Hi guys,i have a 2000 model a4 on which i have replaced the maf sensor as i was getting problems on cold starting the car in the morning.Having done this the car was fine but now each time i drive it and push the revs to 2000 or more i can hear an odd sound like air being released and in third gear i cant seem to go past 60 mph without the car starting to miss and go flat . Have been told it could be the turbo radiator? Or a loose turbo pipe . Howver the power seems to be there *just that when i get the hissing sound i get a *flat spot in third gear then the power seeems to kick in again.Any thoughts anyone?

booooost leak. could be caused by some loose turbo pipes. Check the connections, pie clamps whatever make sure the pipes are tight.

Many thanks for the input jonbonesjones,but have now taken it to 3 garages,one of them an independant vw/audi specialist but no joy.they can hear air leak but cant locate.Took car on mway today to see performance and again if i give smooth acceleration then it picks up speed ok but then at around 90/100 tends to mis and lose power.am totally cheesed off and surprised that it cant be pinpointed.

There's a DIY about Pressurizing the System.

Youtube link;
You'll find the leak yourself and it won't be expansive

Have you scanned for codes? Has your check engine light been flashing? Could just be a strange misfire...

you should install a boost gauage. your problem sounds much like a boost leak, and probably a decent sized one at that. but you need to check the entire charged piping area, as well as the intake manifold. this means the turbo outlet pipe, the metal pipe that runs to the intercooler, the intercooler itself, the map sensor oring, the pipe from intercooler to throttle body, throttle body gasket, intake manifold vacuum ports, and the intake manifold gaskets. there are many areas where boost can leak. and if you got the boost gauge, you could measure vacuum too, which im just guessing is low as well.

I had the same problem on my '99 A4 a few weeks ago. I only realized it after driving close to a concrete barrier for a few miles. On start up in the morning, I could hear the hissing sound quite loud but it went away after driving for a few miles. Turned out I had a vacuum leak. I got it fixed and it's gone. I used to hear a high pitched sound especially on a mountain climb and was accompanied by some engine hesitation. The B5's are notorious on vacuum leaks. Get it checked out every 65K miles and you'll sleep easier.

No engine light flashing at all,just cant seem to work it out.

Thanks for the reply redline,but sureley would'nt the indie vw specialist havedone this?

i wouldn't imagine they would check all the pipes for a boost leak, usually if your car runs they are done. Did you ask them to check for a boost leak? they should have a tester or a way to test it. A boost gauge is a good indicator tho, and you can find a boost leak yourself