Table of Contents

• Rational
• Removal
• R and R
• Replacement
• Recap

Rational

Ever since I’ve owned my ’77 Jeep Wagoneer, the fuel level gauge has been inaccurate. It’s pretty close at empty, but when you fill the tank, it only registers 3/4 full on the gas gauge. I know the tank is full because I’ve put over 19 gallons into the 22-gallon tank. This was annoying, but as long as the empty mark was accurate, it really wasn’t worth losing any sleep over.

As we started driving the Jeep on more long trips after the engine rebuild, we noticed that you could smell gas fumes inside the cabin after a while. You couldn’t smell them if you stuck your head under the hood, so I assumed that they were likely caused by a problem with the fuel tank — perhaps by cracked rubber hoses.

Unlike the CJ’s and California FSJ’s of that era, which used a fuel vapor canister in the engine bay to vent gas tank fumes into the carb’s air intake, the 49-state FSJ’s in 1977 simply vented the gas tank fumes into the atmosphere via a vented gas cap. It was suggested that perhaps my old (non-stock) cap was non-venting or had a clogged vent, so I bought a new, stock-style cap. No change in the fumes, but it looks nicer than the previous one.

Removal

The fumes were starting to get annoying, so I decided it was time to drop the tank and see what I found. You can do this job by yourself, but it’ll go a lot faster if you have a second person that knows what they’re doing. The first step was to run the tank as close to empty as I could, since gas weighs about 6nbsp;lbs per gallon, and the tank is rather awkward even when empty. When the engine stalled on my wife about six blocks from home, I figured that was close enough. 🙂

Note that the ’70’s FSJ’s gas tanks are a different style than the ’80’s tanks, so the instructions here are only vaguely accurate for 1980 and newer Wagoneers, Cherokees, and Grand Cherokees. I’m not sure how far back this tank style goes, but I do know that it was used from 1974-79. For more instructions on this era and the newer FSJ’s, see the IFSJA tech section.

Removing the tank is reasonably straightforward, but it still took me about 2.5 hours and left me rather sore. Even though my Wag has about 2-3″ of suspension lift, I found it helpful to jack up the driver side of the Jeep to gain a little more clearance. How high you can raise it depends on how high your floor jacks will reach when it comes time to support the tank. I raised my left side frame about 3″, which put it about 5-6″ higher than a stock Wag would sit. Be sure you block the tires, because you’ll have to disconnect the emergency brake later on.

The first step for me was to remove skid plate. This plate exists on both my ’77 Wag and my ’78 parts Wag, but it’s not mentioned in my TSM, so you might be able to skip this step if you don’t have one. The skid plate isn’t all that heavy, so I supported it with one floor jack while I removed the dozen or so bolts that held it in place. To aid in this step, I recommend spraying all the bolts/nuts with PB Blaster before you begin. The skid plate mounts are completely separate from the gas tank mounts, so you don’t have to worry about the gas tank falling on you just yet. There are six or so bolts (9/16″ heads) along the frame rail, which gave me the most trouble because they’re exposed to the most road grime. There are also two bolts along front edge about half way up, two more at the top of the bracket half way back on the passenger side, and two more at the back end of the tank near the frame rail.

Once the bolts are removed and the skid plate is mostly lowered, you’ll find the tricky part. The emergency brake cable runs through a hole in the bottom of the skid plate as it passes from the tensioner to the frame rail. In order to remove it, you have to remove the “ball” end of the cable from the metal clip that connects it to the rear portion of the cable. This clip is located inside the frame’s C-channel about half way back along the gas tank. To remove it, I had to free up some slack in the cable by disconnecting the guide that holds it to the frame up where it first contacts the frame. This guide is held in place by a pin outside the frame with a cotter pin through it. Once you’ve got some slack, you should be able to move the ball at the end of the cable toward the center of the metal clip and then out through a hole in the clip. On my clip, the hole wasn’t large enough, so I had to pry it open a bit farther using vice grips and a small pry bar. With the cable free, you can thread it through the hole in the skid plate and then get the skid plate out of your way.

