switching tanks = air bubbles?

[M.L.S.C.]

This is something the previous owner of my truck warned me about, dunno how true it is, so I gotta ask - say I'm running off the rear tank, engine is idling, then I hit the switch to go to the front tank to check how much gas I got there, then quickly switch back to the rear tank - is that gonna put an air bubble in the fuel system? Actually when am I allowed to switch tanks (say go from rear to front and run off that one), can I do it while driving down the road, or do I have to be stationary, or even with the engine off? What if I turn engine off and let truck stay for a while, then hours later before I fire it up again I hit the tank selector switch and then turn engine on - is that gonna put air bubble in the system?

I realize these questions may be quite retarded, but all my other vehicles being gassers I'm a bit paranoid about messing up the diesel fuel delivery system... Thanks!

[dakotajeep]

I prefer to change tanks while moving. Someone will chime in with more precise knowledge.

[1991F250]

Unless you bleed a tank completely dry before switching, you shouldn't get air in the system just from switching. It also doesn't matter if you switch while in motion, stationary, or even running! Happy Truckin'

[M.L.S.C.]

cool, thanks guys - I'm draining the rear down to 1/4, then switching to the front one as it's been full to the brim since last week

[M.L.S.C.]

Aight, tried the tank switch today, worked like tis supposed to, with one exception - when I go to t he front tank the gauge reads empty. What do you think can be causing that issue? Can a faulty ground wire for the sender be blamed, as in, does an open circuit default to empty?

[1991F250]

I think you usually get a false-full in that case. Could be a bad sending unit, or your float could have sunk. When you switched the tanks, did the needle move around, or just fall straight to empty?

[Close-Call]

Don't know how Ford does it but here's how Chevy did it in the mid 80's.

Broken Ground For Sending unit = False full (way over full)
Broken Positive wire for sending unit = False empty (below Empty).

As I said, I don't know if it's right for Ford, but it may be somewhere to start looking.

I had the same problem as you did. Midship (front) tank had a very slow reaction on the fuel gauge eventually it read empty always. Turned out the little brass float had developed an invisible hole. Filled up with fuel and sank.

I can tell you for sure that the float is the same for the trucks with a 460, though the rest of the sender is different. I also replaced the midship fuel tank while I was down there. I thought $150 for a brand new tank was cheap insurance.

[M.L.S.C.]

Originally Posted by 1991F250

I think you usually get a false-full in that case. Could be a bad sending unit, or your float could have sunk. When you switched the tanks, did the needle move around, or just fall straight to empty?

Needle went straight to emtpy, fairly quickly too, but not like it just dropped, it was a gradual movement, like I see it go up when I switch to the rear tank.

Close-Call, since you confirm his suggestion about the float, then maybe I should look at that - I know RockAuto has them for fairly cheap, so I may replace the whole sending unit when it warms up out there. Did your new tank come with a new sender unit too?

[Close-Call]

No, the tank did not come with the sending unit. Sender was $300+ . I didn't like that idea so I tried one from the local boneyard. That's how I know the difference between a sender from a 90 with a 460, and a 93 with a 7.3

I think the used sender cost me about 50 bucks.

[M.L.S.C.]

How hard are they to get out and in, are they like on top of the tank? I'm heading to the j/y tomorrow for a bumper and a grille, I'll see if they got something with a 460.

[Close-Call]

You'll have to drop the tank as the sender is in the top, right about the middle. DO NOT CUT THE PLASTIC LINES. They are sorta press fit (shrunk?) onto the metal tubes and are a real PITA to replace if you cut them.

There's a metal ring around the outside of the sender. I used a chisel to break the ring as it was to rusted to turn.

I replaced the tank because it came with a new ring & gasket and was in stock. They didn't have just the ring & gasket stocked and I wanted to be finished with the job before returning to work.

It seems to me that the ring & gasket would have been about $30.

As I said before, the 460 was similar but the tubes ran in the opposite direction. That may have been a difference in the model years though. I'm not too sure about that. The pickup strainer is also different. Diesel has bigger holes. IF you find a 460 sender that has tubes running in the same direction as yours, you should be able to just swap the pickup, as far as I can tell.

[M.L.S.C.]

Well can I replace just the float then? Will that still require dropping the tank and fighting the ring?

[Close-Call]

Yes And Yes.

Float is in a little wire loop (hook like thing) on the sender arm.

Once you have the sender out, pull the float out of it's loop, and give it a shake. You'll hear/feel the fuel sloshing about in it. The other thing you can do to test it is put it in a small container (1lb. margarine container or similar) full of fuel. If it floats, it's ok. If it sinks, replace it.

My father used to be able to let them sit (and drain) and then re-solder them. He had a pretty good success rate, but I've never been able to pull it off. It's usually an invisible hole.

When you pull the sender look for the little brass can (the only brass part in the entire unit). You can't miss it.

[M.L.S.C.]

Darn, so no easy way out of the situation... well that sucks, guess I'll have to deal with the tank then, but I will wait till weather gets nicer, for now I'll make sure I fill up the front tank every 100 miles of driving.