web space | free hosting | Web Hosting | Free Website Submission | shopping cart | php hosting

Troubleshooting Fuel Tank, Fuel Pump, and Fuel Gauge Problems with Ford F-150 with in-tank pumps.

Background: There seem to be numerous complaints about fuel tank, pump, and gauge problems on the message boards. I experienced some of these same symptoms with my 88 F-150 dual tank system. Because I have extensive previous experience with in-tank pumps, I decided to document my current project. While I donít have all the answers, I am making a serious effort to not state any guesses or theories only experienced facts in this article.

My experience began several years ago when I replaced an Ď83 2.0 carburated engine with a í91 2.3 EFI in my Ranger. The replacement included the complete drive train as well as the fuel tank and wiring harness, etc. Next, I installed a Ranger tank in my í54 F100 with 5.0 EFI engine. I did this one twice, because the first (plastic) tank I installed sat too low under the í54, so had to do it again with a metal tank. I have also dropped and opened up the tank in my í91 Explorer.

This project involves troubleshooting the common problem in my í88 F150 of one tank pumping fuel into the other tank, complicated with only one tank fuel pump working, and neither of the gauges working properly. This model has the low-pressure in-tank pump and external hi-pressure pump, with the external six-port reservoir/valving system.

CAUTION! Working around gasoline is very dangerous! Here are some critical doís and doníts when working with your fuel system.

DO Ė Insure there are no motors, relays, electrical switches which can be activated in proximity to your project. A compressor comes to mind here as a potential culprit.

Have a known good fire extinguisher handy.

If possible, have someone near by while you are working in case an emergency occurs.

Make sure you have a clean clear work area.

DONíT Ė Use an electric (120v) trouble light.

Let anyone smoke nearby.

Let kids play nearby (any age kids 2- 100).

Spill fuel.

SYMPTOMS: The engine dies after a few miles when running on the front tank, although the tank is almost full. Runs ok when switched to the rear tank. Both gauges are erratic. Sometimes showing over-full and sometimes empty, never in-between. Moving the fuel to the rear tank by siphoning was a temporary measures, as the tank would refill from the rear pump via the return line.

Troubleshooting steps: Take the easy option first Ė check the reservoir.

In order to check the reservoir, the fuel pressure must be relieved, the lines disconnected, and the fuel must be caught in a container, as the tank will gravity feed for a while, depending on how much fuel it contains. I used a plastic gasoline can, and wedged the line into the can opening, checking every few minutes to make sure it didnít overflow.

All the documentation I have states that the reservoir is non-serviceable. I agree. I took it apart as much as possible, found no contamination, corrosion or foreign particles, but cleaned it with carb cleaner anyway. Still no change in the symptoms.

Since the easy option didnít work, the next option is to drop the tank, remove the pump/sending unit/filter assembly, and see what is broken.

Before dropping the tank, empty it by siphoning or other safe method. I used a hand-operated vacuum bleeder and a piece of clear plastic tubing to get the flow started. Holding the pump below the bottom of the fuel tank will allow you to prime the tubing without allowing gasoline into your vacuum pump. You may need several gasoline cans. I got by with two by pouring the fuel into the rear tank from one can as the other was filling.

Once the tank is as empty as possible, relieve the fuel pressure by pressing in on the schraeder valve on the fuel manifold. Hold a rag over it to prevent exposing yourself to gasoline spray. There should only be a few drops of fuel, if the pressure is released slowly.

The next step is to loosen the filler hose from the tank. To me, disconnecting and reconnecting this hose was the most difficult part of the project. Once the hose is loose, loosen, but do not remove the support straps. The bolts were not captive, so I had to use a 13mm wrench on the bolt head while I loosened the nut with a 5/8" deep socket.

Just before you take the straps loose, place a jack under the tank. You may want to put a piece of plywood or other cushion between the tank and the jack to prevent tank damage, depending on the jack and how much fuel was left in the tank. Take the straps loose. The tank will come partway down.

Next, disconnect the supply and return lines and the vent line, as well as the wiring connector from the top of the tank. Mine had the plastic clips which can be removed with your fingers to allow the lines to slide off. If you have other types, you may need to use the special tool to disconnect them. I found that it was easier to pull the whole vent assembly from the rubber grommet than get the hose off the plastic piece.

After the connections are loose, and the filler hose is disconnected, lower the tank and pull it from under the truck. I immediately covered the two openings and the inlet/return lines with aluminum tape to keep trash out of the tank. Here is what mine looked like:

OK, if you have done all the above steps in one session, itís time to take a break. Get the kinks out of your leg muscles, the dirt out of your hair (I donít have that problem Ė no hair on top) pour another cup of coffee, etc, here is where it gets fun. CLEAN the area around the assembly! You donít want your new pump to fail because of grit falling into the tank.

Use a BRASS or STAINLESS STEEL brush to clean the dirt, grime and rust away from the assembly before loosening it. Here is what mine looked like after brushing:

Loosen the retaining ring. I used a hammer and non-sparking drift pin to gently tap on the ears until the ring loosened. Then I removed it carefully by hand, as there was still some trash to fall into the tank. Remove the ring, then gently pull the assembly (if itís stuck, you may have to tap it lightly) from the tank by bending the filter sock as necessary. I found that it was a really tight fit on the float arm, even bending the sock to gain space. Be sure to cover this large opening to keep out trash until you are ready to re-install the assembly.