Now it’s time to remove the tank itself. The tank is held in place by two bolts through a flange on the front edge, a strap under the middle of the tank that’s supported by one bolt at each end, and a bracket at the left rear corner of the tank. Begin by squirting down both the bolt and nut end of each of these bolts with PB Blaster. I’d recommend you then loosen each of them part way before you clutter up the work area with jacks to support the tank. It’ll make the job of breaking them free easier. I’d recommend removing the bolts holding the center strap first, since they gave me the most trouble and were the hardest to access. On my ’77, the bolt heads (facing downward) were 5/8″ and the nuts (facing upward with very little clearance) were 11/16″. The bolt by the frame required about 6″ of socket extensions and a wobble joint, plus a box-end wrench for the nut on top of the frame. The bolts at the front and rear of the tank have 1/2″ nuts and bolt heads. On the rear bracket, I recommend removing the bracket at both ends (one bolt at the tank and two at the frame rail). It makes removing the tank easier.

Now that you’ve broken loose all the bolts, it’s time to disconnect the four hoses from the tank. Probably not a bad idea to first remove the gas filler cap to relieve any pressure that may exist inside the tank. The two supply and return lines that run to the engine bay are connected to the tank via short rubber hoses near the frame rail at the front of the tank. Remove these hoses from the steel lines. At the top of the rear of the tank is the fuel filler hose. The filler vent hose is accessed from inside the left rear wheel well, just in front of the tire. For now, just disconnect the rubber hose from the metal line in the wheel well, and leave the rubber hose connected to the fuel tank. It’s not very long, and won’t really get in the way. This leaves only the sending unit wire connected to the tank, but it’s difficult to access until you drop the tank several inches.

Now it’s time to support the tank so it doesn’t fall on you when you finish removing the bolts. I was able to do the job by myself with two floor jacks. I used some 18-24″ 2×6 boards on top of the jacks to distribute the weight a little better and avoid denting the tank. Once it’s supported, you can finish removing the bolts.

The front of the tank is supported by a flange that rests on top of the bracket, but the rear of the tank will start dropping as soon as you remove the bolts and lower the jacks. Once the rear of the tank has been lowered a few inches, you should be able to reach up and unplug the wire that goes to the sending unit. It has one of those safety clips on it that requires a screwdriver to pry free, so you probably can’t do this blind while the tank is fully raised. With the wire disconnected, you’re free to remove the tank. You’ll have to move it rearward a couple inches so the front flange can clear the mounting bracket, but otherwise it should drop straight down. You can’t move it sideways right away thanks to the bracket to which the middle strap attaches. If you’re using two jacks, be careful to keep the tank mostly level so that any fuel still in the tank doesn’t run to the low end and shift the weight suddenly.

This much of the removal process me about 2.5 hours by myself, the majority of which was spent removing stubborn bolts. You might be able to do it faster if your bolts are in good shape and you pay attention to the tips mentioned above, some of which I learned the hard way.

R and R

There were several things that needed to be done while the tank was out. Most obviously, I needed to replace all the short rubber fuel hoses that connect the hard lines together. All of them were bad, but one almost resembled the floor of a dry lake bed. The larger (roughly 1″ ID) hose that connects the tank to the filler vent line seemed to be in fine shape, surprisingly. It had the factory hose clamps on it, so I’m guessing it’s original.

The most annoying hose problem I found was with the main filler tube. The filler path on a ’77 Wag is quite long. Originally, the filler goes from the plastic filler mouth to a right-angle rubber hose that goes from the quarter panel to under the body, where it connects to a metal tube. This tube snakes past the spare tire, where it connects to another rubber hose with two bends that eventually connects to the nipple on the gas tank. The problem is that, after 26 years, both these rubber hoses tend to go bad, and nobody makes them any more. The solution I came up with was to have an exhaust shop bend a piece of aluminized exhaust pipe in the shape that I needed, then splice that piece to the adjacent pieces using short, straight lengths of rubber fuel hose. I already did this a couple years ago with the elbow pipe closest to the filler neck. When I inspected the hose by the gas tank, I found that it had a bad crack and probably wouldn’t last much longer, so I did the same thing with it. Making the two bends at the proper angles proved difficult for the shop, but they got it close for $20. I’m not wild about trying to mate the end of my new pipe in mid-bend to the neck on the fuel tank. Time will tell if it’s sealed well enough.

