Diagnosing the fuel system after you confirmed no leaks in the main lines

[TurboJoe]

I'm at a bit of a loss here on where to go next with my M817.

I don't have the truck on the road yet been working on it when I had time. It would never run right, always take a long time for the RPM's to go up, and then back down. The air compressor kicking on would make the idle fluctuate all over.

This is very indicative of an air leak in the fuel system. I pulled apart the selector valve and rebuilt it with new o-rings. The lines on this valve were just very recently replaced by their looks. The main fuel line that went from the filter housing to the IP was shot, so I replaced that. I then drained all of the fuel out of the tank, and changed the fuel filter. I greased the fuel filter o-rings, made sure the square plate on top was flat and the oring under it was replaced. This was snugged up, and I primed the system using a vacuum break bleeder.

Got the filter full of fresh diesel I had filled the tanks up with. Put the main line back on the IP and she fired up after a bit of cranking. It ran great! Sounded good, idled good, the RPMS were as responsive as I would think the truck should be, both up and down. It did hunt for idle after coming down, but I figured it needs to drive a bunch. Then it died.

At this point I assumed I didn't put enough fuel into the tanks, so I added 15 gallons and vacuum primed. No start. Next I pressurized the fuel tank and instantly found a leak on the hard line going into the fuel filter. I replaced that line, capped off the supply line from the IP and pressurized the system. No more leaks, and I got the truck to start. However, It runs like crap again, like it did before I started this mess.

As a side, I posted in another thread. My priming pump pulls out and then gets rock hard when pushing it in to the point I can't push anymore. The gauge does not move ever, and once I crank or start the truck I can push it in again, but the next pull is rock hard.

Any ideas on what to do next? The fact that it ran great in the middle of everything is throwing me way off.

Joe

 

[74M35A2]

I'd check the fuel tank dip tube and its junction to the tank fitting. An issue there would not show up on your air pressure test. If ok, try to run the engine on a hose from a remote fuel supply to determine if the issue is vehicle side or engine side.

 

[TurboJoe]

I'd check the fuel tank dip tube and its junction to the tank fitting. An issue there would not show up on your air pressure test. If ok, try to run the engine on a hose from a remote fuel supply to determine if the issue is vehicle side or engine side.

I had enough daylight left to run the engine from a tank. Luckily I have a few 10-an hoses lying around from old projects so I took another hose right into a 5 gallon container. It ran much better directly from the 5 gallon container. I suppose pulling the fuel dip tube out is the next step. Are these repairable?

I still have to figure out why the primer pump gets rock hard and the gauge never moves.

 

[TurboJoe]

I removed the tank pickup tube, and it was in very nice shape. No cracks, no possible leaks. The double flare looks good, so it put it all back together.

When I removed the line from the IP it sucked air in as the fuel ran out, leading me to believe I do not have an air leak. I had an idea and removed the vent line from the fuel tank. It seems to run much better now, I believe a clogged vent line was my last fuel related issue.

Well with the exception of the primer pump being stiff after one pump and no working primer gauge.

 

[162tcat]

One other thing to check, some older hoses will collapse internally and plug up. They look good on the outside and don't leak under pressure but once the engine is attempting to pull fuel, they will collapse. Just replace all rubber lines before you spend money elsewhere


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