38 Gallon Rear Tank and 1984 Hot Fuel Handling System

I'm planning on upgrading my tank in the rear of my '84 F350, I've asked around in a few groups and did a fair amount of research and came down to these questions.

1. Which tank will work with a 1984, with electric pumps? F26c (listed as correct for 1984 on rockauto) or F26d (listed for 1985 trucks)

I ordered a 1984 sending unit for my rear tank not realizing the amount of rust in it, and it fit but I can't use the tank because of the poor condition

2. Can the hot fuel system sending unit be correctly modified to work in the 38 gallon tank? There isn't much info on modifying the senders at all, and even less for the '84 electric pump trucks.

The 460 running one 19 gallon front definitely doesn't have the range I want, and two 19 gallons isn't that much better at 4mpg loaded. I use the truck for long-ish distance towing and will soon have it in a shop for a laundry list of work since its downtime season. I'd like to see how hard the upgrade is, if it's even worth it. I won't be carrying a spare under the bed, so that won't be a worry.


Thanks in advance, y'all!

There's not much to extending the pickups in these trucks to the bottom of a deeper tank.
I might want to add a piece of Holley Hydramat to the pickup, but it's expensive and all these trucks worked fine without it.

It's not like the Aeronose trucks with a FDM that sits in the neck and has to reach to the bottom of the tank.

 In 1985 trucks went to in-tank EFI pumps and have a larger (4"?) bung.
So, if you already bought a new sender for an '84 I'd think you need the 2 1/2" bung found in the F-26C tank


There are some threads about using a 33 Gal Bronco tank and some threads mentioning the metal tank or big poly tanks intended for diesel.
I can help you search for more information on conversions, but if you're only wondering what the difference is, it's that '85 & up have a bigger hole and retaining collar.

Spectra also offers the LO26A fuel pump extension kit for $31at Summit.
But it's basically an assortment of tubes and clips and bushings that seem like a catchall to fit any of their deeper tanks.

I'd probably just get a piece of tube and swedge it in there...

Thanks for the info, I'll probably end up running a 19 for now just to get it all working (it's been on the front tank for 20 some years and I doubt my shop would be ok with cutting up a sending unit) and upgrade later. Now I know which one to get and how in depth it'll be. Also thank you for the link for the extension kit, that helps a lot!

Glad I could help point you in the right direction.  

I thought my truck at about 10mpg was expensive. I can't imagine 150 mile range at 4mpg....  

What's the gearing in your D70?

4 mpg? What gear is in yours? Darth always got a solid 10 mpg empty or loaded, 7 - 8 towing a 30 ft 5th wheel. This was still running the carbureted 460 and C6 and at the time still had all of the original factory exhaust system.

I agree with Jim, extending the pump mount down so the pickup is in the low point of the tank and hopefully a gauge sender that reflects the actual fuel level. I think I still have the original rear 19 gal pump and sender from Darth, but don't know if the pump is any good.

FWIW, after converting to EFI and E4OD, even with quite low compression (early large dish pistons and early EFI large chamber heads) Darth delivered 12.5 mpg on a long high speed (70 mph) run after the updates.

Since we are talking about fuel tanks and getting more range for a 460, I have a couple of questions about my restore/build of my grandad's truck.

The truck came with only one side mount 15 gallon (or whatever it is) tank.

Since it was new it's had an aux has tank mounted below the flatbed as seen in this picture.



The way it was rigged up to use was having and electric pump mounted on the frame rail and pumped to the filler neck of the stock tank. It was done via manual switch that was easy to leave running.

How would you guys suggest managing this type of dual tank in more of an automated solution?

This tank is mounted lower than the stock tank and I would like to avoid anything like Ford's failure prone method with the switch on the dash solenoids etc..

Sorry, not trying to thread jack, just curious how one would run two tanks?

Off the shelf: I don't know.

Custom (NERD ALERT!): Does the aux tank have a sending unit? If so, if one was skilled enough with electronics one could use a microcontroller (arduino, etc) or Op-Amp circuits to make something that would turn on the aux pump when the main tank got below a threshold but not shut off until it was nearly full (or another "higher" threshold), but only if there is fuel available in the aux tank. If going the analog op-amp route, you'd need a voltage comparator for the latter and a positive-feedback amplifier for the former (to give the hysteresis needed to prevent short-cycling the pump and control relay), plus some buffer amplifiers to keep the low-impedance sending unit circuitry from getting messed up by the added circuitry. On par with some of the things I had to make in my EET studies... Op-Amp circuits controlling a transistor controlling a relay... Fun...

