Help - S3 Boost going mental!

[TanyaS3]

Hi

I have a boost gauge in the car and lately the car has been running oddly. Ive had it plugging into a laptop at an audi approved place but nothing showed up.

It sometimes boosts to just 5 and there is just not much power there and other times it boost up to 25! then back down and the power is great but obviously it shouldnt be boosting that high. It so random and can change so often and quickly!

Can anyone help and suggest what this may be or has anyone else found this problem?

thanks
Tanya

 

[s3ollie]

sound like it might be your N75 valve that controls the wastegate actuator or maybe the actuator or wastegate its self sticking maybe. I'm not sure you would get a DTC for this

 

[ChriS3]

Sounds like its surging? Could be your N249 valve (the one that the DV is plugged into), the N75 as mentioned, or your DV could be sticking and/or knackered. And where is the boost gauge plumbed in to? Is everything else standard?

 

[ChriS3]

Just to add...

My S3 was doing this for a while just before I sold it, again with no errors showing up at first. The surging eventually caused some rich/lean running DTCs, and then a N249 mechanical malfunction DTC. Since the N249 controls the DV, I figured that before they go completely and show up a code, the valve stops keeping the DV closed all of the time and leaks air back into the inlet between the MAF and the turbo. The N75 increases the duty cycle to compensate, and if the N249 starts working and closes the DV suddenly, the over-compensation gives you the surging effect.

If it is the same thing in your car, you can bypass the N249 by taping in the FPR vacuum line and taking a line to the DV from there.

I was using a manual boost controller, not the N75, to control duty cycle so that probably why I got the rich/lean codes. The bypass cured the running and surging problems but the N249 was gone by that point so I still had to buy a new one to keep the engine light off

 

[TanyaS3]

Thank you very much both for your help. This means nothing to me but i will show it to my other half and try sort it