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  1. #1
    Join Date
    May 2018
    Location
    Massachusetts
    Posts
    2

    Default Random RPM drop or engine stall after 2 hours

    My SP135 Ford Lehman diesel started acting up last fall. It would run fine for 2 hours or so, and then the engine speed will drop and return, usually 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, 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 put a Tee in the fuel line just before the injection pump to a valve and then a hose so I can check or bleed the system. When things seem to get close to failure, like low pressure or even a few motor speed blips, I open the valve and get a very good stream of fuel without any bubbles or burp. This would seem to rule out an air bubble.
    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?
    Last edited by KnotMee; 05-28-2018 at 08:05 PM.

  2. #2

    Default Re: Random RPM drop or engine stall after 2 hours

    Hi, Can you tell me the year of your engine ? In the meantime i would try running out of a five gallon container of clean fuel connected to the lift pump to see what happens.
    Charlie W

  3. #3
    Join Date
    May 2018
    Location
    Massachusetts
    Posts
    2

    Default Re: Random RPM drop or engine stall after 2 hours

    Quote Originally Posted by CHARLIE W View Post
    Hi, Can you tell me the year of your engine ? In the meantime i would try running out of a five gallon container of clean fuel connected to the lift pump to see what happens.
    Charlie W
    hi, thanks for the suggestion. Motor is 1988 with only 1500 hours on it.

  4. #4

    Default Re: Random RPM drop or engine stall after 2 hours

    Thanks for the info. doubtful that this is an injection pump problem. I have seen similar problems when fuel tank vent lines droop down over time and if the tanks are overfilled the vent lines fill up and create an airlock shutting down the engine. correction is to keep the vent lines heading uphill to the vent. Also spiders and wasps love to plug vents.
    Please let us know what you find.
    Charlie W

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