I just bought a 1976 F-150 Supercab with a 390 FE and auto transmission

[Problem 1]
I was running the truck in my driveway and it died. It turns out the AUX tank was empty. So I switched to the MAIN tank using the switch
on the dash (Next to the heater) and it wouldn't start, even though it has over a half a tank. So I filled the AUX tank and it fired right up.
Does the switch just tell you the amount of gas in each tank or is it supposed to switch the fuel supply to the engine.
Any help would be appreciated.

[Problem 2]
When running the truck at idle, the oil pressure seems to be at the very edge of the safe zone at the low side.
When driving it reaches a little less than half way up the gauge. Is this normal or is my engine just about worn out.

 

I believe there are two switches. 1 switches the fuel gauge, the other switches tanks. I THINK the tank switch is on the floor? or seat? Near the trans hump? That's what I think I've gathered from info on the forum. I don't have two tanks so I can't say with certainty. Someone will come along soon and confirm or deny lol

Have you checked your oil level?

 

When you switch tanks, not only should you pull fuel from selected tank but your fuel level should correspond. Both of my '79s operate this way. Heck, cruising down the highway there were several times I'd run one tank dry hit the switch and it'd just fire back up and i didnt miss a step.. but i knew that both tanks were good and my switch worked.
Can't imagine that it was much different in '76

 

So typically when the needle goes up on your oil pressure gauge it's because you're building oil pressure. This is good, cause it pushes into the bearings. Middle of the guage should be no worries.

 

Because these factory gauges tend to be "approximate" and an old tired sending unit can cause the gauge to read low too, it would not hurt to at least verify the pressure with a temporary mechanical gauge. Just so you know what your new-to-you truck is trying to tell you.
Yes, the engines do get tired, and the pressure can drop. But it doesn't sound like yours is in the danger zone yet, and it would be good to know if it's even close.

And yes too, you could throw a new sender at it for just a few bucks (not sure how much they cost these days though), but I prefer checking stuff more accurately. Even if it means spending even more money on a new gauge you won't keep on all the time. Or might...

For the gas tanks, the earlier trucks did have a floor mounted valve, but I would think if yours was so equipped you'd have seen it by now. It's pretty obvious from the ones I've seen. Not sure if your '76 would have had them though, or if the dash switch does both like they do on the later trucks.
But you can crawl underneath and follow the fuel lines (especially from the rear tank) to see where they go. And if it's an electric solenoid valve you can test it to see if it's still working or is in it's default position only.

First thought was that it needed to re-prime the pump to fire up. But once you said it fired right up with the other tank, I figured that was not the issue. However, if you had also primed the carb with some gas to get the engine fired first, the running engine pushes the pump hard enough to pull gas from a tank and into the dry lines. So thought I'd at least bring up the possibility.
If you didn't do anything special though, and it just fired right up, perhaps the valve is not working, or the hose has air holes in it causing it to suck air instead of only gas, or it's disconnected.
Hence the suggestion to at least inspect the lines to make sure it's got some, and they're not leaking.

Good luck.

Paul

 

1975/79 F100/350 with factory installed dual fuel tanks have a fuel tank selector valve and solenoid assy mounted on the cross member.

The switch located in the heater control panel changes the tanks & the dash fuel gauge from one tank to the other.
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1973/74 F100/350 with factory installed dual fuel tanks has a manually operated fuel tank selector valve, located on the floor adjacent to the drivers side of the seat.

The switch located in the heater control panel only changes the dash fuel gauge from one tank to the other.

 

My info is accurate just for earlier years. Don't say I'm wrong for saying I think trucks have two switches and then in the same post say 73/74 trucks have two switches... I never said any or all trucks only have two switches... I just said I thought some trucks did... which is correct

 

What truck has TWO switches? NONE! And I never said that 1973/74's have two.

The 1973/74 switch (D3TZ-9A050-A) located in the heater control panel only changes the dash fuel gauge from one tank to the other.

The 1975/79 switch (D5TZ-9A050-A) also located in the heater control panel changes the tanks and the dash fuel gauge from one tank to the other.

1967/72's also have only one switch, it's the toggle type located under the left side of the dash mounted to a bracket.

Where is the other switch?

I've been looking up these parts since these trucks were new, so I know what the hell I'm talking about. You obviously do not.

 

If the 73/74 switch located in the heater panel only changes the dash fuel gauge... then how do you change to the other fuel tank? Pretty sure you said there's a fuel selector on the floor adjacent to the drivers side of seat. Is that not two switches??? Edit**geez! Unless you are being that petty and I am calling it a "switch" and you are calling it a valve... they both mean you "switch" which tank is getting gas**

Never said you didn't know your stuff, never said I was a know it all... just don't always have to be an *** when passing on your knowledge.

 

1973/74 with factory installed dual fuel tanks:

Have a manually operated fuel tank selector valve (see 9189 in pic) located adjacent to the drivers side of the seat, so you have to put your mitt on the lever to change the tanks.

Selector switch (9A050) only changes the dash fuel gauge from one tank to the other.

 

Question 2: My level drops after a few miles of driving. Started to bother me. I put a mechanical gauge on it. Reads fine. Will get a new sender soon.

 

The fuel sending unit float is available by itself and is made from two pieces of brass soldered together.

Solder breaks down, gas seeps in, float settles to the bottom of the tank.

C0AZ-9202-B (replaced B7A-9202-A) .. Fuel Sending Unit Float / Available from Ford & all the repro parts sellers.

Same: 1957/79 FoMoCo Passenger Cars & F100/350; 1961/79 Econoline; 1966/79 Bronco.

 

I ended up screwing a mechanical gauge in the engine and its only got 10 psi in neutral and 5 psi in gear. (At operating temp)
While driving down the road, it only goes up to 15-30 psi max. I'm gonna end up rebuilding or replacing the engine...