I have a buddy that has a 2000 dually with a 5 speed 3.55 and 4x4. He has a 160k on it. He has had it for a little over a year. He bought it in texas, when he brought it home he avg. 17 mpg. but here lately (last couple months) he can't seem to do any better than about 10 mpg, and it is extremaly doggy. We cleaned the AIT sensor, it wasn't to bad. And we checked the fuel pressure, @ idle it is about 10 and if you just goose it neutral, it drops to 7psi. Couldn't get a reading going down the road @WOT with our quick test last night but we will get a gauge in the cab here in the next couple of days to check that. The truck is bone stock. So what else could we do or check. Could the vavles be that far off, that it is causing trouble.
Thanks for your help and input
Heath

 

Oh one more thing, the coolent temp won't hardly come up off of 140* unless your under a hard pull. If you put card board in front of the radiator it well come up but it hasent been that cold out 408 or so. Did the southern trucks have a different t-stat or something?
thanks
Heath

 

I seen a increase in my MPG after replacing the Thermostat. It didn't take me long to do it, and I went ahead and changed the anti freeze as well. My truck was doing the same thing not coming up off 140*, new thermostat fixed mine.

When I got the truck 14.5 mpg, changed oil, rear diff fluid, transmission fluid, new t stat, antifreeze change, and my last tank i got 19 mpg around town , I was happy to say the least with diesel at 2$ a gallon, and I'm almost positive the t stat caused the mpg gain.

Clint

 

clint,
just curious did you put in the 180 or 190 thermostat. 1999 models call for 180 but I have heard of a few putting in the 190.

Just trying everything I can to gain a few MPG.

 

I got it from Carquest, because I needed it ASAP (cold heat), can't even remember the brand, but I do remember it was a 188* t stat, and I think it even said OEM on it. The miles were hand calculated, and only 1 tank, the overhead read 21 mpg, and my tires are a few sizes off, not sure what the long term effects will be, will take 3 tanks to know if it was a glitch or not. I was shooting for 18mpg, that was my dream. truck is still pretty new to me also.

Clint

 

Well we are fixing to put in a new t-stat. How does the t-stat affect mpg so much, just the warmer motor burns the fuel more efficent or what. If thats the case shouldn't you see more smoke?
Heath

 

Not sure how it works, but I know on the overhead my MPG will drop fast cold morning start up, I'm sure the computer will change they way the motor runs when cold, and when at operating temp, I would also think that the engine would run more efficient when at operating temp.

The oring in the t stat came apart on mine, it was stuck in the top of the thermo stat. Hope this helps.

Clint

 

You should always run the hottest T-stat you can that still allows proper engine cooling.

I'd run a 210 if they had one.

JLH