Very bad fuel economy on KW T600 ISX15

Why would it be a better design? I don't know much about Cummins.

 

It does the same thing as a wastegate... except its continuous all the way up to maximum boost. So when operating correctly you have maximum boost all the way through the RPM range of the engine. You don't have to worry about the momentary loss of power you get when the wastegate dumps and then recovers.

Then of course there are all those advantages of not having to worry about all the extra exhaust piping...

 

Do you know how a turbo works?

From what you said, it sure sounds like the ECU is derating the engine when the turbo overboosts. The ecu cant directly control boost. The turbo shaft isnt connected to anything but the turbines.

A wastegate gets rid of boost. It limits boost so that you only get the amount you want to have.

 

Have you had any luck in solving your fuel problem? I have the same motor with the same symptons that you describe. My fuel economy is 3.2 mpg. I would give anything to have your 5.3 mpg. I have reset my valves, set the injectors, changed the air to air rad to no avail. I've talked to the people at Cummins they have offered to work on my truck at $100.00 per hour with no guarantee on any better results. If you have found a solution please share with a fellow broker. Thanks.

 

The OP hasn't been on the forum since 9/30... maybe someone else can help.

 

I have seen unusually high boost (over 35 psi) when my disconnected EGR-valve got... separated from the gears mechanism and started opening freely under heavy acceleration. But there was also heavy smoke and check engine light. I think ECM was letting it happen (high boost) to "fight" high amount of exhaust gases being introduced.
From what I see, EGR-valve is very first step in ANY troubleshooting on ISX, it seems to be fooling many people...