Custom #2 (Less of a nerd alert?): use some relay circuitry (or an Arduino) and a push button with a kill switch. Time how long it takes a full aux tank to transfer (or whatever amount you generally would want to transfer at a time) and set the timer relay (or Arduino code) to match. Push the button, relay latches (wired in a latch configuration) until the timer cuts the circuit or someone hits the kill switch.

ratdude747 wrote

Off the shelf: I don't know.

Custom (NERD ALERT!): Does the aux tank have a sending unit? If so, if one was skilled enough with electronics one could use a microcontroller (arduino, etc) or Op-Amp circuits to make something that would turn on the aux pump when the main tank got below a threshold but not shut off until it was nearly full (or another "higher" threshold), but only if there is fuel available in the aux tank. If going the analog op-amp route, you'd need a voltage comparator for the latter and a positive-feedback amplifier for the former (to give the hysteresis needed to prevent short-cycling the pump and control relay), plus some buffer amplifiers to keep the low-impedance sending unit circuitry from getting messed up by the added circuitry. On par with some of the things I had to make in my EET studies... Op-Amp circuits controlling a transistor controlling a relay... Fun...

Custom #2 (Less of a nerd alert?): use some relay circuitry (or an Arduino) and a push button with a kill switch. Time how long it takes a full aux tank to transfer (or whatever amount you generally would want to transfer at a time) and set the timer relay (or Arduino code) to match. Push the button, relay latches (wired in a latch configuration) until the timer cuts the circuit or someone hits the kill switch.

No sending unit in the aux tank.

I was thinking something along the lines of a timer. I have also used a speed switch in the past for another application, but could hook it up to a relay to trigger the pump at a certain speed and turn off when it's below that with a manual override.

C6 power eater, 4:10 gears, a holley 670 truck avenger (on it when purchased) and a 2.25" dual through flow master super 10s that I need to change. It melted my carpet from the heat on the driver's floor. Can't imagine what the poor fuel tank switch valve is enduring.

Previous owner claimed he towed with it. All these issues and a failing cooling system flared up soon as I put a load on it so I'm saying he lied. 4 radiators five flushes and a thermostat later...

It drinks like nothing I've ever experienced. 10 bobtailing, 6 empty trailer, 4 loaded trailer. Secondaries stay open for 90% of its life even on smaller grades.

Extremely gutless thing, I'd always heard 460s we're powerhouses. I felt lied to when I put the 4400lb goose on it for the first time. Most I've had behind it is ~15000lbs and I wouldn't want that ever again. Usually pulls around 10k gross trailer weight.

E4od is in the works when compushift fixes their supply chain, and the timing adjusted to 12° base vs the factory 8. Really woke it up after that. Carbs been tuned on some, but I'm planning on installing a vacuum gauge and afr gauge to really dial it in. Eventually going TBI or stand-alone MPFI. But that's too big-money right now. I still have to switch the front hubs and rear end out to get the correct stud size to ditch these 16s. I'm ready for my 19.5s, dang it!

I've got 4.10's and a Holley 750....
Yeah, that C6 is very good at turning torque into heat.

But I'm going to suggest you replace your timing chain for one from an EFI 460 ('88-'96)
These don't have the 8° of crank retard (4° at the cam) that were put in for federal emissions and make a noticeable improvement in power.

My truck gets 10.
Doesn't matter if I'm towing a car, but I'll admit that or a big camper is about all I've done.
With the Zf-5 I get 12 or a little more when highway cruising.

Even with the C6 I think it should do better than that, and agree with Jim - something's wrong and cam timing is very suspect.  And if that doesn't make a big difference I'd do a leakdown test to check the rings, valves, and head gaskets.

If you are getting that kind of heat in the exhaust my guess would be extremely late timing. I would check two things, (a) mechanical advance in the distributor (b) vacuum advance.

