M915 low power. Looking for likely problems.

[JH1]

Y'all...
I have a M915 with the NTC400 engine. Don't know if it's a BC1 or BC3. Seems weak. Only develops 17 PSI of boost, and the manual says it should develop about 25 if running right. Have looked through the postings and see things about hot-rodding them for more power, but not seeing a general attack plan for isolating the low power problem. Some things that are obvious are: Clogged fuel filter, clogged air filter, collapsing fuel lines from tank to pump, incorrect valve timing, burnt valves, failing turbo, and incorrectly adjusted throttle linkage. I'm sure there are more. The TMs generally start by having you take out the engine. That's a bit ham-fisted. Is there a rule of thumb for what to check first, etc?

I can test for a fuel delivery problem if I know what the fuel pressure should be at the intake and output of the pump, but can't find that spec. There must be a vacuum spec for the area between the air intake filter and the turbo, too. That would tell me if the air filter is obstructing the flow.

Any help is greatly appreciated.

JH1

 

[NDT]

Hmm, mine only develops 15 psi and I never thought there was anything wrong with it.

 

[wreckerman893]

My first M915 was a sick puppy...wouldn't pull a greasy string out of a fat cat's arse. I took it to an old school Cummins guy and he said the best thing to do was to remove the Injector Pump and have a good pump shop set it up to run. You can turn the fuel to it yourself but you run the risk of melting something down. I have ran the 400 Cummins at 2300 RPM but you really have to know how to drive them. If you keep your foot in it on a hard pull it will build heat quickly. Knock the tip off an injector and you'll have raw fuel hitting the piston....been there and got the t-shirt (this happened on a commercial truck I once drove).
Before I did anything radical I'd check all the things you mentioned.....especially the fuel delivery system. When I drove those trucks in the Army we had one that was an absolute dog......it stayed Deadlined for lack of power. They finally sent it up to Third Shop Maintainence and the mechanic found a kink in the fuel line that kept it from getting more than a trickle of fuel.

 

[Construction]

Is there an adjustment on the fuel pump? My M917 is acting up in a similar way, except it has a lot of power - then a little - then a lot - then a little. I can't make sense of it. Replaced all filters, checked lines, etc.

 

[Ferroequinologist]

Mine was running very sick as well, had the injection pump rebuilt and they set it 15% over, now I have to watch it or the speedo will creep towards 70!

I would do the simple stuff first- check your air cleaners, change your fuel filters.

My truck would run fine till the fuel got low, then would just stall and die, wait a min, and it would go again. Found a plastic cap in the fuel tank, that would block the pickup tube. once I got it out, never happened again.

 

[JH1]

Cummins Northwest told me that boost is an accurate way to judge horsepower. On the ntc400 you get all 400 hp when you have 25 psi. 17 psi means about 300 hp. Thanks for the replies. Jim

 

[greenmonster]

Also try running it with the fuel cap off a clogged vent line will do similar things.

 

[Triple C]

Are you getting black smoke or running clean. That will tell you a lot. If you are smoking I would suspect a exhaust restriction. If the exhaust can't exit fast enough and creats back pressure, it won't let the turbo spool up, resulting in low turbo pressure and unburnt fuel - black smoke. If it isn't doing that, I would suspect the pump or fuel delivery system. If you are running it full out at that turbo pressure and not getting any smoke, I would be careful about heat as wreckerman893 suggested. I had a NTC400 in a 88 freightshaker that we dumped a LOT of money in trying to fix that same problem, only to find out the muffler was clogged! Staight pipe fixed the whole thing..... Good luck. Oh and it might be worth the money to put it on a dyno if you really want to know what it will do.

 

[Construction]

Yes, black smoke when it is not moving like it should. When it is running normal everything is fine, when it loses power it smokes like a train - and will not rev up above 1000 rpms - then it clears and screams down the haul road again. Never thought about a clogged muffler.

 

[greenmonster]

Bad turbo or waste gate will do the same thing. If the turbine side wheel is impacting housing it will come and go as well. Eventually it will stop working but in the mean time it make work on and off.

 

[Danl]

Green Monster is right on my truck did the same thing, I feel the turbo is not big enought and it also dosen't get enought lube, you'll do well to get 60k miles out of a stock turbo set up with week lube , I say this if you keep your RPMs at or around 2000. Yup keep your foot in it

 

[rangereter]

Good point on checking the air filter for restriction, I have chased my tail on a low boost condition before to find out that the operator only changed the outer filter element and not the inner in many years. As far as the boost equating to the horsepower...fits right in with the which came first, the chicken or the egg question. Also check your boost psi with another gauge if you have not already, falls in with always test the test equipment before condemning the assembly.
Regards, Bob

 

[Construction]

On mine, I am still confused. It has full power most of the time, then loses power - truck barely moves 10 mph - then back to normal. Again, changed air filters and fuel recently. Each time it does it I change the fuel filter.

A Cummins mechanic told me to clean the leads on the fuel shutoff switch by the fuel pump. Anyone ever heard of that or had power issues related to that?

 

[Construction]

Found the issue. Semi-loose fuel line connection at the fuel pump. It was sucking air at higher rpm's. FYI for all who are reading. Truck runs excellent now.

 

[rcflyer70]

When I first got 915a1 it would take forever for it to get up to speed - same when pulling a load. I took it to the local Vo-Tec and let the diesel students work on it. In a day they called an said my aneroid valve had a hole in it. $30.00 later I had a new truck. It has all kinds of power and goes from 0 to 62 in a flash.