1979 F100 EFI Gas Tank
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Fuel Level Sender, Gas Tank Swap, EFI Fuel lines, and EFI Fuel Tank Wiring

For my 1979 F100 EFI conversion project, I'm using a gas tank and along-the-frame fuel pipes, high pressure fuel pump, and filter out of 1989 F150 for the EFI conversion. This will work great - except for the fact that the 1987 and up F series trucks use a different style fuel pressure gauge that needs a totally different in-tank fuel sender. The resistance is completely and "inverted" different on the later model trucks. All of the gauges were changed in a similar fashion. I learned a lot about this difference when troubleshooting and repairing the fuel gauge system in a 1989 F150 - see the section on that page titled "Fuel Gauge" for details on that saga.

The older trucks (like this truck) use a "bi-metallic" style fuel gauge that uses a small IVR (instrument voltage regulator) on the back of the instrument cluster to supply about 5V to the gauge. The sender is set up so that "empty" on the gauge is a high resistance value from the sender (about 70 ohms) and low resistance for a "full" reading on the gauge (about 10 ohms). The later style gauges are "magnetic" and the resistance values are different, and reverse so that you low resistance from the sender is for "empty" (about 15 ohms) and high resistance from the sender is for "full" (about 160 ohms). This change in the gauges happened in 1987 in the F series trucks, and somewhat later on the E series vans (at least post-1989, I'm not sure exactly when). So, I can only use a fuel sender up through the 1986 model year and have it work with my original fuel gauge.

The other issue is that the later model EFI tank uses a completely different looking sender and float assembly to clear the low pressure in-tank fuel pump for the EFI system. That means that you cannot easily (or at all) graft the earlier model sender and float onto a later style sender/pump/pickup assembly. So, I'm, stuck, right? Almost, but I got lucky here. The factory started using EFI on some of the 5.0L engines as early as 1985, though the EFI engine was an option in 1985 and 1986, so they are not terribly common. But, the good news is that the factory did make senders and EFI style pickup assemblies for two years, and that replacement units are available. Based on some quick visual analysis of the different senders and pickups, it looked like a 1985 or 1986 EFI sender would fit into a 1989 fuel tank and still have the right electrical arrangement to work with my original gauge. So, I bought a sender/pickup assembly for 1985 F150 with a 5.0L and EFI from RockAuto.com for about $70, and installed it. They come without the low pressure fuel pump installed, so I re-used the one from the original 1989 sender that went with the tank. The photos below detail the process of swapping the low pressure fuel pump onto the new sender assembly, and I tried to show the tools needed. The sender is a reasonably delicate thing, so you can't be a brute when working on it.


This is the fuel level sender that was in the original gas tank. I took it out when I dropped the tank in preparation for the EFI conversion. Check out the windings of the resistance wire - they are completely busted at around the half tank location. No wonder the gas gauge never worked properly! I tracked down a replacement sender from a 1979 F150 via Craigslist before I figured out that the EFI tanks and senders are different. I was trying to see if I could EFI pickup with the earlier sender and make it all work somehow, see above for how I got it to work.

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Sender Replacement

Here's all the parts side by side - 1989 sender and pump assembly on the left, new 1985 sender assembly on the right, with the carious parts and pieces it came with.

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This is the 1989 sender with the pickup filter/sock removed and the pump off of the fuel line. Gently pry the sock/filter off the pump with a screwdriver by getting under the metal ring, and it will come off easily. The rubber hose needs to be worked up the metal line (towards the right in this photo) far enough so that the bottom of the pump and it's rubber isolator (just above/behind the brass float in this picture) so that you can swing it to the side and out of the metal holder. I used the pliers to work the rubber hose back and forth while pulling it up the metal hose. Be gently, and go slowly, and it will come off.

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The wires to the pump come off with a 6mm wrench and some care. The nuts and lock washers are small, so don't lose them.

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The wiring polarity is stamped into the top of the pump, + for red, - for black - a refreshingly smart use of international color codes in wiring. Good job, Ford! Seriously. Mount the wires where they came from and you're ready to re-mount the pump.

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This was the hard part of the entire process. You have to slide the rubber hose up onto the metal pipe far enough to get the pump on and swung over into it's metal support ring - with the rubber isolator installed and without forgetting to put the hose clamps over the hose first. I had to trim about 1/8" to 14" off the rubber hose to get this to work - it was simply too long as it came in the sender replacement kit. It was still a tight fit and I ended up bending the metal ring slightly and had to bend it back into shape, but it worked pretty well.

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The clamps are use-one clamps - use a pair of diagonal cutters to gently crimp them under the raised area. This will draw the sides of the raised area together and tighten the clamp. No need to go nuts here, just get it snugged up.

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Finally, you can install the new filter sock from the bottom. It pushes over the bottom of the pump inlet and is a snug fit. If you gently compress the sock from the bottom, you can feel the inner metal structure and find flat spots to push on. Be careful not to tear it, and work is on gently and evenly. It will go on, trust me.

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You also need to position the sock so it clears the float the full travel. It will rotate if you apply gently pressure from the bottom - again, be careful not to tear it. If you do, it's junk and must be replaced. Based on comparisons with the original, the float arm will almost touch the sock when it's in position correctly. Operate the float through it's full range of travel to ensure nothing hits, and be careful not to bend the float arm or mess up the sender itself - it's the white thing the float arm attaches to at the top/pivot end.