Perhaps a better solution would have been to have a qualified shop weld a longer, bent neck onto the tank that would allow the use of a 12″ straight rubber hose between the tank and the intermediate filler tube. I didn’t think of that until just as I was writing this, of course. 🙁

As I was removing the fuel pickup to inspect it, I managed to break the “sock” filter that fits over the end of the filler pickup. This is actually a cylinder about 4.5″ long and 7/8″ in diameter. The outer surface is two layers of plastic mesh. Inside is a foam-like tube that extends from the metal tube down to the bottom of the tank. Nobody local carries anything close to that, and I didn’t feel like waiting to mail order one (BJ’s Off-Road sells them for $5). The route I took was to ditch the in-tank filter and replace it with a 5″ length of 3/8″ rubber fuel line. I positioned it so that it would just barely touch the bottom of the tank (it goes in at a 45-degree angle). To keep the hose from curling up, I sleeved it with a spring, which I secured in place on the pickup tube/hose with some 20 awg wire. To keep the end of the hose from mating flush with the floor of the tank and causing a vacuum, I cut two small notches into the end of the hose — just in case.

You still need a filter prior to the fuel pump, though. I bought one of those transparent in-line fuel filters (Purolator brand) with the replaceable filter elements and placed it in the supply line to the fuel pump. The filter is 4.5″ long, 1-3/8″ of which is hose nipples. I wanted to place it just in front of the tank where the hard line starts to run along the frame to the engine bay, but it was a good 2″ too long to fit there. I ended up putting it at the other end of that hard line — in the engine bay at the base of the rubber hose that supplies the fuel pump. It’s only a few inches from the front of the exhaust manifold, which concerns me a little, but maybe it’s no big deal. We’ll see. I’m having some occasional problems with vapor lock, so I may have to do something different with that entire line anyway.

The most vexing part of this job was fixing the inaccurate fuel gauge that I mentioned earlier. The gauge circuit works like this: in the instrument cluster is a voltage adjuster that takes the 12V accessory current and converts it to 5V, which it then provides to both the gas gauge and temperature gauge. The other terminal of the gas gauge is then grounded through the sending unit via a pink wire to the sending unit and from the body of the sending unit through the tank to the frame. The sending unit provides a variable amount of resistance in the ground circuit, and this resistance is what changes the reading on the gauge. According to the TSM, the sending unit provides 73 ohms at empty, 23 ohms at half tank, and 10 ohms at full (give or take 5%). A short circuit will peg the gauge past full, while an open circuit (or no accessory voltage) will peg the gauge past empty.

After pulling the sending unit (by using a screwdriver & BFH to drive the lock ring counter-clockwise), I first cleaned up the electrical connections by wire-brushing all the rust off the sending unit body and the lock ring (be sure to use a brass wire wheel so it won’t spark). I then dug out my trusty, 4-yr-old, mildly-expensive multimeter to test the resistance it provided. To my surprise, it provided somewhat less than half the expected resistance across the board. After beating my head against that for a while, I eventually determined that my multimeter wasn’t quite so trusty after all. A borrowed multimeter indicated that the sending unit was functioning exactly to spec. I tested the circuit between the gauge and sending unit by placing multimeter leads at the chassis ground and at the pink wire terminal on the big, round connector at the back of the instrument cluster. Same resistance, so there was no significant resistance between the gauge and the sending unit.

The next step is to test the gauge by bypassing the sending unit and putting a known resistance in-line with the ground circuit. I bought an assortment pack of potentiometers (variable resistors) at Radio Shack, one of which had a range of 0-100 ohms. I set the “pot” to a known value with my borrowed multimeter, then turned the key to see what the gauge registered. The gauge seemed to test out OK, so I’m assuming the extra 5 ohms of resistance I was getting (enough to force the needle from “full” down to 3/4 tank) must have come from a bad ground between the sending unit and the frame. In addition to cleaning up the sending unit housing and the bolt holes on the tank with a wire brush, I also replaced two of the flat washers at the mounting bolts with star-type lock washers, which will dig into the metal and provider a better electrical connection.

