Last few days i have had a following problem: When accelerating hard (pedal to the metal) RPM's go to 4K and then boost's gone. After it drops a bit it picks up the RPM's again and all the same happens all over again. This is what I found out so far: It happens when turbo pressure gets higher then 1,6. I have heard there's some kind of protective circuit that cuts off fuel pump when pressure gets too high? I tried this also. I boosted with my tranny on second gear (tranny is auto) the RPM's went up to 6400 with no problem cause pressure never got over 1,3. So it is the pressure protection issue. More history: When I bought the car few months ago it didn't have any boost at all. Finally I found out that the hose going from the turbo to radiator (with metal rings around it) was broken. So we fixed it temporarely with... it's a long story but the boost came back ok and I could feel the turbo doing it's job. Before the problem with pressure I never got pressure higher then 1,5 at full boost (6000 rpms) I hope that kind of describes my problem. Additional symptoms: It sometimes starts hard. Sometimes it won't start at all. The starter works fine but it seems there's either no fual or no spark. Then after 20 mins or so it starts just fine. New spark plugs last week.
Haven't changed fual filter.
Any ideas? Mechanics here (Estonia) are mainly just parts replacers :( I don't trust them. Specially on turbo matters.
Thank you in advance

Sounds like you are definitely hitting overboost cutout. You've already replaced the throttle body hose, that's good. That hose is connected to the intercooler, not the radiator.You may want to check the lower wastegate hose for any rips or tears. May also be a torn wastegate diaphram.

Your hot start problem is probably not related to the cutout under acceleration, more than likely it's a fuel pressure problem, perhaps a leaky check valve on the fuel pump, or leaking fuel injectors. If you've got a high mileage car and the injectors are original it would be worth replacing them now anyway. Try to have the fuel system pressure checked after a warm shutdown.

www.sjmautotechnik.com is quite comprehensive for diagnosing boost related issues for your car. Definitely bookmark it!

You should also susbscribe to the Quattro list, www.audifans.com, it's a valuable resource for older cars like yours! Good luck!

Allen
89 200q Avant
91 200q20v Avant

I had the same problem on my 87 5KTQ. i did all the same things you did. it turned out to be a leaky injector that was blowing the compression out where it was connected. if replacing the injectors doesn't fix it, then the next thing to look at is the wastegate. i'm betting its the injector, because if only one of them is blowing, you'll still have compression in the other 4 cylinders, which will feel like power cutting around 4000rpm.
Bisqueth
87 5KTQ
941CQ