7 replies to this topic

[Michael_101]

Hey new to this site, i have recently bought a B4 and for the first week it was driving fine but now when i take the revs past 4500-5000 in first and second gear (havent tried any others) but once it gets to that rev range it feels as tho there is an extreme surge in power and like the brakes have been jammed on it jerks back and forward until i release the accelerater or change gears. Pretty new to these cars so any help will be appreciated!!

[Barbbachello]

The surge in power would be the secondary turbo coming online. Sounds like its misfiring on the secondary. The twin turbo system needs some love and care

Do some reading here
http://www.subyclub....-to-twin-turbo/

[Robbks]

Secondary changeover and then boost cut.

Welcome to owning a TT  

[Michael_101]

Secondary changeover and then boost cut.

Welcome to owning a TT  

Would this cause the car to jolt back and forth?

[Reevesy]

If it feels like you would be thrown out the front window then you know you're hitting boost cut

[LukeFranky]

I had a exact same problem. Do you have a boost gauge and code scanner?

 

My primary turbo had seized and the ECU detected this going up long hills and would throw an engine light. Once the engine light came on, any time I got into higher RPMs the car would surge forward with full boost from the secondary and then a second later it would suddenly throw me forward in my seat as if I jumped on the brakes. If I kept on the throttle it would rock back forth between full boost and slamming on the brakes, like you described.

 

I didn't have a boost gauge at the time and chased my tail trying to figure it out. Eventually realised it had to be the turbo, bought a new pair and swapped them over.  This highlighted other problems (O2 sensor), but the difference was amazing regardless.

 

Not too long later I actually blew up the new primary turbo because it wasn't suited to my model, so I pulled my old primary turbo apart. Discovered that oil had been leaking into the exhaust side and had eventually baked itself on like charcoal. Wire buff cleaned it out nicely. Rebuilt it with new bearings. Put it back to together and it has been boosting reasonably since despite not re-balancing the turbines.

 

You are definitely seeing a boost cut, but why is harder to answer. Could be a failing primary, or something else. First thing to do is scan the codes (Supercheap can do this), and like I said, a boost gauge won't go amiss.

[Robbks]

Would this cause the car to jolt back and forth?

Yes,
the jolting is the ECU cutting power to the engine to stop it from pushing in too much boost and risk damaging itself.

There's a lot of helpful threads about cleaning the solenoid box (Black Box Of Death, BBoD) and the vacuum lines in the system.

A very good temporary solution to keep it driveable is to unplug the factory boost control from the turbo on the passenger side,
And simple connect a short piece of vacuum line from the vacuum line nipple on the turbo's outlet, directly to the wastegate actuator.
It will run the engine on low boost (7-10psi) but will stop the jerking and risk of engine damage while you sort out it's problems.

If it still jerks, then you have bigger issues