My 2000 S40 is reporting the P0234 "Overboost" code. I notice sometimes the engine stutters around 3,000 RPM when accelerating onto the highway and I lose power. Other times it is fine. It also seems the RPMs raise slower than they used to.

I have a Manual Boost Controller and some heavy duty Turbo Control Valve (or something like that) installed on the car. I only raised boost about a psi or two and it's been running fine like that for about a year.

Should I remove everything and see if that works? Or is my MAP sensor bad?

 

Have you tried turning the boost down with your manual boost controller to see if the code goes away?

As you suspect it certainly could be the MAP sensor. I would try lowering the boost first just so you don't spend money needlessly in case it's not the MAP sensor. Doesn't the 2000 model have 2 MAP sensors?

 

When one is playing around with boost one also should be able to monitor it.It doesnt seems like you actually know what you are boosting so please install a cheap boost gauge.The manual controller sometimes gets dirty and either blocking the boost or letting too much boost occur.In your case the code could have been set earlier as well so you just have to be able to monitor the boost every given second while driving.
High boost could blow out the spark hence the hesitation but high boost at low rpm is what kills the engine so again you have to monitor it.

 

I took off the boost controller and reset the CEL and the error hasn't come back yet. Now it feels like there is no boost. Is this the car trying to counteract the Overboost code? If so, how do I reset it so the turbo is at stock boost level? Perhaps I should try installing a boost gauge again.

 

Did you reinstall the stock Boost Control Solenoid?


(BCS) Boost Control Solenoid

 

I reinstalled the stock BCS and installed a boost gauge. With everything stock, I am boosting at 5 psi. What's the stock level supposed to be, and how much can I raise it with a MBC without any exhaust modifications?