Very bad fuel economy on KW T600 ISX15

Guys,
please help me to overcome my fuel economy problems with my truck! I bought a KW T600B with Cummins ISX15 engine (art of crap) 475HP, w/o roof top wind deflector, 3.9 gear ratio, 2004 year.

Initially when I bought it had 4 MPG, hauling semi light loads with dry 53' van. After putting in there around $13K I got average of 5.3 MPG.

I replaced the coolant and air coolers, turbo and injectors. Also shut off the EGR because it farts all the time and a lot of black smoke comes out.

I have a few bad symptoms left and probably there is something is the motor system that is causing that, eventually giving me bad mileage and poor experience with the ISX.

My turbo very often goes up to 48 psi, though I know it should not go over 38 psi. No smoke out of exhaust pipe at all.

Have done full service, changed intake air filter.

Fan goes on very often, my oil temp shows 250F mostly, though coolant is between 180-210F. I think I do have faulty oil temp sensor.

When turbo goes over 38 psi up to 48 psi I feel the engine is derated because I see loss of power.

That is mostly all I can state for the beginning of the discussion. Guys, please help me define the problem. I want to fix the problems potentially bringing my fuel economy to 7-7.5 MPG. Kenworth dealer guys have so many opinions that I can't trust these ###-perts. They just take my money, not the problem.

 

The oil does not smell burnt, turbo is my big frustration. Why the ECU lets it go that high? No technician gave me a definite answer yet.

 

thanks for your answer. I go mostly through ND, SD, MN, WI, IL, IN, OH, Canada - MB, SK, AB, BC and ON. Hills are everywhere, and a lots of crosswinds with head winds. My speed is never over 62 mph, mostly 60 mph, when passing - 65 mph, since it is my top governed speed.

Also, I noticed if I do 50 mph, having about 1300 rpm, that is my best fuel economy getting on average 6.5-7 mpg, but that is a dry land turtle speed! My gross weight averages between 50,000 - 78,000 lbs.

What I think is that my gear ratio plays significant role in mpg, and I guess if I have around 3.36 I would do at my worst 6.5-7 mpg, but... climbing every hill I would have to downshift to 8-lower. That is not the best option.

As to the turbo speed, I need to find an answer why it goes up to 48 psi, and I agree with you that it is running over its designed working temp. and speed. The old turbo had the same problem. So, I conclude that is not turbo problem. Something else is causing the turbo overboost and thus ECM derates the engine.

The engine mileage is 818K km (510K miles). It idles smooth at my lowest setup 750 rpm. I do not idle the engine except for warmups and cooldowns.

My other guess is that the cylinders do not have good enough compression as they should and that might waste extra fuel.

 

when turbo overboosts, the boost jumps rapidly from 38 to 48 psi and engine pitch goes low freq., seems like it is lugging.