E320 CDI Goes into Limp, real-time data shows 0 boost

[Diesel315]

Looking for any guidance here.....2005 E320 CDI OM646 in line 6 sweet engine normally runs great
Has started doing: On acceleration, feel all acceleration lost, with Code reader real-time hooked up, show 0 boost. After couple runs ECL comes on with P0101 MASS OR VOLUME AIR FLOW A CIRCUIT RANGE/PERFORMANCE.
I can reset ECL and will go fast once or twice before going to limp again.
Here is the weird thing, with ECL OFF, reading Boost Pressure on code reader, in idle neutral 2000-2500rpm boost goes up to about 20psi (1.5bar) approx and moves with throttle up and down to 0(14.7psi, 1 bar)--normal
In limp mode, same idle conditions, going from idle to 2500rpm, boost reported by code reader barely moves from 0 and actually drops a few % , 14.7 to 14.5 to 15psi, turbo is spinning so how can there be 0 boost?
Scanner says change Hot Film Air Flow Unit in intake, which is expensive assembly....wouldn't the Boost Sensor also be suspect as something is telling the computer (and Code Reader) there is no boost available "dont feed fuel"
I hate to throw parts at it ...any ideas?

 

[supernoodle]

I've never seen a scanner than tells you replace a part as the first action. If it does, bin it.

If you have a fault one mitigating reaction the ECU might take its to disable certain functions to prevent damage or undesirable behaviour. So it's quite reasonable here that it has disabled boost pressure control. It could do this by opening VGT vanes or the wastegate (depending on which type is fitted).

 

[onefortheroad]

You've no boost pressure because it's in limp mode. Find out why it's in limp mode and boost will return. According to your findings it's the MAF. Try disconnecting the MAF and run the car , see what happens.

Time and again on these forums we see , " my gearbox isn't working". When it's infact the engine at fault. It's all in place to protect itself from damage.