Ford Lehman SP135 Stalling

[Raven Odyssey]

My SP135 Ford Lehman diesel has plagued me for several years, but now things are at their worst. The engine will run perfectly for anywhere between 2 and 50 hours, then the engine speed will drop and return, it usually drops around 2-300 RPM and then back up immediately. It sounds just like a filter is clogged. Needless to say I changed both primary and secondary filters and it didn't help. I've tightened up all fuel connections, installed a new lift pump, had the fuel polished, rebuilt the injector pump, serviced the injectors, and blew out all the fuel hoses with compressed air, and checked the tank vents. While the motor is running, my vacuum slowly increases to about 3 inches after starting close to zero. Correspondingly, the fuel pressure just before the injection pump starts at 6 and slowly works its way down to 4. It doesn't act up at this point but I do know if the pressure gets less than 3 then I'm living on borrowed time. I've looked carefully for air entering the vacuum side, and definitely don't have any fuel leaks on the pressure side. I've had a hunch that the vacuum side is very slowly sucking air and ultimately resulting in an air lock on the pressurized side but haven't been able to find it.

I have considered the fuel pickups getting clogged, but I blew them out with my air compressor, and both tanks exhibit the same behavior, so I doubt both tank's pickup clogged at the same time. I do wonder if there's something in the injection pump that's amiss, but it has to either be time sensitive (more than 2 hours of running OK) or temperature sensitive (as a result of running for a few hours. Any ideas?

[a64pilot]

I had a similar problem with my boat on delivery when new, but it was much more frequent.
I was getting a clog in the elbow of the pickup tube, of course I could disassemble and find the culprit.
The fact that your losing pressure / increasing vacuum indicates a clog in the fuel supply, nothing in the engine downstream of the pressure and or vacuum gauge can cause either to increase / decrease.


It could well be an air leak that doesn’t occur until the vacuum gets high enough to cause air to be sucked in, it could be just a loose hose clamp.
You won’t find that by pressurizing or blowing out the hose, but you May see it if you temporarily plumb in a clear section of hose just before the engine.
Temporarily, cause I know of no suitable clear fuel hose, and it’s bound to fail if left in long enough.

You may also want to watch the return fuel to see if it’s getting aerated or if supply of return fuel drops off when the engine sags.
Aerated means air leak of course and supply dropping off means fuel starvation