Engine Locked? | 855 Cummins

[Zoidsfan77]

My experience with the NHC-250 is somewhat different. I think he is a little low on the power spectrum compared to my truck. I have an M923A0 that I put 14R20's on. I can get to 60 in about 30 seconds and have pulled hills at 55 mph with 26,000 pounds in tow. I had modified the pump by drilling out the tamper proof ball in the throttle shaft and unseating the fuel screw by about 3/4 of a turn. When the pump puked the front seals out and filled the crankcase with diesel, I had it rebuilt. The shop that did the work flowbenched it and turned it up even more. I asked if my attempt to turn it up resulted in the failure and they claimed it wasn't even turned up to factory Cummins specs. One thing they noted was that the "emergency shutoff" valve did not stop at center when pushed all the way forward. Could just be the one on my truck but likely many more of them. Before I reinstalled the pump, I inserted a drill bit backwards and added a mark with a hacksaw blade so that I could index where I need to set the "emergency shutoff" to on the top of the pump to "true center" of the slot in the shutoff. They also upped the fuel rate 10% and I certainly noticed the difference in performance. The heavy towing comparison was done before the pump work. For what it's worth, having the pump remanufactured locally only cost $350 or $375, can't remember exactly but that's in the ball park.

I was under the impression that turning the screw was to be avoided, and the button should be replaced if you wish to increase fuel pressure. I could be mistaken though.
Something I had wondered is how far is safe when turning up these engines. I know it depends on how you drive it and what you use it for.
Thanks for sharing your experience.

 

[Jbulach]

...It struggles to hit 55mph, even on long, flat stretches of road. It must be driven at full throttle to reach anything above 45. I am downshifting at the slightest inclines. I estimate 0-55MPH time is greater than 60 seconds, unloaded on level pavement....

Does your truck still have the small dual tires? The reason I ask is, I thought top speed was 50-55 MPH, if so I think it would take full throttle to get above 45.
Also, what RPM are you shifting at, with no load, while trying to get up to speed quickly?

 

[Zoidsfan77]

Does your truck still have the small dual tires? The reason I ask is, I thought top speed was 50-55 MPH, if so I think it would take full throttle to get above 45.
Also, what RPM are you shifting at, with no load, while trying to get up to speed quickly?

Yes I have the stock NDCC 11.00x20 tires. If anything, the smaller diameter would make it easier to get to top speed, as top speed on these vehicles are limited by their low gears.
I usually shift at 2100 when accelerating.

 

[red]

When you say some were turned down, would that mean a larger button? I am curious as to what size button our trucks are supposed to have.

Which fuel button is in the injection pump varies. Those who have opened their pump have found anywhere from a 18 up to a 31. Same thing with the pressure, it's all over the place.

The NHC250 in my wrecker is an example of one that was turned up. Happily cruises at 2000rpm/55 mph on my 395's up here at roughly 5000ft elevation. If I floor it up a hill it will go to 1400 degrees F with the exhaust and while in gear the engine will rev to 2500 rpm, way to hot and way to high. Don't want to take these motors over 2100rpm and a max sustained temp on the hills of 1200 F.

Proper fuel adjustment takes a combination of a better flowing button and adjusting the pressure.

 

[Floridianson]

I found the best way if your not taking it to a shop is have a good pressure gage. Myself I don't go over 200psi rail pressure on the stock 250 motor. So it would be decrease button number open up throttle shaft and use the pressure gage.