I've got a '90 L8000 6-wheeler dump with the 7.8 ford. We bought the truck about a year ago and I believe it sat for a year or two on the dealer lot before we purchased. After about 6 months the truck started losing power sometimes while trying to upshift through the gears. For instance, it will pull up to 6th gear and as soon as it hits 2000 rpm and 35 mph it feels like it falls on its face like it's nosing over and leaning out. Any ideas what causes this? Pump? Injectors?
I'd start with the simple stuff first, fuel filters and a peek inside the fuel tanks to make sure they're reasonably clean and there are no rags or junk floating around in them. If you have a way to cut fuel filters open do so and if there lots of little black rubbery pieces in there you may have a fuel like deteriorating. Also you can take your fuel line going to the filter off and blow it out back to the tank and see if there's any garbage in it. More often than not a motor that ran ok will do so again without major parts. On occasion you'll have certain styles of pumps quit after sitting long periods of time. My old farmall did that to me. Junkyard
The problem w/ the 7.8L is, if it has the Bosch fuel system made in Brazil, the fitting aren't mechanic friendly.. & that makes for diagnostic nitemares. I normally find the rubber hose from the tank to the filter head is collapsed.. OR the racor filter behind the passenger side mud flap is full. Another thing is> if the hi speed stop on the injection pump is missing or run in to far, it will cut the speed/rpm's back in half..
I have a 91 F800 with the 7.8l engine. It seems to like having a new primary filter about every 3 months or so. tanks are clean and the vents are clear, but I get the same symptoms as the op. Transfer pump getting weak? I've had the truck since 2001 and that's been the case since I got it.
Weak rubber line from fuel tank to filter, weak supply pump, weak overflow valve on injection pump return or just plain ol' dirty filters.
I've changed fuel filters several times over the past year. There were a lot of little black chunks in the filter behind the mudflap the first time I changed that. I will try blowing out the lines next. It's definitely taken a major turn for the worse since getting colder. Now today after another filter change itll barely reach 40mph pulling a 10T trailer and mini ex
As per thepumpguys advice , I checked the fuel wheel on the one we worked on earlier . I adjusted it as he specified and it really woke it up. I still have some adjustment left and its not smoking either , when it warms up its going all the way in. Would one be about as well off to replace the fuel line from the tank to the first set of filters? A napa made hose couldn't cost that much , if it fixed the problem for sure. I had another one of these , it was a runner , pulled my 943 with it on a 20 ton tag even with a 6 speed trans.
All the lines from the primary filter all thru the system are steel except 1.. and you say the tank is clean but your getting black specs/chunks in the filter. Theres ONLY 1 black hose in the system.. change it.