As for exhaust, Darth has the factory mid 1984-1987 system, dual 2.5" pipes to the muffler and then a 3" tail pipe over the rear axle and exiting through a 3.5 or 4" outlet that originally had a pair of spark arrestor screens in it. I did add some extra insulation when I redid his interior. A strip in addition to the factory one over the exhaust route.

One item, the factory systems frequently used double wall pipe and at highway speeds the pipes will get quite hot near the front, to the point that hitting a puddle at speed can chill the outer layer enough to make it contract and squeeze the inner layer enough to collapse it and severely restrict the flow.

I'll be recurving the distributor and possibly doing a timing set (kind of under a shade tree so anything beyond a few hours work is difficult) I know my vacuum advance is fine because I can see the change on the timing light and hear it in the exhaust when I pull the hose off. Also still has the temperature activated vacuum solenoid so it will noticably change over when warming up. I've dug mildly into the weighted advance and found everything I think should move moved. I'm still learning from coming out of the small engine world where "timing is this, jetting is this. that's it."

That being said I've had some great help along the way and learned a ton on carb tuning for these holleys with their wizardry. I'm also going to be changing my entire exhaust system soon to something quieter and all passenger-routed. Still true-dual but gets the heat out from under my feet, and the fuel lines.

The entire truck has been a process of undoing neglect and "cool mods" to make all the tow pig stuff functional again, as well as slowly deleting the smog system, which the pump did itself by seizing and roasting the belt.

My goal is to have everything fully functional about the time I'm ready to rip the engine out and have it built to maximize the amount of power I can get out of 87 octane and dump an overbuilt E4OD in. It can guzzle all the fuel it wants if it makes acceptable power. I like being different, don't really care for diesels, and enjoy passing them with a trailer usually larger than theirs just to see their faces!

460's have 2 1/2" headpipes from the factory.
I can't imagine you went any smaller.

Have you considered adding a heat shield to the floor?
I see a bunch of them on the U-Haul surplus eBay page (U-techcenter)

I've seen a bunch of true dual setups where the owner or shop put two oval mufflers on edge under the passenger side of the bed, but you'd really be better off with an X or H pipe before the mufflers.

The water temperature vacuum switch in the distributor advance only comes in above 230° to step up the idle if the truck is overheating in traffic.
Otherwise the advance sees vacuum through a tiny restriction (V-REST)

Doing a timing set is definitely not something you're going to accomplish in a couple of hours.
There's just too much that needs to be removed and a 35 year old timing case is likely to be corroded to hell (as well as the water pump backing plate) but it will definitely improve torque and top end as well.

BTDT, bought the tee shirt.... many times.  😉

My switch must be tired, as soon as the thermostat opens it bumps the timing up to full advance at idle. I wasn't the one that installed the exhaust, the previous owner used it to try and hide massive manifold leaks that I've since had fixed (I pity the guy who did all those helicois and snapped bolts.)

The truck recieved a "new engine" in 1991 at just 50,000 miles and the internals are a mystery. It sits around 95000 now. Consumes about a quart of oil every 3000 miles, more if it spends any time on a trailer. Never smokes, though and the oil pressure is typically toward the high end of the gauge, it'll almost peg it when cold which tells me the oiling system is at least in good health. Temps stay low (now, at least) right around 190.

It has been overheated to destruction-level temps due to failing radiators and a freak issue with the thermostat shutting mid-trip, but never complained and came right back to life. Cooling has been my biggest issue, but now all components are well sorted and with corrected timing it sits happily never getting over 220 even on those long mountain grades coming out of the valley.

Yeah, that's not right.
I suspect that someone has the distributor vacuum line hooked to the EGR thermal valve.

These are color coded but I'm not certain of the colors used for 1984.
Do you have two switches, blue and yellow?
Do you have the emissions and vacuum routing diagram in place on the radiator support?

ArdWrknTrk wrote

Do you have the emissions and vacuum routing diagram in place on the radiator support?
Is there a compliance sticker on the valve cover?

There are six possible emissions calibrations for 1985 MY 7.5l automatic trucks.
I think Gary has the 1984 version of the factory Engine/Emissions Facts Book

The one thing I can say about those VCV's is that in all cases direct manifold vacuum goes to the port closest to the thermostat housing. Restricted vacuum goes to the farthest port and the distributor is connected to the center port.

But I can't begin to look up the vacuum restriction number without a calibration code.