NHC 250 goes past idle and dies when letting off throttle

[LCA078]

Truck starts and runs solid. But when I let off the throttle to turn a corner or such, the engine rpm drops below idle and sometimes dies on me. Not a fun time when turning so I generally keep the throttle pressed a bit.

I looked through the forum but only found the this thread that says the right way to diagnose/fix this is to pull the injector pump and send it off for bench testing. Is this the only fix or am I missing something really simple first?

NHC 250 fuel problem

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...

www.steelsoldiers.com

 

[doghead]

Maybe your torque converter is not disengaging?

 

[LCA078]

I didn't think about the torque converter as a possibility......hmmmmm. So, how would I test or rule out the TC?

 

[Floridianson]

Has nothing to do with your truck but. Back in the mid 70's working in a shop on for a customer. He says the car run great when going straight but cuts out in a turn. I take it for a test drive and yep cuts out in a turn. Bring it back to the shop and start to wonder / think. Open the hood and think, look and scratch my head. Ok everything looks good so lets try this put it on the rack and see if we can make it do it with all four wheels off the ground. Yep does it when we turn the steering wheel. Wait a second what is at play here. Wait the power steering ram the cylinder rod is bent just a little. No way what happened ok put on a new ram and the problem was out the door and a happy customer.

 

[TheQuaker]

I had the same problem when I first got my M923. It wanted to die when dropping to idle when decelerating and the trans did the rough 2-1 downshift.

Start by completely checking your fuel supply to the engine if you haven't already.
Start at your tank. How's your fuel quality? Got anything growing in there? Have you added anything like Howes to the tank to condition and stabilize your fuel and help clean out your fuel lines? Poor fuel quality will create poor idle/stalling.
Drop the filter and see if what it looks like. A dirty filter will restrict flow and cause poor idle/stalling.
Check all the supply/return lines from the tank to the engine for air leaks. Check the condition of the rubber lines, all the hard lines for cracks and loose fittings and make sure the manual primer pump is not sucking air. The NHC250's are very sensitive to air in the fuel lines and could cause your problem.

After that, check your throttle linkage and and transmission modulator cable and make sure they are adjusted properly. Improper adjustments can contribute to the problem.

Is your idle speed correct? Wrong idle speed is an issue.

All the above trouble shooting is well covered in the TM's...I checked EVERYTHING (takes some time to do it right).

My problem turned out to be air sucking in through a mis-aligned connection where the fuel return line connected to an elbow at the pump and allowed air entry into the pump and thus, the stalling. I never thought the return line would be the problem but it was. There was never any diesel dripping from the connection but it would suck air. Both male and female threads were trashed by someone not paying attention and aligning the hard line with the elbow correctly before reefing them down so I needed to order parts and replace both. Problem solved.

So before you start contemplating pulling your pump, check everything else carefully and put on your Sherlock Holmes hat...the problem may be something much more simple.

This connection right here was the culprit on my big girl:

 

[LCA078]

Maybe your torque converter is not disengaging?

Can I test this by sitting in neutral, revving up to driving RPM and then letting off the throttle? I vaguely remember testing it this way and the truck responded normally so maybe the TC is suspect. I won't be around the truck until early next week but wanted to get all the knowledge needed to rapidly test the truck.


Quaker- thanks for the detailed write-up. I'll check all as suggested. Since the truck starts easily and idles/drives well, I mentally ruled out a fuel supply issue but it wouldn't hurt from some PMCS on the truck.

 

[TheQuaker]

LCA078

No problem. Trying to save you some headaches and hopefully some cash.

I spent the time to write this up for you as my truck would start and idle fine in neutral as well. But in gear and decelerating it presented the same issues as you are describing.

Your problem sounds identical to what I went through and it took me a lot of time to diagnose it.

I wanted to dismiss fuel/air issues as well in my head prior to starting the troubleshooting process. It just HAD to be something much more sinister than air in the lines (my "I know better" syndrome). But these trucks are more complex and surprisingly sensitive to small things than we think they should be.

I buckled down and went through everything I described to you step by step and found it when I put a wrench on every single connection on the supply and return lines to check them.

Had I not followed the procedures and checked all the lines and connections I know would still be chasing the issue a year later and throwing parts at the problem and having no positive results...

