1990 f150 xlt lariat with duel tanks / fuel pumps in tanks

The truck sat outside for 3 years parked. Rear tank read 1/16 of a tank, front tank read empty. The truck started on the rear tank. I added more fuel to rear...and have run 20 gallons through the rear.

When I switch to the front tank, the engine stalls (gauge says empty). So, I put 5 gallon in it. Gauge still says empty, and truck still stalls. When I switch to the front tank, I can hear the fuel pump running in the front tank.

Runs fine on the back tank.

So now I am confused what to try next. I have no idea if the front tank was working before it was parked or not.

Should I pull the fuel line by the filter to see if its pumping ?

Could the fuel be gelled from sitting? If so, how do i check?

What would you do next?

 

Could be a bad rear fuel module in the tank.
You need to put a fuel gauge on it and see what psi you get from each tank.

What engine do you have?

 

subford, do our trucks (I have the 4.9) have a diagnostic code that will display if the module is bad? Thanks Sandy

 

No, but they do have a code that will show if the fuel pump in the modules circuit is open or shorted.
But nothing for the valves in the modules.

 

Thank You. Sandy

 

OK.

Its a 1990 F150 XLT Lariat 4 x 4 with the 4.9L I6 EFI.

First, I confirmed with the previous owner that he seemed to remember both tanks were working when he parked it in 2013, and also unfortunately confirmed no fuel stabilizer was used.

Today I hooked up a fuel pressure gauge and the rear tank (which I have been running it on) pump is putting out 50 lbs with engine off and key on. Manual says 50-60 psi, so that one is working and the Motor runs great on the rear tank.

Today we changed the fuel filter, and confirmed the same engine off pump on fuel pressure from the rear tank and ran the truck.

We then checked the fuel pressure on the front tank. The fuel pump in the tank runs (can hear it), but no pressure at the fuel pressure gauge at the fuel rail.

The fuel gauge is one empty when we switch to the front tank. I knew there was fuel in it (because I had added 5-10 gallons), but I didn't know how much was in it prior. So we siphoned/emptied the front tank ..... as it turned out, the tank was nearly full....and the fuel didn't smell normal. Smelt varnishy. Am not sure what affect gas sitting in the tank for 5 years would have on the fuel pump

So here are a few of my new questions..... After considering that the rear tank works just fine..... Is there anything besides the front fuel pump that could be causing no pressure at the fuel rail, even though I can hear the front pump running? For some reason I thought that when the fuel pumps fail, they quit running .... but I can hear it running.... ....do these pumps quit putting out pressure but still turn on? Is there are valve or something mechanical that is supposed to also switch when you change tanks and the pump starts running?

Any answers or additional suggestions are much appreciated.

I guess what is throwing me is when we switch to the front tank, the tank fuel pump comes on but we are getting no pressure.

Thank You

 

There is no valves outside of the fuel tanks. All valves are in the fuel module inside the fuel tank.
A bad rear fuel module can lower the pressure of the front running pump but you should have some pressure (10-30psi) even if it does not run the engine.
At zero pressure when on the front pump I would say a bad pump or stopped up sock (inlet filter in the below diagram).

Attachment 280308

Attachment 281295

/

 

Thank You,

I will put a little fuel back in the front and check the front fuel pressure again to confirm 0.

Next question, any suggestions on where to get the complete drop in replacement fuel pump and sending unit? OReilly wants over $400 for one for the 19 gallon mid tank (8 foot bed)...had no idea.

Another question ... if it was you, would you drop tank or pull bed?

Can't thank you enough.

 

The parts person misquoted you or misunderstood you I think. The parts are no where near that price. Try again or a parts place or ebay online. Sandy

 

Price sounds about right for the whole drop in, FDM, Hanger & sender.

Most just replace the FDM as in the image below:

================================================== ================================================

If you have an extended cab I would pull the bed, if it is a regular cab I would drop the tank because of the front fuel tank hanger.

As far as the fuel sender showing empty it either has a sunk float or a shorted wire in the gauge wiring to the sender.

Diagram below is for a 1991 but a 1990 should be the same.
Attachment 285111

Fuel sender:




================================================== ==========================================

Fuel Hanger:


/

 

Does the 1990 I6 have the dual fuel reservoir on the frame? If so maybe it's not switching causing no fuel to be sent on to the engine even tho the front tank pump is running? And then the in-tank pump isn't the FDM of the later model F-series.

 

No it does not have one.

 

Update with new information

Engine running fine on rear tank OK, I retested the fuel pressure with engine off and key on and hearing the front pump and it max's out around 9 psi. Retested the rear fuel pressure with engine off and key on and its like it was (50 last time, 53 today). So I was incorrect stating the front fuel pressure was zero.

Additional information. I noticed the rear tank fuel level dropping way too quickly while operating the truck on the rear tank. We emptied the front tank and figured out why it was full when we drained it the first time. The rear tank is pumping into the front tank and running the truck at the same time.

So where I am now is =

Engine wont run on front tank, can hear the pump running when switched on, fuel gauge reading not accurate on front tank, and fuel pump showing 10 psi with engine off and pump on.
Engine will run on rear tank, fuel gauge reading accurate on rear tank, and fuel pumping 53 psi with engine off and pump on .... and is also pumping gas to the front tank.

What fuel module should be replaced? Rear, Front, or both?

(At a minimum, I have a bad float or stuck float and/or resister in the front tank in addition, right?)

Any suggested next steps are greatly appreciated.

 

I would say both. And that is what a Ford dealer would do so they would have no come back on the problem.

By the way the fuel gauge is a separate system and has nothing to do with the fuel pumps, the electrical to them, there pressure or fuel flow to tanks. They do share a common ground and selector switch but have separate contacts in the switch.

 

As for the gauge on the the front tank.
The floats can be replaced is it is full of fuel and sunk.
If the resister was open the gauge would peg passed full or if shorted to ground the gauge would peg passed empty but a shorted to ground resister is not likely.
Wiring between the gauge and resister shorted to ground would also peg passed empty.
If someone has been in the tank then a hung float arm or bent arm could also give a bad indication on the gauge when that tank is selected.