M1078A1 driving me nuts (Very Slowly)

[Mikes Bikes]

I have a fleet of FMTV's A0's A1's & A1R's - All run great except 1. It's been flashed to 330hp and has 307 gears, like the other A1's in the fleet. This truck, runs great for about 4 miles, then slowly loses power. Pull over, shut it down, let it sit for a minute or two. It fires right up and takes off like it should. A mile or two down the road, same thing. Over and over again.

No check engine light or strange sounds, just no power.

I've changed the filters and flushed the tank. Primer bulb pumped right up. (This gave a slight improvement, but still behaving the same way. However, It will recover when idling at a light now and take off as it should, only to lose power once again).

Fuel lines all look good. Tried running it without the gas cap, Thinking maybe it's sucking air in the fuel feed? Checked all the connections. All look good and are tight.

 

[Lostchain]

I have a fleet of FMTV's A0's A1's & A1R's - All run great except 1. It's been flashed to 330hp and has 307 gears, like the other A1's in the fleet. This truck, runs great for about 4 miles, then slowly loses power. Pull over, shut it down, let it sit for a minute or two. It fires right up and takes off like it should. A mile or two down the road, same thing. Over and over again.

No check engine light or strange sounds, just no power.

I've changed the filters and flushed the tank. Primer bulb pumped right up. (This gave a slight improvement, but still behaving the same way. However, It will recover when idling at a light now and take off as it should, only to lose power once again).

Fuel lines all look good. Tried running it without the gas cap, Thinking maybe it's sucking air in the fuel feed? Checked all the connections. All look good and are tight.

Have you checked to make sure the PAC Brake isn’t getting stuck on? When I first got my truck it was sticking and resulted in a dramatic reduction in power.

 

[Mikes Bikes]

Have you checked to make sure the PAC Brake isn’t getting stuck on? When I first got my truck it was sticking and resulted in a dramatic reduction in power.

Exhaust brake functions as it should. Turns on and off without issue.

 

[GeneralDisorder]

Have you checked to make sure the PAC Brake isn’t getting stuck on? When I first got my truck it was sticking and resulted in a dramatic reduction in power.

Can confirm - mine stuck partially closed - that took a minute to figure out. It was just mechanically stuck with rust and carbon, etc.

 

[Mikes Bikes]

Mine runs great for a short time. It'll run right up to 70 MPH, just like my other A1's. Then, slowly loses power till I'm a hazard on the HWY and pull over. Sit for a minute at idle and she takes right off, only to repeat the same scenario. I'm thinking that at high power, the demand for fuel is, out pacing the lift pump, maybe drawing air in through a leak in the fuel feed?

 

[NDT]

Time to hook ecm to CAT ET.

 

[Ronmar]

I would start with baseline fuel pressure, then Cat ET looking at the HEUI oil pressure regulated pressure…

 

[dmetalmiki]

HA!....Puzzle all you might..This is a 'losing fuel', supply, or pressure problem?.
Hope this > tip < gets you to the root of the fault. Let us know the eventual solution.

 

[Guruman]

Fuel tank vent clogged, creating a vacuum in th tank, restricting fuel flow?

 

[Ronmar]

Here is what a small air leak looks like in the primary fuel system.

Your symptoms sound like a fuel restriction, but it could be air also. I have seen restrictions happen 2 ways: I had an old pickup with something dancing around in the fuel tank, it would occasionally find its way to the pickup tube and partially plug the line. It would start to first loose power then get rough and misfire above idle. The next instance was a failed hose, the inner liner was cracked and would occasionally collapse, again partially blocking flow and delivering insufficient fuel to meet demand above idle… The hose looked great on the outside…

You can test baseline pressure with a simple mechanical oil pressure gauge kit from any auto store. It attaches between lift pump and second filter. You will have to crimp/restrict the line to the gauge to dampen out the pulsations. I show this in one of my other fuel system videos. You will have to of course mount the gauge where you can see it while driving.

What I would expect to see if you have a primary system issue would be the pressure to jump up to 60 as you first accelerate from a stop. Then as you sustain engine load, the pressure will steadily drop off until engine performance also falls off…

 

[Mikes Bikes]

HA!....Puzzle all you might..This is a 'losing fuel', supply, or pressure problem?.
Hope this > tip < gets you to the root of the fault. Let us know the eventual solution.

No link attached to ">tip<"

 

[Ronmar]

Fuel tank vent clogged, creating a vacuum in th tank, restricting fuel flow?

He tried running without the cap, so no Vacuum lock…

 

[Ronmar]

No link attached to ">tip<"

I think that WAS the tip… And I agree with it…

 

[Mikes Bikes]

Update: Problem resolved -

• Pulled the feed line off at the tank (also removed the dipper in the tank) and removed the line at the feed side of the water separator.
• Blew it our from engine side to tank side with compressed air.
• Removed the fitting from the feed side of the water separator and blew that out as well.
• Removed and drained the primary filter/water separator & cleaned with fresh diesel.
• Reassembled everything using light oil on all the fittings & O-rings.

Fired it up - Problem solved. Have no idea what came out of the line, as I was working alone and couldn't see the discharge. Or maybe, it was a poorly seated fitting and I resolved it upon reassembly.

Anyhow, thank you for all the responses!

 

[Ronmar]

Sounds like it was probably an air leak… at any rate, good houses have good foundations, and baseline fuel pressure is your diesel foundation…

 

[BERZERKER888]

Update: Problem resolved -

• Pulled the feed line off at the tank (also removed the dipper in the tank) and removed the line at the feed side of the water separator.
• Blew it our from engine side to tank side with compressed air.
• Removed the fitting from the feed side of the water separator and blew that out as well.
• Removed and drained the primary filter/water separator & cleaned with fresh diesel.
• Reassembled everything using light oil on all the fittings & O-rings.

Fired it up - Problem solved. Have no idea what came out of the line, as I was working alone and couldn't see the discharge. Or maybe, it was a poorly seated fitting and I resolved it upon reassembly.

Anyhow, thank you for all the responses!

"...Have no idea what came out of the line,.."

Hillary Clinton ?

 

[Mikes Bikes]

"...Have no idea what came out of the line,.."

Hillary Clinton ?

That would require the truck to be destroyed.