Couple of problems.

1) Can not switch from front to rear tank. The truck only draws on the front tank.

2. Front tank fuel guage not accurate. It stays on full until almost the empty, floats for a a gallon or two and then jumps to empty.

Any ideas?

Thanks, Dan

This might be a repost, but I don't see the original so maybe I did something wrong.

 

So if you switch to the rear you run out of fuel?What year is the truck?

 

Correct. I can no longer switch the the rear tank. The way I know this is that I fill the front tank and "switch" to the rear. But its the front tank that needs refilling, and the rear tank guage doesn't move.

It's a 1988 F250 with the midsized V8 (smaller than the 460 and larger than the 302)

Thanks, Dan

 

I am no expert on the matter but here is a couple of ideas.If your system is the same as my 88-f150 you have 3 pumps. One low pressure pump in each tank and one high pressure tank on the fuel rail. The fact the front tank works ok tell me the pressure regulator is ok and the high pressure pump is ok. So the problem directs us to the low pressure pump in the rear tank, the dual function reservoir on the fuel rail, or the electrical switch on the dash.When switched to the rear tank can you hear it come on indicating power.I would first check the electrical connector on top of the tank.It lives in a hostile enviroment.Mine needed to be replaced as the plastic had pretty well fallen apart.

 

Hi,

I have two pumps on the rail under the driver's seat.

But, wouldn't the truck stall if I switched to a tank that had a faulty pump?

At this point, I am only taking fuel from the front tank - regardless of which tank I'm switched to.

THanks, Dan

 

I think his switch is not working. It switches the gauge but not the tank. Is there a relay downstream that does the actual tank-switching? If so, that would be the culprit.

 

You know I was thinking the same thing. Is it possible that the electrical switch in the cab is defective and even though you are switching to the rear tank it is still supplying power to the front tank pump.Are you sure you have 2 fuel pumps on the rail?

 

This is a late reply, but I didn't know enough to reply earlier.

I only have one pump on the rail, the other pump is actually the fuel filter.

I also learned that the switching valve that is behind the high-pressure pump, is mechanical only. In other words, the switching occurs due to the pressure that it receives from on of the low pressure pumps. The mechanic thinks that the electricity is getting to the rear tank, because the gauge is working, but the since the rear pump I am not switched to the rear tank.

So the truck runs off the front tank by gravity, even though it is switched to the rear.

I find it hard to believe that the front tank will deliver fuel by gravity, but who knows.

One final question. Does anyone know how the lines from the two tanks are supposed to enter the switching valve. The lines are color coded, each tank having a unique color. Both lines from the front tank go to the bottom of the switching valve, and both lines from the rear tank go to the top of the switching valve. Since my front gas gauge doesn't work properly, I'm wondering if the input/output lines from each tank have been correctly connected.

Thanks, Dan

 

This is a very common problem. First, have someone sit in the cab and switch between tanks on the switch while you listen to each tank as it is engaged. You should hear a whirring noise in the selected tank. If not, it is either the switch or the in-tank pump. If you can hear each pump whir, then it is the dual-function reservoir. This part will run you about $80.00 at Ford and will take all of about 30 minutes to replace. If you need to change a pump, you will have to either remove the bed or drop the offending tank. Not too hard, really.

 

I will go along with what mike2112 said. But I vote is a bad rear tank pump.

Here is the diagram of the lines you asked for:

 

You can do this with the truck running, or turn the key off, and back to on, (not start) every time you switch. When you turn the key on, the fuel pump will, er should, run for ~5 seconds to prime the system. You might need to put your hand on the tank if you can't hear it over the high pressure pump.

If the switch isn't working, the same pump will start for both front and rear. If one doesn't come on, that pump is probably bad.

 

The "mechanic" that told you the fuel selector switch works because the tank reading followed the fuel selector switch action is ignoring half of the tank switch. Notice in the drawings posted by subford that there are two sets of contacts in the switch. One set of contacts routes power to the chosen fuel pump and the other set of contacts routes the chosen fuel level sensor to the fuel gauge. My vote is for a fuel tank selector switch problem.

 

Not to stomp the idea. But I don't think its mechanically possible for that type of switch to fail that way. The two sets of contacts are normally mounted to one moving part, so they both move or neither moves.
You can check it easy enough. You can either take it out of the dash and check it at the switch, and pull down the tank connector and check it there too.

The gauge moving wrong for the front tank is probably a bent or broken sending unit float bar. Easy to do when replacing a pump.

So to summarize.
Neither gauge works right:
The front stays at full, until nearly empty, then drops quickly.
The rear never moves.
Pumps:
You refill only the front tank, the rear always stays full of gas, even when you drive with the selector switch set to rear?
The truck runs with the switch set to both positions?

Does this happen durring short trips 1/2 tank or less, or long trips, or both. There is a common problem with fuel returning to the wrong take in the high pressure pump systems.

 

Thanks to all who replied, and the diagrams from subford were a great help.

clasrfbc: Your summarization is correct. Regardless of whether I've selected the front or rear tank, the truck only draws down the front tank - and it doesn't matter if the rear tank is full or 1/4 full.

While there seems to be some disagreement about whether the fuel tank switch can "half fail", with the guage working, and the pump failing, I'll check that first.

Thanks again,

Dan

 

I think there is a good chance when you take the dash apart you will find a couple extra wires or at least a couple splices. I'll even venture a guess that they pulled the power into the switch and plugged it straight to the power out to the front tank.

Normally the truck would stall when switched to a tank with a dead pump?
If it doesn't someone must have changed the wiring so that the good pump always runs. Effectively giving you a 19gallon ballast tank over the rear axle.