Diesel Truck Poor Mileage?

Question

1994 Chevrolet K3500 6.5L Turbo Diesel (Detroit Diesel)
- Dual Rear Wheels
- 189,000 Miles
- 4X4 ( I drive it in 2WD though)
- New Tires. Properly Inflated.
- Had Oil Changed Recently (used Shell Rotella 15W-40)
- 4L80E Transmission with Overdrive.
- New Green Lifetime Air Filter (very similar to K&N Filter)

This truck only gets 10 MPG city and 11.5 MPG on the highway just driving, not towing anything. I know trucks like this aren't known for their fuel economy but this stuff seems odd to me. The RPMs at 55 MPH is around 2250 RPMs. The RPMS at 65 MPH is around 2750 RPMs. This seems awfully high to me. Also, at 65 MPH (with cruise control on) it will go faster, then slow down. Then it will pick back up to 65, speed up over 65 and then go back down. It repeats this pattern.

Something seems wrong here. Any ideas what it could be? Differential ratio to high? Any suggestions would be greatly appreciated

Answer 1

Theres a dozens of possible reasons for poor economy, dirty injectors, worn drivetrain components, etc etc. It seems to me it should be getting better milage than that, so I would take it to a reputable shop and have it serviced.
You might consider the single most common reason for poor milage too, its also the simplest, low tire pressure.

Answer 2

Dually probably came with high gear ratio for towing--check the option label in the glove box for the gear ratio--Chevy parts dept. can tell you what option code to look for--- 3.23 or 3.55 should be good ratios for power and economy if you're not hauling lots of heavy stuff. Cruise problem sounds like the servo is bad or needs adjustment--could be vacuum leak in the hoses or diaphram too.

Answer 3

Two things that rob diesels of mpg are dirty fuel filters and dirty air filters. You can also run some injector cleaner through it. That may help some.

Answer 4

Cummings 5.9 Diesel would not have spark plugs or wires, so bypass that. i might advise on your broker that perhaps your valves might desire to be adjusted.did you spot that the exhaust grow to be black and sooty, or white? once you initiate up it? i've got considered leaky injectors in this motor vehicle formerly inflicting numerous distinctive circumstances beginning from adverse gasoline financial equipment to a no start up situation. it fairly is purely 2 opportunities,there is many extra.

Answer 5

change your fuel filter thats a bigie on diesels should change about every 13 to 17k miles! it is located on back center of engine top. you may have to remove engine shroud that says 6.5 turbo diesel.

Answer 6

I would suggest you try changing the fuel filter.