8.3 running rough/weak, only sometimes

[CBR Metalworks]

This is the 2nd truck I have had to do this (first one gave these symptoms when I was on the way to sell it), so I figure it might be a common enough issue to make a thread. If anyone has any ideas let me know, otherwise I am just going through the fuel system to try and find the problem.

The symptoms:

Truck starts/runs fine, but sometimes in 4th/5th gear it starts to sputter/die/no power, it has not actually cut off on me, it clears up and starts to run fine if I work the throttle, but may start to sputter again within seconds after it clears up.

It has fresh diesel in it.
I just replaced the engine mounted spin-on filter
I removed the canister filter because I could not find one locally, (plus they are about $50...), I am going to do an in-line filter before the fuel pump like is common on a lot of the A2 model trucks (ones that do not have the canister filter). The canister filter housing had some crud in it, but didn't appear to be too excessive

It still runs the same.

Next I am going to remove the fuel feed/return lines and blow air through them to blow out any crud in them.
After that I will try replacing the fuel pump itself (I have a new one on hand, only about $50 from Cummins).

Any other ideas?

 

[Rusty Nut]

Did you check the fuel tank vents ?

 

[Carlo]

I have not seen these symptoms on a A2 but gosh I sure have seen them on A1's! fuel starvation from fuel lines sucking air. I have seen the lines look like new but if you Ben them you see they have dried out. Try pressurizing the fuel tank and look for wet marks on fuel lines. Confirm or eliminate this for it could be 60% possibility of your symptoms.

 

[5tonman1971]

Sounds like your sucking air. Have you checked the bottom of the tank to make sure the tank isent rusted out? I know it's hard to do on m939 series cuase the filler screen is not removable

 

[162tcat]

The filler screen IS removable. Sounds like air to me. I had a Mack E7 400 hp that would do the same thing occasionally, replaced the fuel lines and the problem was solved.

 

[5tonman1971]

The filler screen IS removable. Sounds like air to me. I had a Mack E7 400 hp that would do the same thing occasionally, replaced the fuel lines and the problem was solved.

How do you get the screen out? I couldn't figure it out.

 

[162tcat]

Large pipe wrench, oil filter strap type wrench, hammer and chisel. Depends on how tight it was installed in the first place. It will unthread just like the fuel cap.

 

[5tonman1971]

Thanks for the info tcat I'll have to try that

 

[wreckerman893]

How do you get the screen out? I couldn't figure it out.

I used the business end of a Tanker Bar. Four or five whacks and it dropped right out.

 

[wheelspinner]

I used the business end of a Tanker Bar. Four or five whacks and it dropped right out.

No need to whack it out. Same threads as the cap, just unscrew it. Then you can just put the cap on if that's how you want to set it up.

 

[74M35A2]

Could still be water in the fuel. My truck did this and I had a big slug of water in both filters. Would run partial throttle, but starve at 3/4 or more. Changing or cleaning the filters once may not get rid of all of it, should that be it. Everything should have drains on them, mine does. Re-drain again all 3 and look at the samples taken: Engine spin-on filter, large canister filter (behind driver wheel), and fuel tank itself. There is a drain plug at the bottom of the fuel tank, remove a small amount of liquid into a clear container and look for water. Same when you drain the other 2 filters.

In addition, the larger canister bolt (on top) one has a square washer atop an O-ring. It is very easy for that square washer to not sit down in the recess of the housing. Not sure if it will leak air or not when not seated correctly, but something to look for.

When you press the hand primer on the driver side of the engine, can you feel a nice firm squirt of liquid circulating past the pump relief?

-Written from Japan, on work assignment

 

[CBR Metalworks]

The problem seems to have been fixed. The return line inside the fuel tank was completely rotted and falling apart. I think pieces of that rubber were getting sucked up in the fuel line and starving the engine of fuel. I drained the fuel tank again and wiped it out as best I could, I think anything that is left is probably small enough to make it up to the filter but not clog the line.