A '79 Full Size Joy

[Simo]

Tank Mods

The posted tank dimensions didn’t tell everything about the tank as they all have two plateaus or levels at the top of the tank and this is important because the raised portion has to fit between the rear axle and the transition in the FSJ floor where it steps down from the rear seating and cargo area to the front floor pans. After comparing them side by side I felt the short bed one had a better chance of fitting and proceeded to scratch up a new short bed C10 tank (I got from summitracing, SUM-255008, $93). The length and width were good but it is a little taller than the FSJ tank and its raised portion is longer meaning when elevated up into position it crashes into the FSJ crossmember that secures the oe fuel tank strap. Also, the C10 tank hung down below the frame rail. I wanted to reuse the original FSJ fuel tank skid plate without losing any ground clearance. Committed to making the best of it without cutting into the FSJ floor or crossmember, I proceeded down the path of modifying the C10 tank to fit.

Let me just pause for a moment here with a cautionary disclaimer whenever working with fuel tanks. This was a brand-new tank never exposed to fuel and devoid of any smell or other volatile chemicals. Still, special care was considered before the first spark was created. While a used tank can be carefully cleaned and have oxygen deprived exhaust gases flowed through it to displace combustible vapors, I cannot stress enough that putting a source of ignition anywhere near a used fuel tank is extremely dangerous.

Ok, that said, I knew about how much the C10 tank hung below the frame rail and therefore about how much I wanted to take out of it, about 2” should give adequate clearance. I debated on where to cut the tank so the top and bottom portions would realign best because the tank does have some tapering so it wasn’t going to be perfect.




After a butt welding attempt blew through the thin ~21 gauge material, I separated the halves again and ended up sliding the top half of the tank just inside the bottom half. The seams formed together for a much easier lap weld but it gave up another ~1” of tank height I hadn’t planned on losing. So, the 16 gal tank is down to about 12 gal capacity after taking about 3” out of it.

Taping the halves in place just before stitching it back together.


Because the ends weren’t perfectly vertical, some mating edges had to be folder over or pressed together in spots.


All glued back together

[Simo]

Tank Sealer

To check for leaks Id fill the tank with water which also doubled purpose to give the raw untreated metal seam a nice flash coat of patina, I mean rust. For the purpose of preventing future rust and sealing any future pinhole or stress crack, there are several fuel tank sealing options out there and most seemed to have decent reviews. I chose POR-15 fuel tank sealer 49208.

I first cleaned the inside of the ‘new, yet modified with some flash rust’ tank using an acid wash then rinsed several times and allowed it to dry for a few days, letting a hair dryer blow through it for several hours to ensure the overlapped seams also cooked out. The sealer covered the surfaces well and needed only a few hours to harden, yet the instructions require 96 hours to cure before use. For what it’s worth, I also painted the outside of the tank’s raw metal seam with the same stuff as its gray color matched the tank’s original grey paint pretty well.

One mistake I made was putting all 16 oz into the small ~12 gal tank, it was probably enough for a tank twice that size. This tank’s ports are somewhat recessed meaning I couldn’t simply drain the tank of the excess sealant and trying to slosh it out of the opening for the fuel pump module just made a big mess. I tried wiping some out with a towel but eventually just resorted to repositioning the tank every so often thinking that would help evenly distribute the contents as it dried. Instead it just created some uneven ripples and globs as the partially dried excess goop wasn’t self-leveling out again. So next time I’ll know better; at least it’s not going to leak.

[Simo]

Tank Mounting

I chose to mount the tank to the FSJ skid plate. Several pedestals of angle iron retain the tank fore/aft and laterally including metal straps fastening it down to the skid. The area is lined with some thin rubber matting to prevent metal on metal abrasion. Once mounted into the rig, there is enough clearance to pull the shortened fuel-pickup / sending-unit without dropping the tank. The full FSJ skid is temporary, when and if I even put a centered transfer case and rear axle in I’ll lop ~6-7” off the side of the skid and make new brackets to make it clear a different drivetrain.



The fuel tank sending unit (NAPA PN STPFG05F), was shortened to fit the modified tank. The GM sender is 0-90 ohm unlike the bosch sender 270-33 ohm I used in the XJ tank paired with the autometer quad gauge so Ill still need to decide what to do for fuel level monitoring on the C10 tank. Im thinking about installing either a dash pad, A-pillar, or overhead mounted pod for a few extra gauges including the new tank’s level.

