P0234 updated with waveforms

Hi guys. I'm from the sunny UK. I have a V6 2.8 vauxhall vectra C 2006 with a p0234 boost pressure circuit positive deviation fault code. I can produce the fault on request. You have to push the sport button and bury the accelerator pedal you have plenty of boost pressure and just as you reach around 4000 RPM the light pings on and you go into limp home mode. It has a turbo by bass valve and a wastegate control valve that work perfectly when testing and revving the engine staticly. All good powers and grounds, PWM on the waste gate control. MAF reaches 4.1 V on a static WOT. Fuel trims are good around 4% in total. Won't produce a fault under normal driving or WOT tests. No lack of power at any point until fault code appears boost pressure sensor hits about 3.4V on a static WOT. I can create and disable boost pressure on a static WOT test by enegizing and de energizing the actuators manually. No obvious visual defects on turbo intake vanes (should lead to low boost pressure anyway) it would appear that it is over boosting at the very max rev range in sport mode but the two solenoids which control this boost pressure have passed every test I know of. Any diagnostic advice would be much appreciated?

Hmm.. Nice feedback Richey. Does your scan tool support bi-directional control? Any chance of posting a freeze frame?
Sounds like crap, since you did quite some tests with zero result.
I have access to manufacturer database, but honestly these stupid flowcharts are not much of a help. I can always take a look with your VIN if you want me to.

The car is coming back in and I will try and gather as much info as I can and upload it. The tech 2 doesn't even list control of 1 of the boost control actuators in the live data, I can't remember which one. It does have a boost bypass valve test that doesn't work, as in it doesn't ground the circuit. At that point I thought I had an ecu problem as there was bias voltage up at the bypass valve with it unplugged. So I hooked up the scope and drove it and much to my surprise the ECU had full control over both the bypass valve and the waste gate solenoid. And if you drive it normally it will not post a fault. If you put it in sport mode and give it all the beans (you have to hold the steering wheel with two hands there is that much torque steer) ping the light comes on it then shuts down the waste gate control solenoid and you are in limp mode. There is even a TSB for that perticular fault code but it describes a whistling from the turbo and is says that rust from inside the manifold breaks off and damages the turbo fins. I have no whistle and certainly no lack of boost. And to inspect the exhaust fins it would be major surgery.

The car is coming back in and I will try and gather as much info as I can and upload it.

Nice

The tech 2 doesn't even list control of 1 of the boost control actuators in the live data, I can't remember which one. It does have a boost bypass valve test that doesn't work, as in it doesn't ground the circuit.

That sucks. But I've had similar problems with actuator tests on Tech2.
Considering the overboost DTC I'd focus on the wastegate control, valve and boost pressure sensor. But like I said, you already did different checks like I would've done :S

Hi Richey

Not sure if this will be any help but there is a case study on The pico case study page relating to overboosting when letting off the throttle
www.picoauto.com/library/case-studies/the-picoscope-journey

I have had several sticking turbo vanes causing very similar issues to what you have described

Hope this helps

Gary

I have had several sticking turbo vanes causing very similar issues to what you have described

Think you're talking about a VGT? These petrol engines don't have that. No variable geometry.

Thanks for your link Gary.

Ah my apologies, I was skimming through the post and thought it sounded familiar

Oops

Hi guys car is back in and I have managed to recreate the fault. Here is a known good waveform notice the yellow bypass valve at the top gets told to open when the throttle valve shut's.

Here is where the fault occurred and fault code gets logged and it instantly goes into limp mode. NOTICE the boost pressure isn't even that high but when the throttle valve shut's the bypass valve doesn't recieve any command to open

This is also where the fault occurred and the boost pressure did go super high again no command for the bypass valve

The red trace is boost and the blue is MAF. The green trace is the wastegate control solenoid which looks to me like when enegised it allows boost to occur, I see no fault here. Other than a software or driver issue I am struggling with this one any help or advice will be much appreciated. THANKS for your help.

The air bybass valve is a valve bolted to the turbo which looks to me like it allows the intake air to bypass the turbo vanes altogether, (taking the turbo out of the intake system when it is not required.) Does anyone notice the down ward slope from the boost pressure sensor when the fault occurred??? Does this mean anything to anyone. Boost pressure Leakage perhaps???? Still struggling to see why there is no command for bypass valve. can't be a wiring issue as there is a zero voltage drop. And it wouldn't only NOT work at a a high boost pressure range and work every other time.

If the bypass valve is not operating the cold side of the turbo gets a huge punch in the face during the throttle closing as it is still spinning at high speeds and now the air has no where to go. Maybe it is some kind of a limp home mode that disactivates the bypass but it sounds strange to me. So if i understand everything right you get the limp home mode during WOT acceleration, not when you release accelerator pedal. To me this sounds like a wastegate problem or its 3way solenoid valve so when the system waits for the wastegate to lower the boost it doen't get the response. You should check basic boost pressure and check for cracks and holes in the wastegate membrane and its connecting tubes.
PS
This looks like a pull to ground design for the bypass solenoid. You still can have a wiring problem then cause you are reading 12v through the windings of the solenoid and can possibly lose ground control from the pcm.
pps
also i would log the intake manifold pressure during normal operation and when (or during) it comes to a fault

Hi thanks for your help. The fault only occurred when you would release the throttle with high boost pressure. I had already checked waste gate diaphrams. In the end I put a new waste gate solenoid on it as it was only 13.50 and I cleaned the not very dirty throttle body (wondering if it was not snapping closed quick enough to get a response from the ecu to open bypass valve) I also heavily lubricated all the pivot points on the wastegate mechanism and stood there operating it with an air line. After that whole process it would not misbehave, no matter how hard I boosted it. I am fairly sure it is fixed but not 100% as it was very difficult to replicate the fault. If it comes back I will be sure to let you know.

Nice, hopefully that's the end of that one. Ive been reading this one, but have had nothing to contribute.
Sounds like all your testing methods and logic was solid.

Glad to hear all is fine. It could certanly be the wastegate solenoid fault. But to my mind if this was happening as you released the throttle then you can not blame the wastegate only. You can easily lose pcm control of the bypass solenoid and in case of a pulldown design (and it looks exactly like this) you will read battery voltage constantly through the windings of the solenoid (and it looks like this on your waveforms).