98 GMC t6500 no start cat 3116.

[edognight]

I had to move my fuel tanks back on my t6500 and I ripped open one of the fuel lines. Stupid, I know...The truck had been running perfectly before then. I fixed it with new stuff and got it back together and now it wont start. I did some troubleshooting and the lift pump works when jumped off. as far as I know, there is only one lift pump on the right frame rail just by the right side tank. But I pulled the fuel line as it goes into the engine, and there is no fuel there. zero fuel pressure. It seems on a diesel, no fuel to the fuel rail, and no start, they don't suck fuel very well...Am I missing something? is there another lift pump or something?

Another thing I noticed was the pressure from the right tank goes to a t fitting where one line goes to the left tank, and the other line goes to the return to the right tank. bizarre. maybe this is a last stop measure in case the right fuel pump runs incorrectly?

I took out the sending unit on the left tank, and there are just steel lines that run to the bottom of the tank, so jumping the right side fuel pump did nothing...

there must be another lift pump, I am totally baffled...does the high pressure pump suck fuel thru the fuel filter and up to the engine?

[DmaxMaverick]

Most, if not all, late model Diesel engine on-highway vehicles run without a factory fuel lift pump. Fuel is drawn from the tank to the high pressure pump with suction only. GM added an electric prime pump to the van models, due to the location of the fuel filter, but it only runs for a few seconds during a start cycle. All other models have a manual fuel primer pump on the fuel filter assy. If you have 2 tanks, the pump you found may be the transfer pump, that balances fuel between the tanks. If your high pressure pump is like most others, it will not self-prime. Find the primer pump to get it primed and started.

[edognight]

Thanks for that. there is not a lot of info about this specific truck online. I am looking to install a hand primer pump of some sort, and try and replace both filters. previously one filter was clogged, and would not prime at all. I had been told by many people there would be some sort of lift pump, but there is not one that I can see...There is a transfer pump on the R tank, and nothing on the left...

[DmaxMaverick]

I'm not very familiar and, you're right, there isn't a lot of info, anywhere. By your description, the left tank is the supply to the high pressure pump, while the R tank is transferred to the L tank. That is typical late model GM in the Chassis/Cab and medium duty chassis with dual tanks. If it does not have a primer pump on board, then it figures that the high pressure pump should prime itself. If it doesn't, and there is no priming method on board, it makes no sense. The truck would be dead at the first filter service, let alone any fuel supply issues that inherently come up over time. The obvious conclusion is, the pump will self-prime, or it has a priming system on board. In any case, if it won't prime, by any means, I'd suspect a leak or blockage, somewhere (wow, that's helpful).

Can you open the fuel supply just before the HP pump, and apply low pressure air to the tank? That would push fuel to the opening, and may identify a leak, if there is one. You could use the same method to prime the system (reconnect the open line, first).

[edognight]

Yes, thats what I am going to do. When I first got the truck, I took off the fuel filter (the one just after the tanks aka a fuel water separator) and it was empty, so the pump wouldnt even fill the fuel filter after all that cranking, so I will check that it is full and then cap off the tank's fuel lines and return lines and hit it with about 5psi and see if I can get fuel to the engine, that should do it....hopefully. I think the first filter might be clogging a bit. might have to replace it. I had heard these were tough to crank when the fuel lines are opened, but it did ok last time. Ill cap all the lines and then loosen the flare nut where the fuel line meets the fuel pump...My mind was going all over the place instead of the simplest answer...the durn thing won't prime.

Maybe also find the hand pump for it. thanks for the help.

[edognight]

For anyone following, in case this helps, the lift pump on the right frame rail pumps to a t fitting on the left tank, one line of the t goes back to the r tank, and one goes to the left tank. As far as I can tell, there is no lift pump from the left tank to the engine, so it must be primed some way to remove air bubbles because the engine's fuel pump doesn't suck as well as it pushes, and it won't remove enough air and create enough of a low pressure for the atmosphere to push up the fuel, so it must be done some other way, like air pressure on the right tank or a hand pump on the right side of the fuel line to the engine...

[DmaxMaverick]

It should (theoretically) have an onboard method of priming the system. It must, somewhere. I've searched the interwebs, no dice. It isn't enough to just fill filter canisters, as even that little bit of air is enough to break prime. If the high pressure pump won't self-prime, it's either worn out, or was never intended to. You have a few options.

You can install an aftermarket fuel filter assy with a built-in manual priming pump (like the Duramax trucks have). It takes a LOT of pump strokes to get a dry system filled, so be prepared for that.

You can install an electric fuel pump, inline, for MANUAL activation, for priming. Full time operation will require a power supply system that ONLY supplies power while the engine is operating, like an oil pressure switch, or electronic solution. This is a necessary safety feature (we don't want to pump fuel onto a crash fire).

Lastly, you can install a valve (simple ball valve, not much is required), as well as a manual power supply, at the tank balance pump, that diverts fuel to the main pump supply line. This would allow for pusher pump operation for the purpose of priming, and can be ran until the engine starts, then returned to normal operation position.

[edognight]

I had some time to mess with it today, and I plugged all the valves into the tank (return line and crossover) and shot 5-10psi into the left tank and it did flow fuel up to the connector with the engine (back of the cyl head where the return line is) it flowed on the slow side and I ordered a new fuel water separator/filter, I think it is clogged. I then hooked up the fuel lines at the back of the engine and loosened the flare nut where the fuel line meets the fuel pump and hit the tank with pressure, and I was able to get fuel up there, but very little. I am going to order a new fuel filter and try again, thanks for all the help....I am going to take advice from Dmax above and look for a fuel hand priming system...

[edognight]

Hey all, I fixed it and wanted to report back in case it helps someone. The water separator seemed to be clogged, and it had a nasty air bubble. replaced the filter and added a priming bulb in line to the filter/separator. I primed it with the compression nut on the fuel filter loose pushed out a lot of air...and it was sooooo close to starting, so it hit it with a shot of either and it fired right up. The priming bulb really seemed to help, because you can push fuel right to the pump, it had lots of air in the line, and the boost of the pump spinning at engine rpm seems to do it. I know the manual says to not add fuel to the filter, but even doing what I did, was a LOT of cranking. thanks for the help...

[DmaxMaverick]

Thanks for the update!

Used to be, filling the filter can during a service was the standard. That was a time mechanical fuel systems were a lot less sensitive to particulate contamination. Later models, and especially HPCR and ultra-high pressure systems, have much tighter tolerances, so every drop of fuel needs to pass through the filter to reduce the opportunities of particulate contamination.