 

[LCA078]

Quaker-

Thanks- I understand now! I'll take your words to heart and give the fuel supply a thorough look. It should be easy to pressurize and see any leaks. Hopefully my fix will be as easy as yours but without the 'learning' headache.

 

[TheQuaker]

Good luck

Make sure to let us know what you find (or don't find).

There are a ton of folks here on the site with way, way more experience than me with these trucks that will be willing to chime in and help as well.

 

[WillWagner]

I had the same problem when I first got my M923. It wanted to die when dropping to idle when decelerating and the trans did the rough 2-1 downshift.

Start by completely checking your fuel supply to the engine if you haven't already.
Start at your tank. How's your fuel quality? Got anything growing in there? Have you added anything like Howes to the tank to condition and stabilize your fuel and help clean out your fuel lines? Poor fuel quality will create poor idle/stalling.
Drop the filter and see if what it looks like. A dirty filter will restrict flow and cause poor idle/stalling.
Check all the supply/return lines from the tank to the engine for air leaks. Check the condition of the rubber lines, all the hard lines for cracks and loose fittings and make sure the manual primer pump is not sucking air. The NHC250's are very sensitive to air in the fuel lines and could cause your problem.

After that, check your throttle linkage and and transmission modulator cable and make sure they are adjusted properly. Improper adjustments can contribute to the problem.

Is your idle speed correct? Wrong idle speed is an issue.

All the above trouble shooting is well covered in the TM's...I checked EVERYTHING (takes some time to do it right).

My problem turned out to be air sucking in through a mis-aligned connection where the fuel return line connected to an elbow at the pump and allowed air entry into the pump and thus, the stalling. I never thought the return line would be the problem but it was. There was never any diesel dripping from the connection but it would suck air. Both male and female threads were trashed by someone not paying attention and aligning the hard line with the elbow correctly before reefing them down so I needed to order parts and replace both. Problem solved.

So before you start contemplating pulling your pump, check everything else carefully and put on your Sherlock Holmes hat...the problem may be something much more simple.

This connection right here was the culprit on my big girl:

View attachment 798273

That is gear pump return. Should have zero effect on running, only a leak. Flow only, no pressure there and return to the tank. It does sound like air. Install a looped clear line at the pump inlet and look for bubbles coming from the chassis.

 

[TheQuaker]

Hi Will

I agree but I don't know what to tell you...replaced the cross-threaded elbow and return line, aligned the connection properly, reassembled and bingo! back in business with no stalling. 20 bucks for parts. Been running like a top ever since.

Just trying to share my weird, frustrating experience chasing down something that "should not" have caused a problem and that was not easy to diagnose, even as I followed the TM's as well as Cummins' own trouble shooting literature.

My intent was to suggest to the OP other options to look at before he pulled his pump and give an example and encouragement to him of how trouble shooting following logic and the TM's can diagnose and address most problems...albeit with spending the time to do it right.

OK...I did not intend to hijack the OP's thread (Sorry LCA078 - just trying to help) and I will silently crawl back under my rock at the farm.

 

[WillWagner]

No I get it, hence the word "should", sometimes one off things happen, they are mechanical of course, and if something came into the shop with this complaint, and had a leak there, the first thing I would do was to address the leak. If it fixed the symptom, awesome, a quick easy fix with minimal cost to the owner. That is 50-60% of troubleshooting, easy/obvious/simple things first.

 

[LCA078]

Install a looped clear line at the pump inlet and look for bubbles coming from the chassis.

I'm heading down to my truck early next week and I'd like to have the fittings on hand before I get there to save another trip to town. I looked in the TM but it doesn't show sizes. What size fittings do I need to add a loop? I'm assuming regular tubing fittings on a piece of tygon tubing or do you recommend something else?

 

[WillWagner]

I'm heading down to my truck early next week and I'd like to have the fittings on hand before I get there to save another trip to town. I looked in the TM but it doesn't show sizes. What size fittings do I need to add a loop? I'm assuming regular tubing fittings on a piece of tygon tubing or do you recommend something else?

2 #10 swivels, a #10 union and 24 inches of i/2 in id clear hose. If they don't have hose that the reusable fittings will work on, the push on swivels work fine. Keep in mind this one is 25 or so years old.

 

[LCA078]

Thank you, Will- that info really helps. I have a feeling you've used that loop more than once since you built it....

 

[WillWagner]

I think once....