Hey guys,

Rebuilding my carb today and while I was waiting for the kit to come in the mail I was going through the rest of my fuel system to see if there was anything else that could potentially be starving my engine for fuel during operation. Truck is an 84 XLT with a 302 V8.

I still have vacuum leaks to seal, timing to be adjusted and of course the carb to be rebuilt today, but I also remembered the P.O. telling me that the fuel selector switch was busted and only the front tank worked. That info combined with another member posting photo of old rotted fuel line from tank causing fuel pressure/flow issues I went underneath to check things out and indeed the rear tank had a very rotten line coming out of it, not leaking but looked like it was probably crumbling inside. Since I knew the switch was broken I disconnected the top (front) fuel line and a little bit of fuel came out, but not a ton, just what was in the short line. I went to disconnect the other side of the switch closer to the engine and fuel started gushing out. I ran the line into a 5 Gallon gas can and it filled it all the way up and was still coming out so I reconnected the hose.

I thought it was strange that A, the fuel was coming out so much without the front mechanical pump running, and also that the switch was actually on the rear tank and not the front like he thought…I ended up cutting/grinding the end of a carriage bolt to fit inside the tank end of the rear tank line and plugged it with a tight hose clamp. No leaks so far. I then routed the front tank with a longer hose directly from the tank line to the hard line on the engine side of the switch. Then I unplugged the vent Tee connections on the passenger side rail towards the back wheel and connected only the front tank vent and left then rear tank open to avoid pressure build up.


Does this all sound safe and like it will work? I don’t think there’s anything else in the fuel tank switch besides a valve essentially right? Just want to make sure it’s not going to cause any fuel pressure issues. I’m rebuilding my carb tonight so I haven’t started it yet but just wanted to post for extra peace of mind.

thanks y’all

 

A few things to consider. The PO may not have been correct about the tank selector switch. For all we know, the switch and valve may be working fine. Could be a blown fuse for the circuit. Could be a plugged line or who knows what. I think all he really knew was one of the tanks wasn't feeding the engine, for reasons unknown.

Your truck has a 3-port valve, just like mine. It has an input from each tank, and one outlet to the fuel pump and engine. Unless the valve were to jam mid-travel, only one tank is connected at a time to the outlet.

This type of valve is spring-loaded to one position. On later models like yours and mine, the rear tank is the default position. (On earlier models, the front is default.) With power off, the valve automatically moves to the default position, in your case the rear tank. To feed from the front tank, you must apply power via the selector switch. But with the ignition switched off, the valve will spring-load to its default position regardless of the switch position. That's why you got 5+ gallons (from the rear tank) with the line disconnected downstream (forward) of the valve.

The mechanical fuel pump had nothing to do with all that fuel draining. It is also downstream of the valve and was disconnected from both tanks once you removed that line.

You may have had a slight pressure built up in the tank. A warm day perhaps and some thermal expansion? When you disconnected that line, pressure in the tank started the flow. There was probably also some siphon action to keep the flow going.

 

If the line in the tank is full of fuel, it will flow out and keep flowing till the tank is almost empty or level with the fuel line outlet. It's just like siphoning fuel out of the filler cap. You stick a hose down in the tank and suck the fuel out, and once you get it going you quickly drop the hose down and it will keep going as long as there is fuel in the tank and the hose is lower than the fuel in the tank.

If the tanks are old and crusty, you may have a battle on your hands until you get it figured out. You might end up getting a new tank.

 

That valve and system is the easiest of all our trucks have.

As said the valve defaults to the rear, at least all the ones I have had off Ebay.
Yes do check the fuse to make sure it is good, test light works best.

Using a test light hook it to the wire at the valve.
Key on move the tank switch between the 2 tanks.
You should get power with the switch to front tank.
If you get no power at all for either tank then more digging is needed.

I would check the frame harness for any cuts or open wires.
There is a plug under the booster that connects the trucks main harness to the frame harness.
Check if it is clean and then check for power there for the tank valve.
If still no power going to have to check at the switch.

May try flipping the switch many times because if not used they get a funk on the contacts.
Sometimes the flipping will get it to work and sometimes not.
I think you can still get the switch from local parts stores.

Oh do you know if the fuel gauge works between the 2 tanks?
The dash switch besides sending power to the valve also swaps for the gauge.
Dave ----

 

Dave, I'm old too, but doesn't the switch control the valve and there's a switch on the valve that swaps the sender?

 

That's true on the 6 port valves with the return system. The simpler systems with just a single fuel supply line did the sending unit switching in the dash switch, and one simple wire going to the plumbing valve.

 

As Dave said and what I have on my truck.

What shocks me is he has the 3 port valve as I thought he would of had a 6 port?
Dave ----

 

How come? You only need the 6 port when you have a return type fuel system. Looks like he has a 302 with a carb, which would not have had a return fuel system. All 460's with the hot fuel handling had a return line, and all the diesels had a return line. The fuel injected trucks had a return system but they seemed to use the mechanical fuel reservoir for the switching. I am suspicious though, that some of the very early fuel injected trucks did use the 6 port on the low pressure side of the system before it went to the high pressure pump