International Transit Bus 3400 T444E air from fuel tank

Maintenance/Repairs
1

1991 International Transit Bus 3400 T444E 7.3L turbodiesel.

Been trying to diagnose an air in the fuel problem, traced it back, fixed a few things, now everything runs fine …if you bypass the fuel tank and pull from an external can of fuel, but not from the stock tank. Installed a lift pump right after the tank, but when hooked up to the stock tank it just pulls in air. Put in one of those little clear mini fuel filters (hate the threaded opaque mini filters they give you with those little green aftermarket electric diesel pumps). The clear mini filter just shows the residual fuel left in it and a lot of bubbles when the pump is on. Probably not a vacuum from a venting problem, otherwise there would just be no movement, no bubbles, I figure. Pump is really close to the tank and low down an claims to be self-priming, so I doubt it’s having trouble priming (and pulling from the portable tank sitting on the ground and thus lower was still no problem with this pump). Even tried a hand bulb primer pump in line with it, still air from the stock tank (though this hand primer is kinda cruddy, always breaks and I have to patch it up, hard to find a place that sells them around here).

I’m going to double check and redo all these tests I guess… But in the meantime, what could possibly go wrong in the tank?? What’s in there? (Or not in there, that should be? :slight_smile:

The stock tank is huge and mounted under behind the big doors between the exhaust and side panels with not much space around it in an otherwise spacious undercarriage. It’s got the fuel out and fuel return connections at the center of the top of the tank, there’s barely enough room to get my hand up in there, much less a wrench, so there’s no way to undo the connections or remove the assembly without cutting a huge hole in the floor of the bus (if there even is a fuel assembly, it looks like it might just be two right angle connections for each just going straight into the sheet metal of the tank, but I can’t tell because there’s 20 years of this weird black coal-dust stuff piled on top and caked on.) Looks like there might also be a pair of wires, pretty sure thats the sender unit for the fuel gauge, I don’t get the impression this had a lift pump inside the tank.

Since the connections are on the top, wondering if there’s some sort of pipe going down inside to the bottom where I would assume fuel is picked up. Does anyone know if this is the case on this model? If so, do those things ever rot off or come loose? And, again, if so, what to do about it? (Besides the obvious of cut a hole in the floor, or sever everything and drop the tank). If not, well, I’m stumped as what else could possibly cause this.

If it is that, maybe just siphon the tank, drill a hole in the bottom of the tank, and thread in some cheap connector, basically make a new fuel out port?

2

How old is the diesel fuel in the tank?

Old diesel fuel can develope micro-organisms that grow into algea which can plug the pickup screen in the tank.

If you don’t know how old the fuel is, the tank will probably need to be dropped to clean out the crud.

Tester

3

The air (or gaseous substance, whatever it is) must be coming from somewhere. If it is not already in the tank as posted above by Tester, it either is coming from the air at the top of the tank or from the air outside the tank; i.e. from the pump you installed. Since you don’t have this problem with a different fuel tank, that pretty much eliminates the pump as the source of the air. So there must be air getting into the fuel line going to the pump somehow. A leaky connection anywhere above the fuel level in the tank could cause this. Or if the fuel path goes in a tube up, then down, there could be an air pocket in the top of that path.

4

Rubber fuel lines will deteriorate and become porous and allow air to be pulled through.

5

Remove the in-tank pickup tube and inspect for rust. Air is easier to pull than fuel. Inspect all steel lines for rust. Replace all rubber with new.

6

We thank you all for your responses and help!

Tester: Fuel in tank is fresh. Can’t really see inside, but doubt it’s algae, since it’s pulling air pretty rapidly instead of like a blockage. Unless you’re saying algae could eat through the fuel pickup pipe inside??

George: Could it really be an air pocket? If I’m pulling hard on the line with both an electric pump and a hand primer, wouldn’t the pocket of air just get pulled through and siphon action take over? Gotta be a leak of some sort, right?

Rod Knox: Yeah, I was worried about old lines, that’s why I put in the lift pump just below the tank, so it would run positive pressure the rest of the way. Problem was, there was no way to get tools in on the tank connection, so I spliced the old fuel line as close to it as possible. That line was terrible stuff tho, not steel hardline, some kind of plasticy tubing, yet really hard, wouldn’t cut with bolt cutters, had to Dremel through it to put in the splice. The splice is solid, so I don’t think air can get in there, but I plat to double check. Unless there’s a leak though in the connection to the tank, I don’t think it can be the fuel line, because there’s only a few inches of the old stuff before the splice, and it’s super hard and non-porous.

Rattlegas: My problem is I can’t get to the pickup tube to even check it or replace it. Are we sure that’s what it is based on the symptoms? (Seems to me everything else is ruled out, so it must be, but I wanted to get the pro opinion here on cartalk :slight_smile: Can I just install a new pickup somewhere else on the tank?

7

Drop the tank if you can’t remove the pickup tube. It takes less time to drop a fuel tank than it does to boot my PC and make coffee.

