M939 Does not Idle

Hi Folks,

This is my first truck.

I have been over the site to see if I could find helpful hints but only found a few similar issues.

Here is my issue.

I was in a parade for homecoming today and after I got back to the shop and turned into the driveway she just shutdown. I tried to start again and was turning over as normal but not starting. I finally gave it a bit of pedal and she started right up but when I let the pedal go she would try to shutdown again, so I have to keep the idle about 600-700rpm to keep running.

Anyone have any ideas?

Thanks
Steve

Injection pump may be going out. Mine would not return to idle after acceleration. Could also be your filters. Try them first.

Check your filter first. After that check your lines. If you were driving, hitting a bump or something could have cause a line to suck air not letting it want to idle. Mine idles in the 600 to 700 range

Which engine does it have? Have you checked your fuel lines for cracks? If it's an A2 does the fuel shutoff solenoid retract fully?

Hi

thanks for your input:

Here is to answer some of you questions:

I have not check the lines or filters yet but will do that in the next few days

Engine is a 505/8.3 Cummins 6 cyl series C cpl 0893 and I looked at the solenoid near the pump and it seems to be staying back if that is the one you meant.

FIFY...CUMMINS

Will..

Looks like you are new here so welcome to the site from Cape Cod, MA. Check your fuel pick-up tube in the tank. They are known to get loose and suck air at the solder joint at the elbow. Also check the tank vent at the top of the intake stack. Bees/wasps love to build nests in the tube and it will block the vent solid. Truck doesn't like to run at idle under a negative tank pressure. Ask me how I know! I actually installed a filter at the vent outlet to prevent it happening again. Hope this helps and good luck.

There is a specified measurement of travel for the FSOV when it is in the run position....leme see if I can find it


Here they are. These are for A and P pumps with RQVK governors. IIRC, this is what the engines came with. If you have the MW pump, let me know and I can find that too
View attachment FSOV Specs004.pdf

Hi,

I have looked for cracked lines and found none which is good.

WillWagner: Thanks for the PDF, as I did look at the solenoid and it seems that I have the Syncro-start and seems to be functioning correctly.

Cape Coastie: As for the now I will check the pickup tube, question thought there are 3 lines coming into the tank are you saying check all three?

Thanks
Steve

Hi,

Thanks for all your inputs:

I have checked all of the lines and have no leaks.

WillWagner: Thanks for the PDF and looks like I have a syncro start solenoid and seems to be okay with the measurements.

Cape Coastie: There are three lines going to the tank, are you saying the lines have a tube inside the tank?

Thanks
Steve

Hey Robinhood, I believe the 3 lines are 2 vent lines, 1 forward and 1 aft, and the fuel pick up line. The only one with the tube is the pick up. Mine has the NHC 250 but I believe the tanks are set up roughly the same. Mine had a problem with maintaining prime when parked and when I removed the fitting and the tube the tube just about fell out of the angle fitting that screws into the tank top. Had to resolder the tube back into the fitting. Prime issue solved! Hope this helps. Good luck.