Bruce M

Does the fuel tank on a 1990 Cummins have a check valve on the pickup fuel line to keep fuel from going back in the tank when the truck sits for a week or so?
Also can the 91.5-93 tanks fit in a 1990 with the different screw on level control cylinder with pickup/return lines?
The 1990 control cylinder uses a clamp on top to hold it to the tank whereas the 1990.5 -1993 has a screw on top to hold it.

jimbo486

No check valves anywhere other than the rollover valve on the top of the canister in the tank. If the fuel drained back to the tank, that tells me there's a leak somewhere allowing the fuel to drain back.

The later tanks will physically fit. Supply and return line connections are the same. The canisters are different from one another though and I can't say that the '89-'90 canisters will work with the threaded ring that the newer ones use. There's also a large flanged seal that the newer canisters use as well. The wiring connector is a flat, 4-way (I think) for the '91-'93 trucks whereas the earlier truck use a round, 5-way connector. The wiring is identical between the 2 so swapping them wouldn't be hard to do.

Unless somebody who's tried can confirm, were it me, I'd just get the tank with the canister and all. I swapped tanks with a '91.5 parts truck I had. Luckily, it had all the parts to go with it. I just bought a new seal and sending unit. As mentioned, I cut and swapped the sending unit wiring connectors.

mknittle

Mine has a small leak at the fuel heater and will lose its prime after a couple weeks or so.

Bruce M

With the fuel tank removed, placed a 3gt bottle of biodiesel on the ground and put the supply to the piston lift pump and return from the injectors in it and ran the engine.
Will let it sit for a day or so to see if the fuel will leak from the lines causing a cranking start.
IF it starts right off, will connect to the steel fuel lines going back to the tank using the 3 gt bottle again in place of the tank for the next starting test.

Bruce M

Connected the 3qt bottle to the steel fuel lines at the tank and started the engine.
After letting it sit for a few hours, had to crank for about 5 to 10 seconds to get it started.
This time air is coming out of the return line in the bottle that I don't remember seeing before.
Has anyone had a bad steel fuel line?
Will have to find a way to pressure test the lines.

BHD

My 90 had rusted out fuel lines on the supply side. So it would take a few mins of cranking after the truck sat for a couple days. It drove me nuts, as fuel lines don't leak under vacuum. I replaced all the lines and fixed that problem.

j_martin

If you can rig up to put about 2psi of air pressure on the fuel tank overnight, the problem will be right over the puddle.

Bruce M

Is the the fuel cap non-vented and what about the vent on top of the tank?
If I pressurize the fuel tank fill line or injector return line will the pressure seal the tank vent or do I need to cap the vent also?

j_martin

IDK about the cap. As you try to figure it out, keep in mind that the goal is to put a head of air pressure (very low, it's a plastic tank) over the fuel so fuel is forced under pressure into the system. If there's an air leak somewhere that'll show up as a fuel leak. Getting air pressure to the tank through the lines would defeat the purpose.

I would figure out a way to get a fitting on a filler cap, then supply it with low pressure air.

Bruce M

If the filler cap is pressurized, will air escape from the vent on top of the tank?
Does the vent need to be capped also?

maybe368

Hey Bruce, if the vent cap leaks just give it a bit more pressure, you only need a couple of pounds to find the leak...Mark

MrFusion

Silly question: is this an acceptable way to pre-prime the fuel system? For example when I drop the tank to fix the frame and install the low pressure piston pump would this work to get fuel into the lines and up to the LP?

jimbo486

Only if you have somewhere downstream in the supply side to purge the air. That could be the bleeder screw in the banjo fitting in the outlet above the fuel filter. Not sure if it would go through the lift pump though. If not, at least you can get it to the pump quickly and then use the priming button to get it up to the bleeder. To my best recollection, it doesn't take very long to prime with the button on the pump.

j_martin

Should go through the lift pump with no trouble.

deere country

I use compressed air in the tank all the time to bleed fuel systems. Never had it not work, just don't apply to much air.