a combinaton of low water level and steep hill and the last time, yesterday actually, it seemed to happen after a long spell in very slow traffic.

 

I suppose my only suggestion is to check all fluids and temperatures before looking for anything more serious. I believe that there is a 'fall back' operating mode which is engaged if a fault is detected so, with a bit of luck, it ain't too serious.

 

Ron.

[littledaz]

"positive pressure deviation", in plain english, is turbocharger boost pressure exceeded.

 

The turbo had indeed dropped out or more to the point, the engine ecu had switched it off to protect the turbo and engine.

The engine ecu requests a certain amount of boost under various conditions. If the boost pressure exceeds the request for too long, the ecu will shut the turbo down.

The ecu will tolerate a certain amount of overboost until it has reached its limit of control. Turning the ignition off then back on resets the ECU, but the fault code will remain.

Common causes are sticky N75 solenoid control valve, sticky turbo actuator control (variable vanes in turbo carbon up and stick) and possibly vacuum pipes.

 

My money is on the turbo or N75.

 

The values of the turbo can be monitored in MVB through VagCom.

Edited March 7, 2007 by littledaz

[JohnR]

"positive pressure deviation", in plain english, is turbocharger boost pressure exceeded.

 

The turbo had indeed dropped out or more to the point, the engine ecu had switched it off to protect the turbo and engine.

The engine ecu requests a certain amount of boost under various conditions. If the boost pressure exceeds the request for too long, the ecu will shut the turbo down.

The ecu will tolerate a certain amount of overboost until it has reached its limit of control. Turning the ignition off then back on resets the ECU, but the fault code will remain.

Common causes are sticky N75 solenoid control valve, sticky turbo actuator control (variable vanes in turbo carbon up and stick) and possibly vacuum pipes.

 

My money is on the turbo or N75.

 

The values of the turbo can be monitored in MVB through VagCom.

 

Thanks. If it happens again soon I'll have to dig around for the cleaning instructions. Sounds like its possible its a carbon buildup on the variable vanes as it has done over 100,000 miles now.

 

John

[Scorpiorefugee]

Stll looking for the simple solution, Biscuit could have had a good point. 90 in fourth is pushing it a bit.... Combine that with lilledaz's info and I think the old advice... "If it hurts, stop doing it." may be appropriate. ;)

[tim-spam]

This has been covered before, and there is an excellent thread started by dt1 who had a similar problem - normally at its worst in cold, damp weather. Most likely cause is a sticking VNT mechanism on the turbocharger. N75 valve problems usually cause boost pressure negative deviation fault codes. However, if you present this problem to a typical dealer, they will usually replace this valve (and sometimes the TMAP sensor as well), find this doesn't cure the problem, and then replace the turbocharger - brilliant!

 

But, if you look at the post started by dt1, you will see that he managed to fix his problem without spending a fortune - took some time though.

[JohnR]

This has been covered before, and there is an excellent thread started by dt1 who had a similar problem - normally at its worst in cold, damp weather. Most likely cause is a sticking VNT mechanism on the turbocharger. N75 valve problems usually cause boost pressure negative deviation fault codes. However, if you present this problem to a typical dealer, they will usually replace this valve (and sometimes the TMAP sensor as well), find this doesn't cure the problem, and then replace the turbocharger - brilliant!

 

But, if you look at the post started by dt1, you will see that he managed to fix his problem without spending a fortune - took some time though.

 

As it was going down hill, and I was pushing it perhaps I over-rev'd? Was not looking at the clocks at the time but it did not seem overworked....was pulling like a good un!

 

If it happens again I'll investigate as suggested.

 

Cheers,

John

[tim-spam]

Over-revving will not cause the car to trip into limp mode due to excessive boost pressure - this will most probably have been caused by a slight sticking of the VNT mechanism. The engine is 'red-lined' at 4700 rpm, and so long as you do not exceed this, you should not experience any problems. In reality, the TDI engine is well into the 'downhill' part of its power curve well before 4700 rpm, and 90mph in 4th is well under 4000 rpm anyway, so you certainly weren't over-revving the engine.

 

My advice would be just to see how it goes - if it becomes a problem, then cleaning the VNT will most probably sort it out.