thermostats and fuel mileage

[hiswarrior77]

I have a 2006 2500HD with the 6600 duramax. I took a trip from South Carolina to New York last year and averaged 20.5 MPG which I thought was pretty impressive for a trip of over 1000 miles. Last week I took a trip to south Georgia and average 17.5 MPG. I took it to the dealer and they ran a diagnostic test and said that there was a "hard" code (happening now) that showed the thermostat was sticking open and not allowing the engine to get up to operating temparature quickly enough causing the engine to run in "cold mode", making it run rich thereby causing the drop in fuel mileage. I was wondering if anyone else has had this problem before I replace the thermostats.

[Mark Rinker]

While technically there is no 'rich' or 'lean' in a diesel engine, the idea that the engine is getting more fuel in an attempt to attain operating temperature makes sense. However, you example siting the mileage decrease doesn't fit the problem - assuming that the truck maintained normal operating temperature on the second trip...did it?

My guess is that there are thousands of 01, 02, 03's out there needing both thermostats replaced (i.e. one stuck open) but they seem pretty happy anyway. Probably wasting some fuel during the extended warmup cycle.

If not expensive, I'd do the pair and start watching your savings at the pump. Covered under warranty yet? While they are at it, you might consider checking out glow plug function - winter is upon us!

[More Power]

DTC-P0128 can set if the engine coolant temperature fails to reach a preset target temperature before a calculated amount of fuel is burned. This can theoretically happen on a cold day and the truck isn't worked long enough or hard enough to bring the ECT up to operating temperature (like in-town driving).

As long as you're not towing, installing the factory winter front would help. :)

Also, be aware that winterized fuel contains a few less BTUs than summer fuel. The various lubes and fluids are more viscous during cold weather and tires are stiffer. All of these can affect winter fuel economy to some degree.

Jim