I recently got my first single axle dump truck, its an 88' Ford L8000 with a 7.8L and a 5+2 speed. When I test drove the truck it had an air leak so I didnt go far and stayed on country roads. It seemed kinda sluggish at high rpm but I figured it was probably a combination of fuel filters, the 5-10 degree temps that day, and the summer blend fuel that was still in the truck. I thought a new set of filters and I'd be ready to go. I worked out a deal on the truck then loaded it up and hauled it home. The past few days I've been working on it I got the air leak fixed and changed the oil and all of the filters. I even drained the fuel tank and filled it with fresh fuel. After doing all this I took it out on the road for a test drive thinking I was ready to go but its still there? The truck seems to run fine in the lower gears and when your very easy on the throttle but when your in high gear and you try to accelerate like at the bottom of a hill it seems like it cuts out? The truck is geared low so you have to wrap it up to like 2500 rpm or so to go 55 or 60. It just doesnt want to get there. If I drop a gear and run like 30-35 it seems like it revs fine and when your in high gear going slower at say 1500-2000 rpm it seems fine but once you go over that it cuts out, then if you let up on the throttle it takes off again? Also if you are sitting still in neutral it will rev up to 2500+ with no hesitation. All this is with the truck empty, I have been afraid to load it until I get all the bugs worked out. Does anyone know where to begin? I'm a young guy actually just turned 21 and about everything I've got is in this truck and my backhoe. I've got a couple small jobs already lined up but need to get this truck going to do them. I went to 1 yr of diesel mechanic school and know my way around a little bit but still know when to ask advice from those who are older and wiser than I am. Any information would be greatly appreciated. Thanks
Could you clarify "cutting out" a little better. Does it make an erratic misfire of one or more cylinders? Or, is it more a smooth loss of power?
Its a smooth loss of power, I havent noticed any smoke or odd noises its just like the fuel is being shut off. I had wondered if maybe one or more of the injectors is not opening up all the way? Like maybe the fuel is just squirting out instead of spraying? I've done a little online research and found a few people say they ve had injector trouble out of these motors. I have some connections with the local John Deere dealer and they have a tester so I thought I might take them out and test them and see where I stand after that.
I doubt it's an injector. One injector failing would give you a vibration, not a smooth loss of power. It's unlikely all injectors are failing making smooth loss of power. Most likely suspect; fuel delivery problem under load. Clogged fuel line or maybe lift pump failing. And I would not suspect injection pump just yet, check fuel delivery first. Another thing that can cause this, if your engine has a turbo and aneroid valve, is a bad diaphram in a turbo waste gate pot.
I was thinking about the lift pump, I may try that route first. I did notice when I changed the fuel filters that it seemed to take forever to get the air bled out and when I finally got it running and moved it around the driveway it died and I had to bleed it again. Could it be getting weak and not be able to keep up when the engine is under load? It sounds fine when its idling and revs up with no load so that would make more since than the injectors since it runs smooth. I dont know if it makes a difference but the truck had sat alot the past few years, I got it from a farmer who used it for a few projects and once they were done it didnt get used much.
The lift pump is a good place to start. They're generally not terribly expensive. If it doesn't deliver the proper quantity of fuel, you would notice engine sagging under heavy fuel loads, high gear or grading hill, but not notice any problems just revving engine with no load.
Well I replaced the lift pump and blew out the fuel lines from the tank to the pump and theres no improvement. Where do I go next?
Yeah, got other ideas, but I get stuck with what you said at post #5, took forever to get the air bled when changing filters. If that's the case, something has to be letting air ingress the fuel system, i.e., hole in fuel line, connection not sealed, bad pickup tube in tank. In other cases when you suspect problems with fuel feed lines from tank, you simply run a hose from lift pump to a fuel can and test engine. But, since your problem only arises under engine load while driving, kinda hard to do that. Check your fuel delivery system well. Until we get past a problem there, we can't go on to other possible causes.
Had this happen before, when pulling old spin on filter, the seal stayed on mount and new filter had it's seal also. double seal would not close off properly. This was not on 7.8 , but happens with spin ons
Filters should be fine, I double checked for that. I've had that happen before on other stuff.
how are the rubber fuel lines ....... we had a dump truck one time that ran fine when empty , but when u loaded it and the need for more fuel , the rubber lines were collapsing restricing the flow of fuel and the truck would fall on its face , check the rubber lines , alot of times they will be dry rotted and not be leaking but will suck air in , it only takes a little for the truck to run out of steam........btw 2500rpms??? its not a gasser why are u insisting reving to 2800.......ive driven a couple older trucks that were 5+2 spds and about 55mph is all you'll get out of it:beatsme
If you have thoroughly checked your fuel lines and are confident the problem isn't the lines, it sounds like it's an injection pump problem. Two things to check though; make certain the fuel shutoff is causing the problem with loose linkage and such, and, does the injection pump have an aneroid valve? Don't think that's the problem, but it's worth taking a look at before removing the injection pump.
I'm not looking to run over 50 or 55 mph, I just wanna figure out why it doesnt have enough power to stay there. I dont wanna run it at 2800 rpm I was just using that as an example to say that it revs up fine in neutral with no load. I will check the hoses out in the morning. I'm trying to rule out all the cheap and easy things first.
