If your testing boost in the driveway just reving the engine, then those may be the numbers I would expect. To accurately test boost the car needs to be under some sort of load(accelerating)

Check the ALDA line from the intake manifold to the over boost switch on the firewall.

Here is a good link http://www.dieselgiant.com/mercedesaldaboostsystemservi.htm

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'83 300D 198,000mi Howard
'03 Saturn Vue CVT 75,000mi(wifeys)