M925a2 running rough

[Cwynott]

This afternoon I ran my truck for a few hours and on the way home it began to run rough. Almost like it's out of fuel. It has 3/4 tank the the filters are new. It wouldn't accelerate past 35-40 mph. And was sputtering like no fuel. It would lose power and slow down till it shut off. But then it would fire right back up and run great for a few mins. And then repeat the issues. Luckily I got it home. Lang ideas as to what's wrong is greatly appreciated.

 

[WillWagner]

Sounds like a plugged filter, something in the tank sucking up on the suction tube or air, I doubt air though. Don't care if the filters are new, if you got crap fuel or something downstream let go and got into the filter, they are just doing their job. They are the sacrificial lamb of the fuel system.

There are a few other things I can think of, but I won't alarm you right now. Easy way to tell for sure if it is something between the tank and the pump is to put a gauge on it and drive it. You'll need a restriction or compound gauge, install at the inlet to the pump, run the gauge into the cab and drive it. The gauge will tell you if the issue is before or after the pump.

You, me or any of the "experts" here can speculate all they want, but until you troubleshoot it, nobody will know for sure what the problem is.

 

[Cwynott]

The local napa didn't have the filters in stock I ordered them will have them tommrow morning. And will replace them and see how it runs.

 

[Scrounger]

As WillWagner has already mentioned the fuel filter is the sacrificial lamb. The paper in a fuel filter when it has picked up water will stop the flow of fuel. It doesn’t take much to cause problems. One of the ways to check for water is to allow the vehicle to sit for some time, overnight is best. Then take a clear container and drain some fuel out of the primary fuel filter. It is called a fuel filter/water separator for a reason. If there is water present it will show as a layer on the bottom of the container. If there is a lot of water one could easily fill a small container with water.

That is one on the reasons the military has draining some fuel from the primary filter as part of the pretirip.

 

[74M35A2]

The suction side of the M939 fuel delivery systems are one of the weakest areas of these trucks. Filter change as mentioned, and also dump out the old ones to look for water in them. If water, drain some of the tank to get it out. Make sure soft hoses on the suction side from tank to lift pump are new(er). If still doing it then, pull the tank pick up tube and check it and the solder joint at the top of the tube into the elbow. These are all common problem areas on these trucks, and they all cause what you are seeing.

 

[Cwynott]

Well after finally getting some time to work on the truck. I changed the fuel filter and it ran fine for a few minutes and started acting up again. I had a spare filter with me and put it on and still isn't running right. The pickup tube all the way to the lift pump is new less than 2 years old. And I don't see any visible issues with it. Could it be the lift pump getting weak?

 

[Tinstar]

Have you ever ran any fuel additive, like SeaFoam, in the tank to keep the water in check and clean things up?

It wouldn't hurt.

 

[74M35A2]

Could also be the fuel tank dip tube, its fitting, or its solder joint into the fitting in the tank. Seeing a lot of issues with these right now.