8

“That line was terrible stuff tho, not steel hardline, some kind of plasticy tubing, yet really hard, wouldn’t cut with bolt cutters, had to Dremel through it to put in the splice. The splice is solid, so I don’t think air can get in there, but I plat to double check. Unless there’s a leak though in the connection to the tank, I don’t think it can be the fuel line, because there’s only a few inches of the old stuff before the splice, and it’s super hard and non-porous.”

How did you achieve a splice with the existing line? Perhaps descibe the connection and sealing method. What makes you so sure you achieved a leak proof connection? Air can leak through holes where fuel does not so don’t assume you have an air tight seal because there is no fuel leaking.

9

To diagnose such problems I use air pressure regulated down to 2 psi via a steel tube wrapped tightly with a shop rag and stuffed into the tank fill pipe… Wherever air is being drawn in fuel will seep out. A few inches of fuel must be in the tank and if no fuel escapes anywhere the problem is inside the tank.

But be advised that the pressure adjustment on compressors are not capable of regulating that low. An in line regulator with a zero to 15 psi range is required.

10

Thanks again, everyone.

TwinTurbo: I did the splice by forcefully threading a slightly-too-big connector into the cut end of the old tube (basically tapping it), then clamping down hard with a hose clamp (I usually torque-limit my hose clamps to prevent damaging hoses, but this stuff whatever it is could really take a beating, and so since you’re right this is the only place besides the tank to look for air to get in, I wanted to make sure it was a tight seal. When I go back and check it, I’ll make certain I remembered to put thread sealer on it for sure. If there really is air getting in at the splice, then I really have no idea what to do other than heavy duty epoxy to make sure it’s sealed, because I have no clue what this hose is or what people use to slice it …compression fittings maybe? The main reason I’m ruling out the splice as the source of the air leak, besides the extra effort to make it tight, is that air was being pulled in somewhere before I even considered putting it in. I put the splice in to put in the lift pump as close to the tank as possible, so I could force fuel out of all the line going to the engine bay to see if I could use the dye in the fuel to spot any leaks where air originally was getting in (and to run positive pressure, because leaking a few drops of fuel is way better than pulling in lots of air, both now, and in case of future small leaks). After putting in the lift pump I realized I was way off-- the line is fine, there’s something at or in the tank that allows the air in.

Rod Knox: That’s good advice. I had actually considered pressurizing the tank slightly to try to prime it, but I had no idea of a good method. Seems I’d have to seal off any possible paths out of the tank besides the fuel pickup-- so I’d have to block off the return tank line, the pressure release in the cap, etc. Does the rag really work? Seems like it would let air through. Is it safe? Any reason why 2 psi is better than 15? If I did that and still only air came through the clear mini filter, that would rule out the splice and such, and tell me whether the problem is really in the tank. Thank you, I may try a way to pressurize the tank.

11

The set up isn’t impressive but it works. The pressure gauge will remain at zero until the wadding is tight enough to actually accumulate pressure in the tank.Your problem is not a new one, @bugnut. As they say, BTDT and got the T-shirt.

12

Just wanted to post the results for others looking at tank problems.

Thank you, Rod Knox, for the advice. I also heard from someone that these old tanks with the connections at the top center often do not have what we think of as a “normal” filter in them. Instead it looks like a screen rolled into a cylinder, and surrounding the uptake pipe. He said the cylinder is open at the top, so sometimes if they get clogged up bad, no fuel can pass through, but plenty of air can get in through the top.

Since the owner of this beastie said she may have had waxing problems last winter, I set to work on coming up with ways to clear such a clog. Rod Knox’s idea above about blowing out with air pressure was pretty brilliant, but if this was a screen cylinder inside which was open at the top, the air likely wouldn’t do much. Since waxing was a possible issue, I thought about how to deliver some solvent to the potential clog. Had already put some winter cetane booster in, but it’s a massive tank with a lot of fuel in it, and felt it might be too dilute and unable to reach the clog well. I decided since I had the new lift pump installed, and had run tests with pulling fuel out of another tank and knew it worked, I decided to reverse the connections and use the lift pump to pull from a hose in a bottle of Diesel-911, and pump it INTO the clogged tank’s OUT LINE, thus delivering a lot of it directly to the clog and possibly also putting some reverse pressure to push the clog back out off the screen.

And that’s when I saw it, the dripping. The source of the problem was indeed as some suggested above-- my original splice to the tank. This was the only way to see where a hole might be in the upstream line, and I can’t believe I didn’t think to reverse the connections earlier to test it.

I redid the splice, and all is well.

I still would like to know what that really hard, thick, plasticy line is made out of, though, and what people normally use to splice it properly.

13

“If it is that, maybe just siphon the tank, drill a hole in the bottom of the tank, and thread in some cheap connector, basically make a new fuel out port?”

Fuel tanks use top connections for a reason. A bottom connection would be prone to corrode and leak. Drilling a hole in a fuel tank is dangerous unless strict safety precautions are taken.

Glad you found the problem.

“…what people normally use to splice it properly.”

How was this tube originally connected at its ends?