I finally got to take the 65 on a proper road trip this past weekend and drove it 2 hours to mom and dad's house. The trip went well and the truck drove great, until I got a bit of a head scratcher. The truck was running flawlessly down the highway and I stopped to clean the windshield/hit the restroom and when I went to get back on the highway, I was accelerating and there was a noticeable loss in power and then it died. The fuel system consists of a 16 gallon mustang fuel tank and sending unit, both of which were purchased new for this truck. The fuel line was also replaced when the tank was put in and is still nice and green in color with no corrosion. The mechanical fuel pump is the 300 stock unit and the carb is a recently rebuilt carter yf. I pulled the fuel line at the pump and had nothing coming from the tank. I put my finger over the pump line and had my daughter turn it over and there was good suction so the pump was working. I tapped on the tank and it sounded very hollow. Luckily we were within sight of a walmart and I bought a gas can and put a gallon in it. Still no starting until I put some gas in the carb, but it took a minute. I took it to the gas station in at the exit and 8 gallons filled it up, so I know I had half a tank. It started right up after the fill up and got on the highway no problem. I stopped on the side of the road a little further since it was going to rain to grab my daughters bag and my tool bag from the bed and when I accelerated it faltered again with a full tank. A down shift bringing up the RPM kept it running this time. This was all between 10 and 11pm. If I take off slowly, it doesn't happen and never at cruising speed, just acceleration. I have also verified there are no fuel leaks. My first thought was maybe vapor lock, but the engine runs at 140 all day long so I'm ruling that one out. Since I had no fuel flow with half a tank, I'm thinking the filter sock on the sending unit came loose and is blocking it. I found references to mustang owners that have experienced almost the same deal, but did not see a definitive solution listed. My next thought is to drain the tank and check the sending unit. Any other ideas?

 

How is your venting? Vented cap?

 

Plugged vent? Not sure if on your tank it's venting through the cap or elsewhere.

Edit: Cheddar beat me to it.

 

Just because the engine is running cool (140 is too cool, you need a higher temp thermostat) doesn't mean the fuel line isn't near a heat source. Are you sure the fuel line isn't routed near the exhaust anywhere?

 

There is no vent on the tank and I believe the cap is vented. Looking at the inside there are holes for what I think is the vent. I don't hear any vacuum sound when taking the cap off unless it is a warm day. I will try changing the gas cap as dad put an old one on there.

The fuel line runs on the driver side and the exhaust is on the passenger side. They don't come close to each other.

 

I agree with either a venting issue or like you said the pickup tube has something wrong. Hope you figure it out soon!

 

The fuel cap is vented, the Mustang fuel tank is not. Neither was the original in-cab tank.

 

Several faults can cause fuel supply interruption:
1) improperly venting gas cap. The vented gas cap is designed to vent small amounts of air into the car gas tank line. The vented gas cap has a one-way release valve that is pressure-activated. When the pressure reaches a certain point (defined by so many pounds per square inch) the valve opens by a very small amount, relieving some of the pressure. In this case, the pressure is formed on the outside of the tank, due to the vacuum formed from the displacement of fuel on the inside. After the pressure inside the line equalizes, the pressure valve closes. The valve is set to balance the pressure with the surrounding atmosphere. Air is allowed in, but no fumes are allowed out of the gas cap, because of environmental concerns.
2) blocked pickup tube: filter sock clogged, displaced or collapsed, broken, bent, misdirected.
3) bocked line to the pump: collapsed rubber, crud in the line, more flowing and blocking until it breaks free
4) vapor lock by overheating line on the way to the carburetor.
5) bad diaphrams in the fuel pump.
6) sticking needle valve occasionally preventing entrance of fuel to the carburetor float bowl

Field trouble shooting for intermittent fuel supply interruption (not all recommended when passengers are present)
1) gas cap/venting issue: slighty dislodge or remove gas cap, if necessary substitute a piece of cloth or other porous material arranged in the neck to allow air in.yet not allow fuel to slosh out. BE CAREFUL WITH THIS.
2 & 3) Apply air pressure to gas tank through neck with fuel line disconnected at fuel pump... slight flow commensurate with air pressure applied should be observed). The old behind the seat tanks with the neck outside the cab are easy to apply lung pressure to. Others, not so easy.
4) pour cool water on the line near any possible heat sources and isolate the line with some sort of insulating material. If fuel starts to flow again while cranking, then vapor lock can be a problem
5) rebuildable fuel pumps used to be easy to disassemble to inspect the diaphragms. Sealed units, not so much. Best to just get a replacement.
6) sticking needle valve can only be solved by cleaning the needle valve. You get to crack open the carburetor for most designs.

 

Thanks for the troubleshooting steps.

 

The gauges on 60s trucks aren't known to be particularly accurate, at least not after 50 years. Mine has never busted 160° and I run a 195° thermostat. When I first got the truck my brilliant powers of deduction determined a new sending unit would fix that. It didn't. I think there is an adjustment to be made on the gauge itself, but I'd probably have to quit coffee for a few days, it looks to be pretty fragile and touchy to do it. It would be nice if it read correctly. "Someday" I'll get around to it.

 

Now that you mention it, it wouldn't surprise me if it was inaccurate. I have yet to release the magic smoke by overheating it and hopefully I won't.

 

I think it is vapor locking. The engine bay is getting very warm and the wife and I took it for a ride last night. Sitting in the drive through it was running fine, but faltered when I started on the road again. Once I got it home, I opened the hood and touched the fuel line and carb, both very hot. I'm going to get some heat shield for it today and see if that will fix it. I have the fuel line routed the best that I can already. I'll update on what I find.

 

Do you have a heat riser on that 300 (I only have v-8s)? Could it be stuck closed routing the exhaust heat into the manifold all the time?

 

No heat riser on there. Here are the only pics I have at the moment for the fuel line route. The line is a little shorter now and it runs behind the oil filler cap, but takes roughly the same route. Is there a better way to run this? I have some heat shielding for cables/lines that I will try on there if there is no other way.

 

Modern gasoline is probably more prone to vapor lock in an old school open system. I like to use Marvel's in the gas tank for a few different reasons, and this is one of them, it seems to help. Also make sure the distributor mechanical and vacuum ignition advance is working correctly all up and down the RPM range and at cruise. Retarded timing can raise temperatures. Is it pinging or knocking? Of course check for more simple defects, like a loose, worn or glazed fan belt & pulley.