M923/A1 Fuel system common issues

I have been working on military trucks full time for a few years now. I have found some of the same issues with the fuel system on M923 and M923A1. Customers will bring me a truck that has hard starting or rough idle or weak throttle response.

First thing I do is change the fuel filter. while i have it out I clean the filter housing and the petcocks. ALWAYS use the new gaskets that come with your filter. Fill filter housing with fresh diesel. I put the filter back on but don't use the primer pump. I instead, take a vent line off. I have made a air line that I run through a pressure regulator at 5 PSI. I pressurize the system. then crack open a line at the pump till fuel comes out. It is amazing how many times this solves the problems. so many times the filter is not tight enough or the seals are not sealing properly. everything needs to be clean, clean, clean.

Second, I always put ATF in the tank and fuel system treatment. A lot of issues come from bad fuel and gummed up fuel system. The ATF lubricates the pump and injectors. The Fuel system treatment bring up the Cetane and cuts the crud. This second part is always a good idea.

third, I check all the hoses and lines for abrasion or ares that could produce holes. Recently I have found a new air leak spot. THE PRIMER PUMP! I disconnect them. put a plug in the fuel line. but I cap the pump and zip tie the hose up, just in case you need it in a pinch. the primer uses a plastic hose. The hose should have a insert to keep the ferrule from collapsing it. So the there will always be a leak.

these are just a few of the things I found. I hope these help.

I have cleaned and resealed all the primer pumps on my trucks.

I have also replaced those fuel lines with new.

I think it's a good idea to have a functioning fuel primer pump.

I also replace the rubber lines. Many times they are cracked and suck air(but do not leak fuel).

The fuel tank selector valves are notorious for air leaks also. Easy to clean and reseal with 2 o-rings.

I've also seen the in tank drop tube(pickup) rust and have pinholes. They run fine with a full tank but run lousy when it gets low.

Thanks. I'm a little confused about the ATF (Automatic Transmission Fluid?). Do you add it to the fuel? How much in a full tank?

I drove my truck from GA to TX and didn't identify any problems. However, after driving a non-turbo duece, I was expecting a little more power. I don't have anything to compare it to, though. It starts great and idles well.

Yes, this would apply to all M939 series trucks with a Cummins 250. The new low sulfur road diesel has less lubricating properties and tends to get stale quick. I put two gallons in a full tank. Your engine with run quieter and stronger! Only problem is that ATF is Red. So I just use it to douch out the system. I don't want us to get into the road tax discussion. The ATF is for lubrication.

Only problem is that ATF is Red.

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Offroad fuel is red to differentiate it from taxed fuel. The dye used can be identified by a test strip used by inspection officers. ATF does not contain the dye. Also putting two quarts in 60 gallons of fuel is not going to change the color of the fuel very much if at all.

Vintage iron said:

Yes, this would apply to all M939 series trucks with a Cummins 250. The new low sulfur road diesel has less lubricating properties and tends to get stale quick. I put two gallons in a full tank. Your engine with run quieter and stronger! Only problem is that ATF is Red. So I just use it to douch out the system. I don't want us to get into the road tax discussion. The ATF is for lubrication.

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Actually, when the government tested they found that ATF did not increase lubricity much enough to matter. It has friction modifiers in it that are designed ot make clutches slip less inside the transmission, and as a result that negates some of the natural lubricity of the base oil

What tested far better was 2 cycle oil, mixed to about 150:1. In addition to adding lubricity better because it is designed to burn as part of fuel it will not leave the deposits and ash that additives inATF can, and it is cheaper.

I wasn't aware of the testing. Very good to know. I will have to try the 2 cycle oil. Thank you

Where did you read this test results?

Vintage iron said:

Where did you read this test results?

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I have a .pdf copy somewhere, let me see if I can find it.

I did a bunch of research into it when we were having lots of issues with the 6.5 injection pumps and JP8 in Afghanistan and were looking to find what was the best additive. A fuels guy from TARDEC hooked me up with the report/

I have attached a study on diesel lubrication additives that may be of interest. It doesn't include ATF, but does mention used motor oil and 2stroke oil. http://www.dieselplace.com/forum/76.../177728-lubricity-additive-study-results.html.

Vintage Iron, i do just like you do, i clean the filter housing, CLEAN, CLEAN, CLEAN, then fill the housing with clean fuel along with a new filter and seals, this makes priming and starting the truck SO MUCH easier after doing a fuel filter change.

What are the fuel tank vent lines made of? I found one of mine cracked. What is recommended for replacement?