I did run into two problems when testing the gauge that weren’t documented in the TSM. First, I tried to measure the resistance on the pot while everything was hooked up, I got a much lower reading than I got if I first disconnected the ground wire from the pot. I assume this is because the temperature gauge (which is powered by the same 5V source as the gas gauge) was providing a parallel resistance path to ground. Second, if I tried to measure the pot’s resistance with the key on, the values jumped all over the board. I’m not quite sure what might be causing this, because as I understand it, it means that there would have to be something in the cluster that was shorting and then opening a circuit to ground several times per second. This meant that I couldn’t accurately adjust the pot on the fly with the gauge active — I had to disconnect the ground wire, adjust the pot, then hook up the ground again. Not a big deal when using alligator clips, but still something to beware of.

While I had the skid plate off, I also decided to coat it with some of that rubberized, rattle-can undercoating (Mar-Hyde No. 4211). It cost $4 a can, and I used a full can by putting a light coat on both sides of the skid plate. It certainly won’t hold up to any scrapes I may get off-road, but it should do OK at preventing rust from road grime. Even with the 26-yr-old factory paint job, the skid plate wasn’t in that bad a shape when I pulled it off. The spray can said that it deadens sound, too, so this was mainly a test run to see how well I thought it might work on the inside panels of my Wag. I think I’ll keep looking. If you buy this stuff, note that it takes well over an hour to cure (at least to the point where newspaper won’t stick if you flip it over).

Replacement

Re-installation of the tank is just the reverse of removal.

Before you slide the tank back under the Jeep, I recommend applying some dielectric grease to both the sending unit stud and to the electrical connector near the frame rail (where the white & pink wires meet). I managed to do this while the tank was lifted up just 3″ from the floor pan, but it wasn’t fun. Dielectric grease will help keep crud out of the connection, which will help prevent rust, which will keep unwanted resistance from skewing your gas gauge.

If you didn’t label or otherwise keep track of which bolts & nuts went where, you’ll be kicking yourself, since there are several different widths and lengths used, and it’s hard to get a good look at most of the holes. As a refresher, the pair of bolts, washers, and nuts at the front of the tank are 5/16″ (1/2″ heads/nuts), as are the three bolts, washers, and nuts that attach to the support bracket at the left rear corner of the tank. The bolts, washers, and nuts that secure the strap around the middle are 7/16″ (5/8″ heads, 11/16″ nuts). The bolts (some with nuts) that attach the skid plate are all 3/8″ (9/16″ heads/nuts).

In my case, this was complicated by the fact that I was trying to retrofit a new fuel filter and filler hose that weren’t there before. See “R and R” for the stories. Lifting the tank into place & connecting it was pretty straightforward. I had a helper (my lovely assistant Stacy) push the jack under the big end of the tank while I lifted it by hand (remember that there was still 2.5 gallons of gas in my tank), but the rest of the process was easy to do by myself. Note that it’s easier to hook up the sending unit wire while the tank is still several inches below its final resting place — close enough to reach, but still low enough that you can get your hand up above the tank to do the job.

In order to help get a good electrical ground between the tank and the frame, I replaced both the upper and lower washers on both of the bolts at the front of the tank with double-star lock washers. These washers dig into the metal as they’re tightened, which will help maintain a good electrical connection long after corrosion would have otherwise set in.

I recommend tightening all the attaching bolts most of the way, but still allowing plenty of wiggle room at first. Once you’ve got everything positioned properly and secured, then you can torque them down. A little antiseize on the bolt threads wouldn’t hurt, since all these bolts live in a fairly corrosive environment. You do want to torque them down well (especially the center strap), since you don’t want your gas tank to wiggle loose over time.

Recap

After having the Wag sit immobile in my driveway for a week doing all this, after I finally drove to the gas station & filled it up, the gas gauge STILL only goes up to 3/4 (or occasionally 7/8) tank when the tank is really full. That’s kind of frustrating, but I think the problem must be in the gauge, not the sending unit. The empty mark is still accurate, so I don’t really care too much about it right now. I haven’t driven it enough yet to build up pressure in the gas tank, so I don’t know yet if I’ve fixed the problem of fumes in the cabin. I’ll know more in a couple days.

The transparent filter is nice to have. After driving only two blocks to the gas station, I checked the filter, and was amazed at how much crud had already collected there. Probably a result of having a slightly longer pickup hose and maybe knocking loose some crud after moving the tank around all week. Just to be safe, I bought some new elements for the inline filter and a new replacement for the stock filter up by the carb.