Pulled a running ntc-400 from a M915a1 with less than 30k Miles to repower a Case Ih Tractor. Truck had been sitting for an unknown amount of time.(Maybe as much as a few years). PT pump still has all tamper proof items. After install, during initial fire up, there was no throttle response at all, would just barely idle. After running for some time and working the throttle, i started to get response although very sluggish. Eventually i was able to get it to 2200 rpm under no load but took almost 2 full seconds to reach WOT under no load. When applying even a small amount of load the engine just falls on its face...(no smoke, no overheating), just no power like its not getting fuel. Put a Gauge on the fuel outlet and wasn't registering anything on a 300 psi gauge.
Being a farmer that likes to mess with things that i shouldn't, i decided to do enough research on the pt pump to get myself a false sense of security and start messing with it. Assuming it might be gummy fuel residue preventing the AFC valve from actuating properly, i decided to turn the no air adjusting screw out one round which i assumed would essentially Bypass the AFC valve function. After this the engine came alive. It would rev to WOT nearly instantly and register a spike of almost 150 psi. I also get a huge plume of black smoke now as well.
The problem i have now is that i get to WOT with only a small movement of the throttle shaft. Idle to WOT only takes maybe 15% of throttle shaft rotation.
Questions i have are...
1. Why is AFC necessary to have and will it harm the engine if bypassed?
2. Does opening the no air adjusting screw essentially do the same thing as removing the AFC?
3. Why would only moving throttle shaft a small amount( maybe 15% of total ) allow the engine to run all the way up to governed RPM?
4. Is there anything that can be checked of fixed without have to send the pump to a shop and spend a ton of money on getting it repaired?
NTC-400 Sluggish Throttle Fix?
Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by Texamwalker, Jan 11, 2025.
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SmallPackage, OLDSKOOLERnWV and Diesel Dave Thank this.
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I ran a 350 BC1 for many years. From your explanation, sounds like you have air in the system and the system is sucking air. Did you remove the plug on top of the pump and prime it ? That has to be done several times. And, with a can of starting fluid, you can spray some of the connections to find where it’s sucking air. @SmallPackage @Heavyd @pushbroom @Goodysnap @jamespmack @BoxCarKidd @Big Road Skateboard can chime in
SmallPackage and OLDSKOOLERnWV Thank this. -
Check all the lines on the suction side to the filter base you were using with the other engine.
The AFC won’t give the engine full fuel till it detects boost from the turbo.
Need to fill the filter with a good dose of injector cleaner to help clean the pump, might help dramatically.
I have a big pump here that I use to run. Built to CPL 625 + 100%. The AFC is blocked so pump is full fuel at all times. Has a V-12 gear pump and the discharge ports are larger than stock.. stock port on top the solenoid is 1/8” npt, this one is 1/4” npt. Got to watch tromping on it to hard, drive it with your big toe instead of your whole foot.SmallPackage, mile marker 27, 062 and 1 other person Thank this. -
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You’re on the right track with troubleshooting, and there are several steps you can take to address the immediate issues before sending the pump to a shop. However, bypassing or fully removing the AFC is not a long-term solution due to potential engine damage and excessive fuel consumption. Start by cleaning the governor and AFC, inspecting the fuel system, and verifying the timing to see if you can restore proper performance without the need for an expensive repair. If these steps don’t resolve the issue, it may be worth having the pump professionally rebuilt or calibrated.
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The fuel tank cannot be higher than the fuel pump. You need a float tank or restrictor valve on the pump outlet if it is. Can you post a picture of the pump? Most Cummins truck pumps use an automotive type governor that controls the high and low idle. The throttle shaft has a hole bored i it that aligns with holes bored in the pump housing when the throttle shaft is opened. What its doing sounds normal for an automotive governor.
The AFC is set by plumbing a gauge into the fuel pump solenoid and with the engine idling snap the throttle open and watch the gauge. You want 75 to 90 PSI max. over that fuel economy will suffer.
Cummins makes a couple options for a variable speed governor which is what you will probably want in a piece of equipment. -
If the pump has a been set with a low number plunger “button” or a stiff spring or a good stack of shims it will go wot with no load and have a quick crack sound in the stack with the throttle and not have to rotate the shaft much. Afc is just a no smoke function. Early turbo pumps did not have them. Afc holds fuel back until manifold pressure is felt then it adds more fuel. Without afc all it will do is just have full fuel throughout the whole throttle range and black smoke more until turbo spools up.
The throttle shafts have an o-ring that will get ate up by this new pelosi ulsd fuel. They will cause air to be sucked into the system thru the throttle shaft. It will not show up in tests from tank to gear pump on the injection pump because that is after all of that. The air from shaft will go straight to injectors. A common throttle shaft leak symptom usually is it not returning to idle very quick during gear shifts. Rpms will linger a little longer before falling back down when trans in neutral with foot off of peddle.062 Thanks this. -
ElmerFudpucker Thanks this.
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