Dmaxcc6spd said:

What are the fuel tank vent lines made of? I found one of mine cracked. What is recommended for replacement?

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I'll second that question. I would also like to ask how exactly you applied 5psi to the fuel system, and where you are cracking a line at the injection pump. Are you talking about an injector line from the IP?

I am currently battling problems on 2 different 855's. One has a weird slow leak from the tank onto the ground. Hoping I can find the leak with air pressure applied.
The second is my 925A1. It idles horrendously, slowing down to the point it almost dies, then coughing and surging back up to normal idle. And I only get a delayed throttle response out of it with the pedal mashed to the floor. Haven't really had time to mess with it as I've been spending all my time on the van, but would sure be nice to get it up and running correctly.

Vent line for the fuel tank is made of DOT air brake hose I belive. As far as applying 5 psi, I installed a Schrader valve in the cap. Just air it up slightly with a tire chuck. The air bleed, follow the steel line from the lift pump. On the banjo bolts that secure them to the pump there should be a 8mm bolt head. That's the air bleed. Mine has it 2 in different places, just look for it at the ends of the steel lines.

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Replace your rubber supply lines, if you haven't already. They are well known for air leaks causing loss of prime and idle issues and slow acceleration.

Also, the fuel pickup tube in the tank is well know to rust near the top of the tank(inside) and cause the same issues.

Thanks for the quick replies! Found a bunch of threads about a problem with a soldered joint in the pickup tube, so I'll check that out. The van just started leaking here a couple days ago after I moved it, so not sure what I disturbed there, but I'll post back. One of the vent line looking fittings doesn't have anything coming out of it, and I'm assuming there's not a check valve in those? So I should probably get that capped. The 934 is different from the 925, whereas the 934 has more ports on top of the fuel tank.

I figured they were the same as my brake line tubing, but wanted to double check. And I will definitely be replacing all the rubber line. As soon as Icepocalype is over anyways.... Still kicking myself everyday for not buying that **** 934A2 when I had the chance... this 250 Cummins won't take the van up a hill over 35mph....

PyroJoe said:

Thanks for the quick replies! Found a bunch of threads about a problem with a soldered joint in the pickup tube, so I'll check that out. The van just started leaking here a couple days ago after I moved it, so not sure what I disturbed there, but I'll post back. One of the vent line looking fittings doesn't have anything coming out of it, and I'm assuming there's not a check valve in those? So I should probably get that capped. The 934 is different from the 925, whereas the 934 has more ports on top of the fuel tank.

I figured they were the same as my brake line tubing, but wanted to double check. And I will definitely be replacing all the rubber line. As soon as Icepocalype is over anyways.... Still kicking myself everyday for not buying that **** 934A2 when I had the chance... this 250 Cummins won't take the van up a hill over 35mph....

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934 should not have anymore than the 925. Its the same fuel tank.

Honestly a 934A0/1 is a pretty slow turtle. When i pulled the box off mine she literally jumped out from under it quickly

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Interesting thread, bookmarked for future reference! (Looking like I might be a future M923A0 owner before 2017 is out). Can we make this a 'Sticky' by any chance? It might help other people with this predicament if it's easily found that way.

Found your ATF comment interesting, Have used it for years, Although I have changed to Marvel Mystery Oil as an additive. For years in England at a gas station (Petrol Station) one would find a small container to add a squirt of lube oil to your fuel.makes sense. Two stroke oil, yup , could see it , although I find it expensive here in comparison to MMO

PyroJoe said:

Thanks for the quick replies! Found a bunch of threads about a problem with a soldered joint in the pickup tube, so I'll check that out. The van just started leaking here a couple days ago after I moved it, so not sure what I disturbed there, but I'll post back. One of the vent line looking fittings doesn't have anything coming out of it, and I'm assuming there's not a check valve in those? So I should probably get that capped. The 934 is different from the 925, whereas the 934 has more ports on top of the fuel tank.

I figured they were the same as my brake line tubing, but wanted to double check. And I will definitely be replacing all the rubber line. As soon as Icepocalype is over anyways.... Still kicking myself everyday for not buying that **** 934A2 when I had the chance... this 250 Cummins won't take the van up a hill over 35mph....

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simp5782 said:

934 should not have anymore than the 925. Its the same fuel tank.

Honestly a 934A0/1 is a pretty slow turtle. When i pulled the box off mine she literally jumped out from under it quickly

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A M934 will have an additional fuel pickup for generator pickup.

We used it often.

Both of our 934s A1 & A2 had them.


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