Turbo keeps cutting out. It comes and goes.

[audiroller]

I have a 2002 Audi A4 avant. The problem I'm having is that the turbo keeps cutting out. Some days it works, others it won't. It's either working 100% or there's no turbo pressure at all.. At first I thought it had to be a boost leak or something, but then it suddenly started working again, so that kind of rules that out. It's not limp mode either. It doesn't help to turn off the ignition and back again. It can last for hours or days, and then it suddenly works again. I can have no boost when I start the car, but after a few minutes then it might start working again on it's own.

I've spent an hour searching trough the forums now and I haven't really found any solution yet. The only thing I can think of after reading about related issues is the control system for the turbo pressure. Could this be the electric valve (N75 ? ) failing? And if so, is there any way to test it?

Thanks for your time.

[customa4]

Clogged catalytic converter. Click

[audiroller]

Thanks for replying. I've read the thread but it doesn't sound exactly like what I'm experiencing. I think that if the cat was clogged then the turbo would spin up, but struggle to produce enough power. Instead, in my case it's like the turbo vanes are just completely shut. It's not even trying. Then, the next time I start the car it might just work again, which lead me to believe that it had to be related to the control system for the turbo. I will try and loosen the exhaust bolts before the catalytic converter so that the gases can escape before the converter, that way i should be able to test if it's the converter or not. :)

[eljay]

Are you experiencing surging? In other words, does this happen when you are accelerating, steady throttle input, but the boost jumps up/down and feels like cutting out? Or is it just not there when you start the car and attempt to accelerate and then next time you start, all is well?
The surging/cutting boost under throttle could be caused by a faulty N75. My surging has been cured by a new N75.
But before throwing parts at it... can you scan the car for any codes?

[audiroller]

Are you experiencing surging? In other words, does this happen when you are accelerating, steady throttle input, but the boost jumps up/down and feels like cutting out? Or is it just not there when you start the car and attempt to accelerate and then next time you start, all is well?
The surging/cutting boost under throttle could be caused by a faulty N75. My surging has been cured by a new N75.
But before throwing parts at it... can you scan the car for any codes?

Thanks for replying. There is no surging. Some days it works like it should, others it won't. It doesn't help to turn off the ignition and back on again. On a few occasions it has started to work again while driving too. While holding the throttle steadily, and then suddenly the turbo just kicks in and it accelerates like hell.. When it's not working it's just like driving a diesel without a turbo. The engine doesn't seem to struggle in any way. It's like if the intake hose has blown off, and the car is transformed from a turbo car to a naturally aspired car, except that I can't hear the turbo even spinning up. It's like it's just staying shut under full throttle.

I used an obd2 scanner on it, but there were no codes. :\

[eljay]

That's interesting. I think that if the ECU would put the car into limp mode, even briefly, it would store a code. You may need to get a VCDS scan to learn more. Depending on the OBD scanner, it may not get you all the codes.
I am no expert, but it sounds like the boost control gets "disconnected" and then comes back on. I would check the vacuum hoses, but first also check the connection and wiring to the N75 since it sounds like your car's boost control goes on/off for periods at a time rather than momentarily changing.

I believe essentially what is happening is at times you are holding boost and then you completely lose all of it. It sounds like the wastegate may be open for a while and then it closes all the sudden. So, that may be sticky too. I would just check the N75 since it's easily accessible.

[audiroller]

I haven't been able to obtain a VCDS yet, but I've done some more testing without it. I checked all the vacuum tubes, they checked out. No leaks. I have confirmed that there is vacuum going into the N75 at idle, but there's no vacuum to the turbo actuator, coming out of the N75. I'm fairly certain that there's supposed to be vacuum there on idle (?). So, there's vacuum going into the N75, but no vacuum coming out from it, even when revving the engine.
Using a multimeter I've also confirmed that the N75 is getting voltage on the connector.

At this point I'm leaning towards a bad N75, but I'm not very experienced with these cars so I could be wrong. Am I missing something here?

[old guy]

The N75 doesn't operate on vacuum. The N75 controls the boost pressure signal from the turbo outlet to the wastegate. When it is deactivated it passes the pressure signal to the wastegate. This pressure pushes on the wastegate and when the boost pressure exceeds the wastegate spring pressure the wastegate starts to open. When the ECM starts the N75 duty cycle it begins to shunt the pressure signal back into the turbo inlet pipe instead of the wastegate and thus allows the turbo to continue to build boost pressure.

[audiroller]

This car is a 1.9 TDI with a VNT turbo, so there's no traditional wastegate on it. I'm fairly sure it's supposed to be controlled by vacuum on this car, and that it's supposed to have vacuum on the actuator even at Idle ? The way I understand it the vacuum to the actuator is holding the vanes open. Allowing maximum pressure. When vacuum is reduced, it closes. So in this case, when there's no vacuum it defaults to it's closed position? If not then I got this all backwards. I'm not very familiar with diesel engines. :D

[old guy]

Oops!
Sorry. I didn't realize you were referring to a 1.9 TDI. I must have missed that somewhere. You might have better luck getting detailed information on one of the TDI specific forums such as this Clicky click®

Good luck!

[old guy]

I just looked at this diagram and it appears to me that the N75 performs pretty much the same function on the TDI? It just activates the charge pressure control valve instead of a wastegate?



Here's another diagram of just the control circuit. Is this similar to yours? Obviously 1 is the N75.