fuso random loss of power under load

[benttofly]

1993 misb, fg439 124711 miles. been in daily driver around town mode for past 10 months prior to that sat around for 18 years, and for 12 years it racked up 114,000 miles on expeditions trouble free. vehicle has been maintained to the letter with sighnings on every single interval in the service book. it also from day one has been started and brought to temp rise and at stop 180 seconds of idle except for the past year when i installed a 90 second relay to shut it off. problem started 7 days ago at highway speed 55 mph i was way past empty on fuel and at a peak a corner and a lane change the enginer simply seemed like it quit for a second. and came back to life. i immediately drove off the interstate and fueled to get home. operated the truck as normal around town four days in a row no problem. wednesday i had to evacuate for hurricane ian from st. pete to tampa. i got a late start and at highway speeds around 55 i noticed a severe drop in perfomance pushing into 55-65 mph gusts and rain we can all guess what i was thinking and so kept going. it never died completely but did suffer brief episodes. thursday returning to st pete again at highway speeds all of sudden about 30 min into the drive severe loss of power and if i hold pedal all way down it simply gets worse. if i mildly pump throttle it seems to help coax i along. the loss of power seems to happen at highway speeds. power loss was repeating so i dropped to surface streets and with normal driving no loss of power. pulled over and without any tools could only determine the following. all 4 wheel drums were not to hot to allow me to touch. the radiator was still touchable (the idiot gauge on dash gave no indication of overheating but that is for idiots. no ticking steaming or any other indication of problem. the loss of power is a constant and not jerky thing. ther are no sounds that indicate any obvious. drove again last night. operated fine at city speeds just fine, but tonight at local street speeds of 35 or so the vehicle did operate normally but seemed to accellerate thru the gears with less power. hard to tell on a vehicle that on its best day might go 0 to 60 in under 10 minutes. the vehicle since the original problem has been refilled with fuel and 3/4 of a tank burned. it has two fuel filters or what i believe to be water seperators and 2 additional filters on the fuel system away from the tank one is a spin on and a smaller drop in filter at the injection rail. all are clean and properly dated for service intervals. when the bogging occurs or loss of power it is smooth like taking your foot off accelerator. not a herky jerky abrupt or sputter and cough type thing. nor does there appear to be any noticable change in the exhaust throat or any sounds that would lead to a clue. air filter is under 1000 miles however, when i pulled it it seemed dirtier than i expected for a highway only driving period. it did seem to have a good bit of suction as i attempted to pull the filter out at idle, i then reinstalled adn checked for suction at the top of the intake tube to see if the filter was clogged or if the same pull could be felt. using a paper towel the intake certainly gave the appearance of a strong pull. i am not an mechanic at all but have some experience with gas vehicles and a working understanding of tools. i.e. there is nothing on the vehicle i would not attempt to do with a manual on hand but i have no frame of reference and scope for diagnosing problems on this motor. and with my youth spent turning wrenches and building vw engines for dune buggys i am 40 years away from that and those are not diesels.

i have all three of the shop manuals but those not really for diagnosing. any assistance would be of great help as we just got smacked with a hurricane and financially i am on the short end of this one.

 

[SkiFreak]

If I condense your little novel... after running the truck out fuel, you now have issues at speed.

From memory (which is sketchy at best), there is a small fuel strainer in your fuel system somewhere. Sorry, I cannot be more specific, because I don't have that model truck, but I remember this being mentioned by others in the past.
It could be that you sucked up a bit of crap off the bottom of the tank and that this little strainer is now partially blocked, limiting the fuel getting to your engine when at higher speeds, effectively starving your engine of fuel.
You could also have an air entrapment in your fuel line.
Just a thought...

 

[Aussie Iron]

My manual says there is a gauze filter in the inlet line union on the primer pump. If not there then it would be worth removing all unions to have a look and blowing lines clean. On my Landcruiser I had a union that got blocked up. Same symptoms loss of power on the highway. Took me a while to find that.

Dan.

 

[kerry]

I agree that its almost certainly a fuel delivery problem caused by the tank getting low and sucking up some debris into the system somewhere. Change all the filters, blow back the line into the tank and see what happens. This is a relatively common occurrence on boat diesels after going through bad weather and having wave action stir up the ******** from the bottom of the tank. Typically on a boat, people plumb the engine into a separate fuel supply to diagnose. It would be possible in your instance too.