Turbo boost pressure related fault... help!

[higgsy]

Hi, I wonder if anyone can help me with this problem:
I have a 2005 diesel sprinter 4x4. It has developed an odd fault which I have been attempting to diagnose using my topdon OBD2 diagnostics tablet and a bit of fiddling about.
The symptoms:
Everything is normal on startup, the van drives and shows good power, boost is normal ( going over 2000mbar at higher revs), boost transducer on/off ratio varying between 85% and about 40% depending on throttle etc which I think is ok.
If you stop, or go into neutral and allow the van to coast, the turbo boost transducer suddenly goes to 4% and stays there, needless to say this removes any boost. (Is this how limp mode works?). Switch the ignition off and on again and the problem goes.
The trigger point seems to be when the boost pressure drops below 1000mbar. I thought it was RPM related, but not so. At 1001mbar boost, all ok. At 999Mbar we are back to 4%, no power, and it stays like that until engine is turned off and on again, even if the engine is revved (which does take boost just over 1000mbar).
The only relevant fault code is implausible signal from boost pressure sensor - however I have replaced this, and anyway the diagnostic machine demonstrates that the sensor is returning sensible readings. It has 5v supply to it which I believe is fine, and the cable run across the front crossmember looks ok.
Any ideas appreciated, I'm at a loss!

 

[supernoodle]

Have a read of this

W203 C270Cdi still limiting revs and turbo after coming to a stop.

As posted previously, the car goes into a sort of "semi" limp mode when you come to a stop after the first 10 mins or so of driving. Drives fine up to that point but then no turbo and limited to 2000rpm. The only code being thrown is MAP sensor reading implausible (read with icarsoft MB v.1)...

forums.mercedesclub.org.uk

 

[higgsy]

Amazing... sounds like my problem and yes there is a fairly large difference between the barometric pressure and map pressure. Do you know if the baro sensor is inside the ecu on this van?

 

[supernoodle]

Not familiar with sprinter but assume it's basically the same Bosch module, so expect it'll be onboard too.

 

[supernoodle]

If you're brave Joe posted this as part of his fix https://forums.mercedesclub.org.uk/...duino-and-sd-card-module.198593/#post-2073437

 

[higgsy]

Thanks. I have found a large discrepancy between what my scanner says is atmospheric pressure, and the map sensor. However I cannot find an inlet pressure figure, or the actual sensor. Is it possible that this van does not have one? I've checked the air box and hoses and no sign.

 

[supernoodle]

I'm not sure I follow, what inlet pressure and sensor are you looking for?