[Simo]

Dual Tank System Concept

I debated between several concepts, the assembled system can better be described with an illustration:

[Simo]

Fuel Filling

I was really trying hard to figure out a selectable fuel fill valve to fill each tank independently through the same exterior body fill port because I did not want to graft another fuel fill port into the body if I could help it. The only selectable dual fuel fill device I could find was from an FJ80 Landcruiser, but they were like $500 to import from Australia if you could even find one used on ebay or a Landcruiser forum. I considered trying to assemble something using fuel safe manual ball valves but couldn’t find much in the 1 to 1.5” dia range and was imagining I’d have to leave an access hole in the inner quarter panel cubby hole to flip the valve anytime I wanted to fill up which would be an extra hassle. So I gave up on the selectable fill valve concept and have a simple 1-1/2” Y-fitting so the tanks will be filling somewhat simultaneously. Hopefully it doesn’t back up once one of them reaches capacity and make it harder to fill the remaining volume in the other tank, this is why I chose the Y-fitting rather than a T-fitting here.

The 1” FSJ undercarriage fill transfer port gets adapted up to diverge through a 1-1/2” Y-fitting. The FSJ fuel filler vent gets adapted from 3/4” to a 5/8” Y-fitting as both tank’s fill vents are 5/8”. I got the Y-fittings from ReplacementBoatParts, PN: PLA1134 and PLA1140.

Here’s a shot of the FSJ ports at the base of the inner quarter panel and the filler hoses to each tank before plumbing in the Y-fitting.


I had originally planned on adapting up to the 1-3/4” C10 hose at the Y, but that would be too big to fit in the gap between the body and frame and then bend to pass through the dimpled hole in the crossmember so the FSJ’s 1-1/4” filler hose had to be reused then adapted up to 1-3/4” only at the C10 tank. That is a lot of hose clamps.

[Simo]

Tank Selection

Some GM trucks have these ‘Pollak’ fuel tank selection valves; with the flip of a momentary DPDT switch the valve shuttles position from (in my case) the Rear spare tire located XJ tank to the Left Hand frame rail mounted modified C-10 tank. We’ll see how reliable the valve is, I see a lot of dual fuel tank troubleshooting and replacement videos on youtube so that doesn’t add confidence. I’ll also try to continue running the mechanical fuel pump for now but time will tell if it has problems transitioning from one tank to another especially if it ingests a slug of air while priming the section of line between the newly engaged tank and tank selector valve. Alternative concepts could include use of an electric fuel pump and/or dedicating one primary tank to supply the engine with no tank selection while the auxiliary tank would have an electric pump to simply transfer its contents into the primary tank once the primary tank level was sufficiently low enough to accept it.



I mounted the fuel tank selector valve (NAPA PN CRB22198) under the floor between both tanks in order to have the shortest suction from each tank possible. For my purposes the valve only needs 2 wires. In one position it shuttles to the Rear XJ tank; reverse polarity momentarily and it’ll shuttle to the Left Hand frame rail mounted C10 tank. The other 3 wires are for fuel level sensing (1 to gauge and 1 from each tanks’ sender) but Im not currently using it for this function since the senders have different resistance ranges from full to empty. I found several helpful wiring diagrams for it online, here is just one random forum thread that I found to have a clear and simplified diagram: http://67-72chevytrucks.com/vboard/s...d.php?t=761187 and the manufacturer also has a good old faded diagram too: http://products.pollakaftermarket.com/Asset/IS-41_b.pdf



Mounted and plumbed


Rear XJ tank plumbed to selection valve.

[Simo]

Tank Selecting Switch

For the selector switch, I stuck with the same C10 application. I didn’t see a great spot to mount the switch vertically as best intended. So I removed one of the manual choke pull knobs (this edelbrock has electric choke) just to the lower right side of the steering column and in its place ground out a larger slot big enough to fit the GM fuel tank selector switch (Napa PN ECHFS500) mounted sideways. I think a cheaper DPDT momentary switch would have worked just as well, I liked having the labeling, albeit sideways, and I only need to remember 1 model application if I ever need to replace the tank parts, valve, or switch.

[Rusty76]

Wow Simo awesome update! Love the roof. If you don't mind I might copy your idea.

[Simo]

Update on Interior Paneling

The interior wood paneling I made was already starting to get dirty and show evidence of moisture getting through in some spots. So I pulled it out to apply some staining & polyurethane to see if it helps the look and water resistance any.

Passenger pre-staining and driver stained.


One of the headliner sections after the ZAR modern walnut 115 stain and a clear satin minwax was applied.


Door panels reinstalled. I still need to find the screws for the driver arm rest and yes that is an old 1-1/16 socket functioning as the driver door’s window crank knob.


One of the rear panels:


While the headliner was out, I added some more sound deadening sheets I bought to the back sections of the roof I had previously only covered 50%.


I made a 6 inch wide piece to bridge the front and rear headliner section together, it secures to the middle roof crossmember.

