Let me tell you what I have to start. 1998 Volvo with condo sleeper, 500 hp N14 Cummins, 18 speed. It goes 70 mph at 1520 rpm or so. We pull flats and haul 75-80k gross frequently. My problem is no matter how my driver drives it 4.5 mpg is all we can achieve. I know not going to be 7 mpg but should be at least in 5's. Any secrets? Only problem right now besides mpg with truck is thermostat is stuck open and only getting 165 max under load. Would that cause more fuel consumption? I am to witts end on this. Something has to give. I already treated for algae. Fully services with overheads ran. Please help!

 

A stuck open thermostat that doesn't let the engine reach the optimal operating temp will cause the engine temp sensors to read a cold engine and cause the fuel system to run rich.

 

So for sure will possibly be my problem. I know cooler it has been outside worse it has gotten. Thanks!

 

Possibly, but not guaranteed. My 2006 ISX had the thermostat stuck open for a few months before I fixed it and I never saw a change in fuel economy. Still, it's a problem that should be fixed. Sounds to me like there's something else going wrong though.

 

Im to the point where I'm open to anything. What mpg you think it should get under average driver? Only 478k actual miles.

 

Do you trust your driver??? I have heard storys how drivers selling fuel.

 

Its gotten same with me driving. This is my best, most trust worthy driver.

 

Have you had your charge air cooler checked? Any leak in the turbo system (most common in the charge air cooler) will kill your fuel economy.

Are you accurately calculating your economy? Meaning that your odometer is accurate and you are dividing miles on the tank by the gallons it takes at the next fill-up?

How much idling does the truck do?

 

It idles under conditions where heat or ac needed while sleeping, otherwise it's off while off duty. The odometer is correct and calculate my actual miles divided by actual gallons. I will check air charge lines tomorrow before he takes off for the week. Thanks for that idea.

 

The best way I've seen the CAC checked is disconnecting one side, plugging the other and pressurising it to see if it leaks down in the cooler, much like you would a coolant system. For a visual check, making sure none of the clamps are loose or broken is the best you can do.