I have a 98 Ford F-Series box truck with a 5.9 Cummins Turbo Diesel, 5 Speed.

Truck runs great but we can't get it over 52 mph unless going down hill. It takes forever to get the truck up to speed.

We have not done anything to the truck because we are not sure where to start or what to check. Any ideas would be great.

 

First thing i would do install a new Fuel filter. Then see what happens.

 

I will give that a try tomorrow, thank you

 

Where I work we have dozens of those engines with 4 speed automatics. We went through a real pita when the fuel changed to LSD. For some reason, fuel filters were clogging on almost every vehicle. Major loss of power.

Is that a 12 valve motor? They ain't no powerhouse to begin with....

 

Yes it is a 12 valve, We are going to try the fuel filter. We not looking to get alot of power out of it, just want it to be able to get up to speed a lot faster & be able to do highway speeds so we are not so slow on the highway

 

I drive one on Saturdays that shows approx 280,000 miles. It will run up to 75 or so, eventually.....

 

You may also check to see if it's getting full throttle.

 

Is it intercooled? Maybe a leaky boot or a hole in the cooler?

Whats the rear axle gearing?

I got a 12V in my dodge and I throw some cetane additive in every fillup. It makes a big difference, especially on the highway. Quiets the motor down a bit too.

Sam

 

There is a black bar on the throttle, is that something i need to adjust to get more speed out of it?

 

I am not sure what rear axle gearing is. We just picked the truck up less then a week ago. But we still have not gotten fuel filter changed yet, that will be getting done Monday.

 

After replacing the fuel filter we gained about 5mpg. Still cant get up to speed to be able to drive highway yet. Any other ideas?

 

What RPM is it turning at that 52MPH speed you posted in the 1st post?

Is it blowing any black smoke? (bad clogged air filter)

Have you verified the speed of speedo with a GPS unit?

 

RPM needle sits on the 20 mark & it says @100 on the rpm meter. No black smoke, but have not checked the air filter yet. Yes i have verified the speed with my GPS.

 

You mean as in 2000 RPM ? Is this in any gear or just high gear?

 

Yes that is the RPM. That is around my average i can hit in 5th gear (last gear i have) and that is the RPM i hit when i am shifting gears.