Turbo problem

[audismittens]

Hi all, I posted previously about my mum's 99 Audi A4 B5 1.8T and some problems with rough idling.

Rough idling problem has been fixed, long story, but it was indeed a fault in a vacuum system.

The car is now running beautifully, brake problem gone, idles smoothly...except now the Turbo is behaving oddly. It's not turbo flutter. The turbo seems to cut in and out on acceleration and sustained revs. Like it's switching on and off.

This is it's third turbo (first one exploded, second one was replaced by mechanics who didn't know what they were doing so a nut fell off and it also exploded, and now this one).

The car is now in the hands of competent mechanics who have done all the testing they know how to do and are now at a loss as to what is causing this. Any ideas?

[MetalMan]

Has the diverter valve (also known as bypass valve) been checked for proper operation?

[Davdraco1]

What are you boosting to?

[tar]

If its in the hands of compentant mechanics they they should not be stumped

[Spanky250]

ive been having the same problem and i still cant figure it out ive checked everything. im starting to think i mine that the turbo needs rebuilt. but like you said this is the third one in your car

[Davdraco1]

Put a new DV in and a new N75 in and see if it still pulses

[walky_talky20]

My guess is bad DV or bad pressure line to the wastegate. With a ripped wastegate pressure line, the boost will spike past request, N75 will shut down to get boost back in check, and cycle will repeat. A worn out N75 could also cause this.

[audismittens]

If its in the hands of compentant mechanics they they should not be stumped

Sorry for resurrecting this thread, but do I have a story for you!

It's long and involved and very expensive, and I might write it up in full later as a cautionary tale, but it ends with me taking the advice of this thread and suggesting that they check the DV and N75 valve. It's looking like it was the DV all along. I suspect the mechanic has just bypassed it but waiting to get the car back to know for sure (can't go pick it up til the end of the week). I've taken it for a short test drive and the turbo problem is gone but now the car is running a bit flat.

So: I've decided enough of mechanics. I've got some experience with working on cars and motorbikes, but not Audis, but enough to be pretty sure I can solve this myself. I'm suspecting that the N75 switch may also be involved in this. Thinking of investing in a boost gauge and one of those scan tool readers with a usb connection and using the ross tech software to do some diagnostics. I already have a cheapy OBD II reader that can read basic codes, etc.

DV and N75 are not too hard to get to, right?

[Davdraco1]

You can sway both of these parts out in under 30 mins. I would recommend removing the air ducts to get better acces to the N75 and I would get the car jacked up or on ramps to get to the DV, as its basic underneath the car. Again, both very easy, just tight spaces so be patient. Good luck!

[Spectre1130]

Sorry for resurrecting this thread, but do I have a story for you!

It's long and involved and very expensive, and I might write it up in full later as a cautionary tale, but it ends with me taking the advice of this thread and suggesting that they check the DV and N75 valve. It's looking like it was the DV all along. I suspect the mechanic has just bypassed it but waiting to get the car back to know for sure (can't go pick it up til the end of the week). I've taken it for a short test drive and the turbo problem is gone but now the car is running a bit flat.

So: I've decided enough of mechanics. I've got some experience with working on cars and motorbikes, but not Audis, but enough to be pretty sure I can solve this myself. I'm suspecting that the N75 switch may also be involved in this. Thinking of investing in a boost gauge and one of those scan tool readers with a usb connection and using the ross tech software to do some diagnostics. I already have a cheapy OBD II reader that can read basic codes, etc.

DV and N75 are not too hard to get to, right?

Hopefully they don't bypass the DV... You want it working to prevent compressor surge which will damage the turbo. You can test it relatively easy. These "competent mechanics" sound quite the opposite.

Also with a boost gauge hooked up to the manifold (even temporary) is very helpful.

[audismittens]

...these "competent mechanics" sound quite the opposite...

I think you may be right!

You're not going to believe this one. The N75 valve was put in wrong way around. Anyone care to speculate what the long-term consequences of this would be?

Anyhow, N75 is now where it should be, right way round. Car is running just perfectly. I'm still going to do my cautionary tale write-up of the whole saga when I get a moment, but for now just breathing a huge sigh of relief this is all over. I also had to replace the power steering pump in my Ford Falcon EL yesterday, twice (first unit had a faulty thread, high-pressure hose wouldn't tighten down properly, power steering fluid all over the engine, it was quite spectacular) so I've sort of had it with working on cars just for the now.

Thanks for everyone's help on this, you were a big part of finally getting it solved :)

[audismittens]

Addendum: How do you tell if the turbo is a genuine Audi turbo? Audi say it will be a Garrett turbo if it's genuine, but Borg Warner is also listed on a lot of sites as genuine? Any way to definitively tell?

[Spectre1130]

Addendum: How do you tell if the turbo is a genuine Audi turbo? Audi say it will be a Garrett turbo if it's genuine, but Borg Warner is also listed on a lot of sites as genuine? Any way to definitively tell?

I've only seen a BorgWarner K03 on a B5 Audi A4.