[Simo]

Glove Box Gauges

I had been thinking of where to mount some more gauges, I wanted a fuel level for the second tank, trans temp, vacuum, and AFR in addition to actually mounting the 4wd selector (it has the part time kit for 2wd and 4wd). I debated on putting a gauge pod on top of the dashboard, on the A pillar, and even one hanging just above the rearview mirror.
The glove box wasnt offering much purpose for me except access to a mess of wiring. Even though I wanted more engine tuning data I don’t want to be distracted by a red flashy AFR number all the time and having these less important clusters in the glove box for normal operation gives the simple option of closing the door - out of sight out of mind. If Im tuning something or feel sporty I can just open it to look at vacuum and AFR, or if Im pulling a mtn ascent with a load I can check trans temp, and if im running off the auxiliary tank I’ll know when to switch tanks or refuel.
The removable panel will end up being about 17 inches wide, most of the gauges are about 2-1/16” in diameter and spaced about 3 inches apart. I don’t have a press brake, but I was able to bend some AL sheet metal using some angle iron, C-clamps, and persuasion.





I do have a spool gun and could buzz the joints together but the bends alone really give it some structure and I don’t feel like it needs to be welded. After a test fit into place, the base was tapered so it angles to face the driver better. It secures at the bottom with some self-tapping screws.


Then made some removeable side panels to cover the gaps

[Simo]

Glove Box Gauges

Despite the walking billboard of gauges I wanted to hold a common theme – black back, white lettering, and red needles but just couldn’t bring myself to spend 2-3x on a VDO or Autometer 0-90 ohm fuel level gauge so this $16 Equus with chrome bezel and white needle is in there until I get over it, take it apart and paint the needle & bezel, or pony up a few more bucks. It’s hard sometimes being a cheap bastard with a hint of OCD. Anyways, the panel is populated.



A 12v barrier terminal block is mounted on the back so all +12V IGN, Light, and GRND are tied to their respective buses with only 1 set of wires needing to connect into the jeep. The fuel sender, AFR wideband O2 harness, 4wd vacuum hoses, mechanical temp, & vacuum gauge tubing also mate up with enough slack to semi comfortably connect everything outside the glove box then install as an assembly.



I positioned the trans temp in the cooler return line at the transmission where the return hard line adapted down for a 3/8" flare to 1/4" NPT. I've adapted the trans, sender, and hard line into a 3-way 3/8" NPT block which was pretty tight clearance to the transmission bellhousing. The hard line did need some massaging to line back up with the new orientation. I heat wrapped the exhaust to keep the radiant transfer down as much as possible.



I originally mounted the wideband O2 sensor in the exhaust after both banks merge into a single 3 inch pipe, between the spun catalytic converter and tubular muffler. A 7/8 inch hole was drilled a little above level for moisture and clearance to driveshaft reasons. Edit - It needs to be upstream of the converter, so I ended up moving it and plugging the first downstream hole.

Instructions on Sensor placement
- At least 18 inches downstream of the cyl head exh port
- Upstream of any catalytic converter
- as far as possible from exhaust outlet
- oriented at least 10 deg from horizontal to allow condensation to drain



In.

[DarkMonohue]

The wideband O2 sensor is mounted in the exhaust after both banks merge into a single 3 inch pipe, between the spun catalytic converter and tubular muffler. A 7/8 inch hole was drilled a little above level for moisture and clearance to driveshaft reasons.

Just a second - is that oxygen sensor downstream of the cat? It should be before the cat so that the ECU sees what the mixture is before the cat scrubs it clean.

[Simo]

Just a second - is that oxygen sensor downstream of the cat? It should be before the cat so that the ECU sees what the mixture is before the cat scrubs it clean.

There's no ECU making adjustments here but you're right. i should have read the directions better before installing it. This was where it fit best with clearance to driveshaft. Ill have to order another weld in bung and plug to move it just forward of the catalytic converter. On my exhaust, the banks merge just in front of the converter so i wont have great mixing but that's the best i can do now unless i choose to put it in the downpipe off just one bank.

[DarkMonohue]

No ECU... sorry, I've been following the thread, but didn't stop to think that the truck was still carbureted and the sensor was just for tuning. Even so, yeah, it should be upstream of the cat.

Sampling from one bank is just fine. Eight cylinders is theoretcally better, but there's nothing wrong with sampling from four.

Those bungs are really pretty close to universal. It's a common M18x1.5 thread that almost every o2 sensor uses. Any corner muffler shop should have dozens on hand if you want to buy one locally for a few bucks rather than waiting for an order to show up.

[rang-a-stang]

Awesome! I plan to do the same thing with my glove box. I also plan to install various switches in there. I think the factory dash is so good looking and do not want to clutter it up with extra gauges, switches, or indicator lights. Your panel could also work for your Brake controller if you wanted...