M915-M920 NTC-400 low power

[JH1]

I have a M915 with the Cummins NTC400. It has low power, as evidenced by low boost (20 psi when it should be 25) and also it just doesn't pull like a known good 400hp tractor. I suspected a leak in the crossover tube, but after replacing the seals and fuel filter, and cleaning the air filter, the problem is the same. I suspect that the military deliberately tuned down the engine, like they did with deuces, to prevent the 19 year-olds from breaking it. The reduced boost is limiting the power to 320 HP, by calculation. The local Cummins guy confirmed that the pump lever is being activated properly.

Otherwise, the engine starts easily, doesn't smoke, runs very smoothly, etc. No other problem, whatsoever. Has anyone else run into this problem of low power, while everything else seems fine? Btw, I also replaced both fuel lines. No change.

JH1

 

[patracy]

I know it sounds funny, but have you burnt out all the fuel yet? My M931 had NO power (ca1517 will vouch for that), I was "happy" that I had two full tanks when I bought it. But there had to be some old old fuel in there. Once I burnt all the fuel and filled up again, it felt like I gained about 50hp!

 

[R Racing]

there is a post on here somewhere on how to turn them up.

 

[WillWagner]

Just because the manifold pressure is lower than max spec, doesn't mean it has low power. I've seen plenty of trucks making 5, 8 psi less than max spec manifold pressure put out more than the 75% of rated flywheel HP to the ground. Find a dyno and see what it is doing.

 

[Vintage iron]

The engine is definitely NOT making 400hp if it is set up like my buddies. Does yours have a single fuel feed like to the heasds? A dual feed line kit from Pittsburg Power is a good place to start and you can send them your pump to have it rebuilt and turned up properly. If you add a FASS to what I just told you, you can expect 475hp and your engine will roar like a lion. I know because my customer did this to his truck and I have done these modifications.

 

[JH1]

Hi,
I just replaced the single fuel line with a brand new one. No difference. I have not measured the vacuum at the input to the filter, though. There is always the possibility of a clogged up intake in the tank, but is very unlikely, according to Cummins Northwest. There must be a place to measure the rail pressure and a screw to adjust, somewhere. I've looked in the manuals online and don't see a procedure for that. Anyone know? Thanks.
JH1