NHC 250 fuel problem

[m16ty]

My M932 has gotten to where when you rev it up to 1500-2000rpm and let off the throttle quickly it will sometimes die. I've changed the fuel filter, checked for fuel leaks, checked the fuel shutoff, checked the emergency stop, and cleaned the screen in the pump.

I'd did do a little reading up on adjusting the PT pump and thought the throttle stop screw was my problem. I did adjust it up and it did help the problem but the engine would still stall sometimes. If I adjust it up more the engine will be really slow at idling down.

The way I understand it, when in the idle position, fuel goes through a different passage way than when above idle and you adjust the throttle stop to allow a little fuel to leak into the above idle passage to keep the condition that I'd experiencing from happening. Even the times when it doesn't stall the rpm will drop to 400 so before it recovers.

Do any of the Cummins experts have any ideas?

 

[wsucougarx]

Sounds like we need to recruit the knowledge of our resident NHC-250 expert. Where are you Willwagner?

 

[jcappeljr]

Sounds like it might be sucking air somewhere in the fuel system.

 

[renovate7]

On a similar note, my new 925A1 idles low when first started. I have to keep the pedal pressed and rpm's up for a minute or so til it warms up, then it seems to idle well at about 650 rpm's. Does this sound normal?

 

[rrrr]

on a similar note, my new 925a1 idles low when first started. I have to keep the pedal pressed and rpm's up for a minute or so til it warms up, then it seems to idle well at about 650 rpm's. Does this sound normal?

yes !

 

[mlwats1@hotmail.com]

your PT pump has a internal leck sucking air in and starving the fuel system

 

[jwaller]

your PT pump has a internal leck sucking air in and starving the fuel system

I was about to say the same thing. does your engine knock like it's running out of fuel?

 

[EMD567]

Are you SURE you have checked the fuel system well? I thought I had, and the 250 still ran like crap. It wasn't until I found a hairline crack in one of the brass fittings, did I finally get all the air leaks.

 

[nf6x]

On a similar note, my new 925A1 idles low when first started. I have to keep the pedal pressed and rpm's up for a minute or so til it warms up, then it seems to idle well at about 650 rpm's. Does this sound normal?

That doesn't sound normal to me. Granted, my experience with M939-series trucks is recent and limited, but neither of mine behave like that. They start immediately and idle nicely, without touching the accelerator.

I haven't experienced any of the problems in this thread yet, but I'm following the thread with interest to learn more about things I may encounter someday.

 

[Ord22]

Is it hard starting? If so, you have air in the fuel, somewhere. try this, go around the whole fuel system and recheck the tightness in the lines.

 

[m16ty]

My truck starts and runs great. The only problem is when you let off the accelerator pedal fast the truck will sometimes stall or at least the rpms will fall to around 400 or so until it recovers.

 

[WillWagner]

If it starts fine, runs good and when it decelerates the RPM isn't slow to return to idle, there is an issue in the pump. The throttle stop screw needs to be set on a stand, you can't do it by ear or by the way it runs. If it gets set too little, on downhill, coasting conditions, you can scuff injector plungers then you'll have low/no power, hard or no start. In short, you will mess things up. The stops need to be set at 37 degrees, any farther past full throttle flow will result in a low power complaint at full throttle, but will run much better if you lift your size 10. The issue you are seeing is probably a governor weight assist issue, could be springs or shims. It needs to come off and be put on a stand.

If it were taking air, internally or externally, it would be hard to start, have slow acceleraton, rev past the governor break point and have slow deceleration, too much throttle leakage...the throttle stops...makes slow decel, but ok starting and throttle response.

 

[m16ty]

Will, The engine is slow to return to idle. I've got a service manual (Chiltons not Cummins) that gave a procedure to set the stop screw running. Before I adjusted it, it would fall to idle fast and the screw wasn't even contacting the throttle shaft (I had to turn it quite a bit before the shaft began to move). The manual said it should fall 1,000rpm in 2-3 seconds upon deceleration, any more or less and your screw wasn't set right. It also said that if the screw is backed out too much it will have the problems I'm having.

I really wasn't wanting to pull the pump but I guess I'll go ahead and pull it and let the diesel shop look at it. The bad part is the only local diesel shop worth a darned only seems to want to rebuild pumps and not just fix what is wrong with them.

 

[73m819]

PUT about 5# air pressure to the tank, you WILL see where it is sucking air in the forum of a LEAK

 

[CUCV85]

Newly purchased M929 dump truck. Has very low power, replaced fuel filter and will clean screen in the pt pump. The truck is VERY slow to return to idle once revedded up!
any help would be awesome. I do notice the Pt pump is not green like the rest of the motor and has probably been rebult. truck is 1986 with 31,000 and is a Gem.
Steel Soldiers mark69k20 email rustytank2@yahoo.com V/r MSGT W USAF "Adamark Farm"

If it starts fine, runs good and when it decelerates the RPM isn't slow to return to idle, there is an issue in the pump. The throttle stop screw needs to be set on a stand, you can't do it by ear or by the way it runs. If it gets set too little, on downhill, coasting conditions, you can scuff injector plungers then you'll have low/no power, hard or no start. In short, you will mess things up. The stops need to be set at 37 degrees, any farther past full throttle flow will result in a low power complaint at full throttle, but will run much better if you lift your size 10. The issue you are seeing is probably a governor weight assist issue, could be springs or shims. It needs to come off and be put on a stand.

If it were taking air, internally or externally, it would be hard to start, have slow acceleraton, rev past the governor break point and have slow deceleration, too much throttle leakage...the throttle stops...makes slow decel, but ok starting and throttle response.

 

[SLOrazorsedge]

Where is the screen located in the PT pump?

 

[Jayco36REQS]

PUT about 5# air pressure to the tank, you WILL see where it is sucking air in the forum of a LEAK

How do you put 5# of air in the tank??

 

[Jayco36REQS]

Where is the screen located in the PT pump?

The PT pump screen in under the flat blade cap on the top of the PT pump.

 

[simp5782]

How do you put 5# of air in the tank??

Drill the cap and tap it to screw an air chuck into and pressurize it with your shop compressor regulated way down. Or you can go thru the vent line with a rubber hose and a npt hose barb.

 

[Jayco36REQS]

Drill the cap and tap it to screw an air chuck into and pressurize it with your shop compressor regulated way down. Or you can go thru the vent line with a rubber hose and a npt hose barb.

Gotcha .. thanks!