2003 379 Issues/Questions

Hello,

I recently just bought a 2003 379 with a C15 6NZ and have been having a few issues with fuel consumption and clutch chatter. Firstly I'll post the specs, 13 speed, 3.70 ratio, 24.5 tires, 776000 miles. The truck has had a complete inflame about 90,000 miles ago. My first concern is fuel economy because I only seem to be getting around 5.5-5.8mpg and I'm pulling dry vans with an average payload of 20,000-35,000lbs. The previous owner said he was getting 5mpg but he'd pull 100,000lb b-trains. I'm kinda stuck because I don't know where to go from here. I just replaced the thermostats because they were stuck open but not much change in mpg's. My second question is regarding clutch chatter, and this happens when the truck is coasting down the road but as soon as I press the clutch down 2" it disappears. Would anyone have any suggestions or ideas what could help me solve these couple of issues? Any suggestions would be greatly appreciated, thanks.

 

Thanks for the advise in regards to the clutch noice/chatter, I'll keep greasing it at least once a week. When it comes to how fast I drive, I do and average of 62mph (1400rpm) with the cruise set and I'd say that the fuel pressure is in the range of 90psi. I recently just had it serviced, new oil, fuel/oil filters but it seems like not much of a difference. The next thing I plan on doing is swapping out the mufflers and putting in straight 8" stacks and see if there is any change in mpg's. I just kept hearing how great these engines were which is why I got this truck, now I'm trying to figure out if there's something wrong with the truck. Also, I'm not sure if it would make any difference if I were to put a whale tail/turbo wing on the roof since it doesn't have one. Anyone have any experience in added mpg's after they have installed one on their truck?

 

In the last week or so I've tried driving around 55-60mph, and that seems to have helped my mpg's. I now get an average of 6.4-7, especially when doing 55. It didn't help that up to now the temperatures where in the -20 to -30's. I'm going to install one of those 'whale tales' on the roof since I've found one thats not very expensive, I can't see it hurting.