The assembly consists of the fuel pump, float and sending unit, and filter sock. Because I knew my pump was defective, I first concentrated on checking out the sending unit. The sending unit is backwards from the older (like my 83 Ranger) type, in that the low resistance side indicates EMPTY, and the high resistance side indicates FULL. I found experimentally (after I repaired it), that the correct resistance is EMPTY = less than 20 ohms while FULL is greater than 200 ohms. This is why if the wire disconnects, your gauge will read over full, and if by chance it shorts to ground, you would read empty. Go figure, still seems backward logic to me.

(Hereís why I say that: The normal fail mode would be the wire breaking or disconnecting somewhere. So the false reading would be full, which is MUCH more inconvenient than the false reading being EMPTY. If you follow, I am saying that the open wire could cause you to run out of fuel, whereas in the old mode it would cause you to stop for fuel before you needed to).

In order to work on the sending unit unrestrained, I cut the wire going to it from the flange, in about the center so I could easily reconnect it. This allowed me to remove it from the assembly. The sending unit consists of a rectangular metal plate welded to the supply tube, with a plastic cover attached. The plastic cover snaps off. One end has two tabs and the other end has one tab and a tiny post. I pressed in on the end with one tab and released it first, then released the other. Here is a picture of the inside of the unit:

What I found after several minutes was that the problem area was the backing plate more than the resistance slider. Just to be safe, I used a small screwdriver with a strip of 1000 grit sandpaper lengthwise over it to burnish the wire grid on the insulator board. It cleaned up nicely. Then I noticed that the wiper also must make contact with the backing plate, which was totally corrupt. Using more of the sandpaper and generous contact cleaner, I cleaned it up too. Just for good measure, I re-tensioned the slider so that it always made contact at both ends. Then I carefully snapped the unit together. See the picture below:

Obviously, the job is too difficult to do more than once in a day, so I tested the sending unit before reconnecting it. I used a simple ohmmeter on the x1 scale. I connected the wire that I had cut to one lead, and the metal frame of the unit to the other. I moved the float arm up and down slowly and watched for intermittents. I held the assembly in various positions, to make sure my repair job was thorough. Having passed the test. I slipped the section of heat-shrink tubing over the wire, stripped, twisted and SOLDERED it to insure a trouble-free connection, then shrunk the tubing over it.

Next step is replacing the fuel pump. Although I had determined that the pump didnít work a few days earlier by connecting a wire directly to its supply wire, I verified it by checking its resistance with an ohmmeter, after cleaning the contacts. The motor showed open, so either the brushes are bad, or the winding is open. Too risky to attempt a repair.

A new pump with strainer is about $40. I removed the two wires using a ľ" nutdriver. Pay close attention to which wire goes where. You donít want your pump running backwards! Also, notice the orientation of the sock. It will make re-inserting into the tank easier, and prevent possible interference of the float.

By the way, AutoZone offers by special order, the full assembly, including the pump, strainer and filter for about $100. I think itís more fun to repair than replace, where practical.

I cut the old hose to ease removing the pump. It simply sits on the bottom support with the rubber boot, held in place by the short hose. Spraying the new hose with lubricant makes installing much easier. Then I reconnected the wires, and finally pushed on the new filter sock.

Next, I completely drained the old fuel from the tank. This is not easy, unless you have a good gasoline proof pump. I had to turn the tank over and over and drain the fuel into a pan. Be sure to dispose of the fuel properly. Remember, gasoline is a hazardous material.

The new filter sock comes with a new tank seal. Carefully remove the old one and insure the surface is clean. Place the new seal on the tank. Place the assembly in the tank, again bending the sock as necessary to avoid stressing the float arm. The flange has tabs on it so that it locks in only one position. Install the lock ring by tightening it to the stop. Here I had to use slightly more force with the hammer/drift to get it fully tight.

Check the wire connector on the harness. Use a 1/8" screwdriver blade in the slot next to each loop connector to bend it slightly to insure a snug fit. I omitted this step and the sending unit didnít work at first. Man I was glad I had verified it before putting it in the tank! I adjusted all of the connectors to insure that the pump and sending unit would have a reliable connection.


I placed the tank under the truck, one end on a jack stand, and the other on the jack, several inches below its normal position. Then I connected the wiring harness, fuel lines and vent tube. Also, I STARTED the filler hose on the port. Then I installed the plastic protector on the front of the tank, and using the jack and my knees (protected by hard cover knee-pads), I lifted the tank enough to start the strap bolts. After I had the tank sufficiently supported by the straps, I removed the jack and jack stand and finished connecting the filler hose and clamps. Then I snugged up the tank.


After reconnecting everything, adding fuel to the tank, everything seems to be working ok, as far as the front tank is concerned. I will have to deal with the reservoir problem or replace it, but as long as both fuel pumps work, I can handle the rest.