Car keeps going into limo mode for turbo underboost condition. I have replaced all boost sensors and turbo pressure compensator with no change. Finally installed mechanical gauge and road tested car with scanner in live data mode. Every time my boost reading drops off from 36psi to 0psi instantly on my ECM reading(absolute pressure), my mechanical boost gauge stays at 23 to 28 psi. My ECM thinks I am losing boost but my newly installed mechanical gauge says that I am not. I have already replaced all of my boost pressure sensors and MAP sensor with no avail. Anyone ever run into this problem?

*New update*
Drivers side intercooler hose did have signs of starting to crack on bottom and I replaced it with a new oem MB hose.
I have tested my vacuum pump and it’s running approx 28-29 in/hg. I removed all associated vacuum lines and vacuum tested. No leaks anywhere. I put tees in my wastegate vacuum supply and boost controller and hooked up gauges for a test drive with my live data stream on scanner. Boost still works perfect up to 36psi off hp turbo but the very second the boost controller starts to dump the hp turbo to get supply from the lp turbo, the sensor reading goes from 36 to 0 psi instantly, then slowly climbs to a maximum of 6-7psi. Let off throttle and boost instantly jumps back to 36 psi. I continue to get error code P0299 and them limp mode on every test run. It’s like the engine only has 1 turbo. I get 0 to 22 psi on mechanical gauge but 23-28 psi is no longer there like before.
Is it possible to check the turbines on both sections without pulling my turbo? Or is there something else I am missing?
I don’t have any experience with these twin turbos, just what I can find online.

 

Take a look at your intercooler hose, the big black one on the driver side. The hose could have a crack sometime visible by the top or press the hose with your hand an you will feel it. This crack will generate an error code P0101, P0171, P0299. Easy fix by under the car, part number 204-528-00-00 150$ at the dealer. Common problem on the GLK250.

 

I'd suggest looking closely at the cables and connectors associated with the boost sensors.

 

+1 for the intercooler hose having at least one cut/tear in it.

$120 and you'll be back on the road.

 

Honestly, I don't think it's the pipe / hose. The car's sensors are going from full boost to zero instantly (not the scenario I'd expect with a bad hose), and the mechanical (read "accurate") boost gauge shows full boost even when the car's sensors show none.