cat 3406e low boost??

Have you checked for an exhaust leak at the manifold?any check engine lights come on?.how does the turbine look on the intake side of the turbo?

 

Have you considered the boost sensor?

 

The boost sensor often slips through the cracks like cam position and engine speed sensors can do. They malfunction without setting a code or check lamp. The sensor is on the left front of the engine in the cylinder head above the air compressor.

 

It's a very common cause of your issue and since there's no practical way of testing it, yes, we replace them with a known good part, a new sensor.

 

Not off the top of my head, but I can check on it. Well, I can't get you a price. You can call a dealer with your eng ser # and AR # and they will give you a price.

If your boost is decreasing only in top gear you may have a fuel issue causing the problem.