It does have an aneroid on it, I also wanted to check that before I took the pump off. I have never done it before but was told that I can take the line off the intake and put 15-20 psi of air to the aneroid and if I can hear it leaking its bad and if not it should be ok? Does that sound right, am I missing anything? I will also double check the linkages on the fuel shut off. Thanks for your help guys
Does it have the mechanical or electronic accelorator pedal? if electronic, has anyone been washing out the cab with a water hose? You can destroy the acccelorator sensor that way. it will cause the symptoms you have described. BTDT the hard way.
I don't think this one has an electronic throttle carogator, I'm thinking this engine has a mechanical injection pump. Hoses are a for sure thing you want to check. I have seen hoses deteriorate and swell on the inside, but look totally fine on the outside. I've also seen hoses where in-line fuel filters are used (probably not on your truck) and inserting the nipple of the fuel filter would tear the inner jacket of the rubber making a nice little rubber flap to restrict fuel. It sure sounds like you have either a fuel restriction problem or a fuel delivery problem. If you've put a new lift pump on it, and verify (or replace) the hoses, that takes care of restriction or delivery problems getting fuel to the injection pump. Now the fault rests with the injection pump. You could have a bad aneroid valve diaphram, but usually when the go...there bad, don't "come and go", that's not for certain though, worth checking out. Not familiar with your injection pump, but most pumps with an aneroid valve have either an inspection plate or large inspection plug that you can remove and make adjustments to the fuel control linkage of the aneroid valve. You could remove this plate/plug, disconnect the hose/line coming from the intake manifold to the aneroid, apply some low air pressure (preferably from an air regulator) to the aneroid line, and as air pressure increases on line you should be able to see the links move through the inspection port, and the links hold while pressure is applied. Also, does it have a wastegate valve that is connected to the aneroid line? I've seen diaphrams go bad in wastegate pots and affect the operation of the aneroid valve turning up the fuel of the injection pump. That's about all I got to help you out. Hope you get it sorted out. :drinkup
Everythings pretty much mechanical on this old truck
There are only 2 lines that arent steel and they are like a low pressure braided hyd line with swivel fittings on both ends, they look fine on the outside but I can take them off and cap one end and apply pressure to see if they leak anywhere. Also it does not have the waste gate valve, the line goes from the aneroid right to the intake manifold. I can see what looks like an inspection plug on it, I'll try to check it out tommorow. I have a regulator on my compressor and thought I'd set it for 15-20 psi and put the blow gun on the aneroid and see what happens. That should be enough pressure to see a result shouldnt it? Thanks Again
One other thought== what is the possibility of a plugging muffler or does it have any king of cat converter. don't think it should have unless someone out a diesel engine in place of a gasoline and didn't change exhaust.
Well I have gone thru everything I can think of and checked everything but nothing seems to help. Its starting to look like a rebuilt injection pump is in my near future. I'll let you guys know when I get it going again, that is if I get it going again:beatsme Thanks for all your advice
Yes, do let us know how it works out, always good to know with problems like this.
I have an update on the truck, I took it over to an old school truck mechanic to see if he could figure it out. It took him a few days but he thinks hes got it going. He found out the fuel filter base was plugged up. I changed the filters but never thought about the filter base itself being the problem.
Good news indeed! Could have had a piece of debris lodged in the fittings of the filter base. Things like that are not easy to find. Glad you got it sorted out.
Yeah I never would have thought that was the problem. Its amazing how you can spend hours upon hours diagnosing and trying to figure it out and it ends up being something simple like this. I'm glad it was'nt the injection pump! It was definitely alot cheaper this way!
another 7.8 Glad to see it is going for you! I also have a 7.8 in an old L8000 similar to what you describe but the box was removed and a crane attached behind the cab. It drove in here two years ago and I hope to acquire it, but cannot get it started. The prime pump is gone, blocked off, and I have not been able to bleed the pump. They shut it off by closing the fuel valve as the shutoff solenoid is gone. I put fresh #2 directly into the pump, all injector lines off and still no fuel. I then pinched off the return line as I see it going back to the tank, tried again but to no avail. Short of pulling the pump which I dread any ideas you have learned?:beatsme I am going to back at it this am Thxks Larry
Welcome to the Forums RVservice1!:drinkup
Welcome to the forum RVservice1. :usa Have you verified the lift pump is delivering fuel under pressure to the injection pump?
Yes, I did, but I did get it to run. The stop lever on the pump was not open all the way. The hand lift pump is gone, replaced by a short plugged hose.I was low on diesel so I substituted WMO filtered and some Canola oil. Runs ok. Rear air brake was seized on, loosened that, moved it to in front of the shop. Alt is not putting out, air is about 100 psi with a minor leak at the rad shutter solenoid. Unit has a telescoping boom crane, 86 vintage, but I do not know the make or model could only decode a CM5001 number and a C11624 which might be serial num I think it about 50' and a cable winch on it I guess with 60' of cable, 2 outriggers, controls on both sides. One control is for a pole puller/tamper but that is not on this crane. Will let you all know If I get it and what I plan to do with it, all off road! Thanks a lot Larry
cam sensor?