Trouble starting the M931, any info appreciated

Our 1983 M931 with the NHC250 has been a reliable beast since we brought her home from the National Guard Amory in Santa Fe, NM in 2012. Last summer she started running a little rough and getting tough to start. She sits for months at a time as her primary job is to pull our boats and RV's down through deep sand to lakeside. She finally died on the beach and it took a couple hours to get her going again. We removed the fuel filter and the fuel had jelled both in the filter and in the tank. We replaced both the primary and small filter in the pump. We wound up bypassing the fuel selector lever, feeling like it might be leaking air in to the system, and we pressurized a 5 gallon fuel tank from a hose connected to the line off the tank. Pumped and primed she finally started up and ran great. Nice smooth idle and solid throttle response just like she had always been. So we got her off the beach, removed the fuel tank and brought it home and cleaned it all out.

So we spend many hours this weekend with the clean tank reinstalled, new filters and 25 gallons of fresh fuel trying to get her to fire up. We went ahead and replaced the main fuel filter again, just to be sure. When we pressurized the tank we were not getting any fuel to flow past the main fuel filter. We dropped the filter twice and reinstalled it just to be sure we had a good seat and finally started getting fuel up to the pump. Figured she would start right up at that point but still could only get sputters, like she wanted to start but just wasn't getting a good fuel source. Of course every time we got to that point the batteries would start to get weak so we had to charge, then go back and try again.

We finally had to give up and will have to go try again. The problem is she is two hours away.

So, what we know. She fired right up a few months ago on clean fuel on a remote fuel source. The remote fuel source was running directly through the existing fuel delivery system, fuel lines, filters, etc.

We removed, cleaned and replaced the fuel tank. We can pressurize the tank and get solid fuel flow to the primer and the hose connected to the pump. She started once or twice for a few minutes then sputtered and died.

Kind of at a loss now. I hate to think its the injector pump, and based on her running a couple months ago still lean to not getting the pump primed enough to pull from the tank, but honestly don't know what to try next time we go down. Can we disconnect the injector lines after the pump and see fuel flow through the pump if we pressurize the tank? Did we cross connect the vent lines, would that make any difference?

Any help is greatly appreciated.

When mine has been slow to start which is only been twice, all use the primer pump and open the throttle a little bit to get better flow. I use the butterfly valve to make sure I'm getting all fuel with no air. You might just be losing your Prime if you've cleaned out all the gel and other gunk.

On my crawler tractor I crack the lines at the injectors and turn it over until I'm not getting bubbles anymore. Then make sure you torque down to spec I found it it's not torqued tightly enough you'll get loss of pressure which will affect starting.

I hope that helps. The other thing I would do if I had A2 hour drive is I would probably replace all the lines just to make sure.

Can you run it directly from the clean bucket of fuel to the pump and it fires? try removing the lines at the tank and blowing backwards through the system, a chunk of debris could have been sucked into the lines.

Run the thumb screw in to bypass the electric solenoid is the first thing I do when I have any trouble not starting. It is the fastest way to exclude that the solenoid is the problem.

This is just a guess without actually looking at it. The jelling is probably from the military using biodiesel. That stuff will jell after sitting, not a problem if the truck is always being run. If there was any in the tank it could have jelled, even if you had added diesel later.


The problem of not starting is from lack of fuel. If the lines and the tank are in fact clean my first area to look at would be an air leak. You mentioned not getting fuel to flow past the filter. I would made sure that line is clear. Something as simple as a leaking O-ring on the top of the fuel filter housing can cause your problem also. That is if you have the large filter/ water separator under the left front wheel well.

Floridianson, can you give me more info on the thumbscrew on the electric solenoid? I'm not familiar with that.

We still have the large water separator filter, yes. We did reinstall the filter twice to verify we had a good seal at the bottom and top o-ring. I did look at that top o-ring carefully when we were tightening the bowl up to make sure things were sealing good. We also were able to pressurize the tank and have fuel flow to the pump with no leaks at the bowl. We removed the lines from the tank and were able to get unobstructed air flow to the filter housing and from the filter housing to the pump by disconnecting the inlet hose off the pump.

If we remove the fuel return line from the tank and pressurize the tank will we be able to pump fuel all the way through the pump and back to the tank?

[FONT=&quot]Something else to check. Make sure the emergency shut off valve is fully open. [/FONT]

Thanks Scrounger, that was the first thing I checked.

Other known problem areas are air leaks and pickup tube failures. Something like a cracked dip tube may not exactly show bad when the tank is pressurized, vs pulling a suction on it. Fitting, filter, and hose leaks can all do the same thing as well.

For or something like this, I'd take a small used outboard fuel tank, modify it for a return fitting and possibly an external Carter vane type fuel lift pump, 12v is fine, just wire it to the ground side battery. This would at least hopefully get it running and moving. The fuel will get warm during recirculating, so watch the temp of it if you are using a small tank and trying to drive it a long distance.

I bought a non-running 931 a few years ago. When I got it home it had 2 full tanks of fuel. I tried everything to prime it but no luck. I pulled the rubber line off between the injector pump and fuel filter and test it. It looked fine but it leak air all over. It was completely dry rotted. Once I replaced it it primed and ran fine.


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I have already done that jjb, thanks for the reply. I have also bypassed the fuel tank selector switch just to make sure I have no leaks there.

I'm going down to the truck this Friday and am hoping that opening the electric shut off solenoid by closing the thumb screw will make a difference.

If that is not the problem, what should my next diagnostic be?

Take a 5 gal can of fuel. If the FSOV screw screwed in won't make it lite, go directly from the pump into the can of fuel. see if it will fire that way. Other issues are the primer pump lines and o-rings, If it won't lite directly from the bucket, isolate the primer system and give that a try.

Carry an 1/8" pipe plug. That will screw into the primer port right there at the inlet fuel line to the pump to get rid of the primer from the system.

simp5782 said:

Carry an 1/8" pipe plug. That will screw into the primer port right there at the inlet fuel line to the pump to get rid of the primer from the system.

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Solid tip Wes. When my injection pump was rebuilt, the shop provided a plug and advised that the injection pump is plenty capable of self priming as long as the fuel filter was full. They were right, it fired up within 30 seconds of cranking after installation.

Turned out to be the fuel pick up tube. Long story short, truck ran fine off of a remote fuel tank. We isolated the problem to the truck fuel tank and when we pulled the fuel pick up tube out it fell apart. Tried to solder it back in, braze it back in, nothing worked. I have searched all my usual parts places, does any one know where I can find one? Or has anyone come up with a better type of fuel pick up system? I don't want this to be an issue again. Thanks for the help!

Member TedCat has them available for sale, new, makes them out of stainless steel thick wall hydraulic pressure rated tube and 90 degree JIC top fitting both, properly TIG welded by Chris Razor (Miller welder spoke’s person). These things look space shuttle ready when done. We’re blessed to live by him.

https://www.millerwelds.com/resourc...ty-fabricator-chris-razor-and-hi-tech-welding

The tank top fitting from Ted is male, not female like stock. So you will need an adapter or just run a new push-loc hose, Ted can send that also with correct fittings, just cut to length and tool-less assembly.

Thanks! I will get hold of him.

TedCat doesn't come in here very often, does anyone have another way to contact him?

He just had his second child born yesterday. PM me a number for you, and I’ll forward it to him. I told him about your situation, so he is aware of it.