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Wrapping Up

After that, it's a simple matter of re-installing the sender assembly into the tank with the new O-ring and lock ring that come in the sender replacement kit. Make sure the o-ring groove in the top of the tank is as clean as possible before you start. Lots of grime tends to accumulate on top of a gas tank, so a good wiping down with a damp rag will help pull out any remaining dirt, grit, and gunk. The O-ring is compressed pretty tightly when you tap the lock ring into place, and if it tears, or has some grit under it that causes it to have an air gap, it can leak fumes or liquid fuel - a hazardous condition for a fuel tank. As you tap the lock ring into place, be aware of any fumes from the opening in the tank - sparks here would be bad, and working in a well ventilated area would be A Really Good Idea. Once the sender is installed in the tank and the lock right tightened down, the tank itself is ready to be re-installed.

Before you install the EFI fuel tank, you need to prepare the EFI fuel tank wiring from the donor truck - once the tank is up and installed, you can't reach up on top of the tank to make connections. That means everything has to be ready to go ahead of time. The EFI tank needs three wires - power for the fuel pump, a wire from the sender, and a good ground. The original tank had a ground strap somewhere and a single wire coming down the frame for the sender. The new tank has a special weatherproof four wire connector to use, and in my case had a second connector in the harness for a second tank. I won't be using the second tank (well, not at this time, anyway), so I'll just tie that up out of the way. The problem is that the donor harness is a rather large and bulky thing with a lot of extra wires in it for things I don't need or want. I just want three lousy wires, one of which will be grounded to the frame somewhere under the truck. I stripped the harness down and removed all of the tape and split loom. Then, I separated the wires to isolate what I needed. The harness was basically in two pieces in my case - one with split loom and one that was fully taped up with a gray-ish tape. The two sections were connected by a lone wire - I suspect the taped one was for trailer towing, rear lights on the truck, and other such fun, and the split loom one was for everything "under the truck" - the wiring for the fuel tank(s), as well as the various sensors (rear axle anti-lock sensor, speed sensor at the transmission, etc.) that are under the truck and need to have wires in the harness along the frame. The wires I removed were all in the split loom section. To get all the other wires out of the way and just have the ones I needed, I had to free the wires + attached terminals at the end connectors by applying the knowledge from my Reusing Plastic Wiring Plugs and Automotive Electrical Connectors pages. Basically, I removed the TPA (terminal position assurance) piece from the "inside" of the connector, then depressed a small tab for each terminal, and pulled each terminal out of the back of the connector. The connector at the back of the truck was a fully weather tight version, with a rubber seal at the back of the connector body that I had to pry out with a small screwdriver. Once all the wires were free, it was easy to separate the few I cared about from the ones I didn't. The ground wires was spliced into one of the sensor connectors, and for now, I just left that grouped with the wires I separated out. I also left in the wiring for the second fuel tank - it was spliced into the ground and if I ever wanted to add the second tank, the wiring will be there. I'll re-tape the harness with the proper vinyl non-sticky harness tape, then add in the terminals I need for ground at the frame and for a connector up near the firewall to connect into the original fuel level sender wiring. By not cutting the wires or destroying the connectors, I can potentially re-use them in my custom wiring work - a handy way to save some time and use a Ford style connector that I know was rated for and worked well in this application - the 1987/1989 trucks I parted out had connectors at the firewall area to connect the wiring along the frame to the wiring under the hood, which in turn connected to the wiring inside the cab.

The vent hose (the rubber tube coming out of the top center of the tank) can be hooked up to the original metal vent line running down the passenger's side frame rail of the truck - though some later years of EFI tanks have a larger vent hose size. In my case, judging by the various parts and pieces I still had laying around, it looks like the gas tank from the 1987 F150 I parted out had a vent line that matched this truck, and the gas tank from the 1989 F150 had a larger vent line that did not match the metal vent line on this truck. Unfortunately, I had already sold the tank out of the 1987, so I'll have to get an adaptor to mate the larger rubber line from the 1989 gas tank to the smaller metal vent line on the truck. Naturally, I didn't discover this until the tank was all the way installed - Doh!

The original metal fuel line going up the driver's side frame rail will need to be removed so that the EFI fuel lines, accumulator, high pressure fuel pump, and filter can be installed. For the tank install, you can just connect the EFI lines to the sender assembly on the tank and let them drape over the frame crossmember for later mounting along the frame rail. To mount the nylon fuel lines, I used the fuel line clips from the donor truck - I just carefully removed them along with the fuel lines. The EFI fuel tank basically lifts right into place where the original one went, and is about as easy (or hard, depending on your point of view) to do as the original tank was. The difference is that you have two nylon lines attached to the sender, and they kink easily if you are not careful. I put a minor kink in one of mine, and hopefully it will be OK - time will tell.

For the filler neck, the newer tanks use a "hose inside of a hose" so that there is a single hose going down to the tank. The older ones use a smaller second vent hose to ensure easy and fast filling - without this, the tank can overfill unexpectedly and burp fuel out the filler neck. My Ranchero has a habit of doing this. The basic arrangement is the same, but the mounting to the plastic housing that in turn mounts to the side of the bed is a little different. The plastic housing is what connects the filler neck to the fender and what you see when you open the fuel filler door on the side of the bed. The earlier units (aka, what was on this tuck originally) mount with a large rubber ring like a grommet that goes into the plastic housing. The later units mount to the under/back side of the plastic housing using three small sheet metal screws driven in from the front/outside. The later units also have a screw on cap with (it's it's not busted off) a small plastic "wire" to hold the cap from falling away and getting lost. The earlier units use a push-on cap.

Comments? Kudos? Got some parts you'd like to buy/sell/barter/swap? Nasty comments about my web page so far? You can email Mike or Debbie.

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Page last updated 06/28/2009 01:44:19 PM