Just for kicks, check your fuel line routing and make sure it isn't near any exhaust pipes or the exhaust manifold. I am seeing a lot of signs that point to vapor lock. Here's how...

1. It does not act up consistently. On cooler days the heat generated in the engine compartment will be more easily dispersed.

2. If it does begin to act up, it is after the truck has been driven for 30-45 minutes...enough time to build up heat in the fuel line and get the beginning symptoms of vapor lock.

3. When he lets it sit with the hood open for 10-15 minutes, he can then get back in and drive for awhile and then it will start acting up again.

4.

Quote:

He did mention that it occurs more often (but not always) on hills when he squeezes the gas to maintain speed. It'll hiccup until he lets go of the gas, then it'll be fine again.

On hills, you are requiring more of a fuel system that simply cannot provide the amount of fuel needed due to the vapor in the line. Plus, on a hill you are generating even more heat.

Is there any pattern to the outside temperature when this problem has been noted? Has it always been 90 degrees or more? Has the problem failed to surface when it has only been in the low 80s? Etc...

That's my $.02.

 

Had a 78 E-150 that vapor locked real bad when pulling a hill on hot days. Fixed it by getting a small trans cooler, mounted it in front of the radiator and running the fuel through it.

 

Make sure all of the rubber fuel line from the front of the truck to the back of the truck is all in good shape with no cracks or has been replaced..I had a 77 that would do that intermittently...and it was a couple of fuel lines with small cracks in them and it was sucking air (Leaning Out)...also base gasket on Carb could have a vacuum leak...or a mis adjusted carburetor (Rich/Lean Needles) my .02 cents! I've played with these dentsides for 20 plus years!! I'll never stop lovin' em!!

 

my dads 73 f259 did exactly the same thing with one extra veriable ....speed the faster u went the worse it would ge.t we changed the little canister filter on the fuel inlet of the carb. theyare usualy a little smaller micron that the filter on the fuel pump or one of those "in-line" filters and stop up faster if theres alot of debris in your fuel.

 

Just an idea, where did/does he buy his gas? Any truck sitting up will acquire condensation in the tank.

Try some heet, the gas additive, and drive it tiil you have put about 3-4 tanks thru it. It may be no problem at all but the build time.

 

WOW, you guys are great at offering suggestions. I just came back from camp, dad is still there. He wants me to go pick up the truck and bring it to my place so I can tinker with it for him.

So far my plan is;
1) Replace old plugs/wires/cap/rotor
2) Elimanate as much unnecessary pollution control junk as I can including vacuum lines
3) Check the entire length of fuel line from front to back for any possible problems
4) I'm going to print this thread and take it with me to the garage

One inconvenience is that he said I'll have to drive it on the highway for at least half an hour before it might start acting up. He's never had a problem just putting around town (but then again, he's never driven it continuously for half an hour around town.....)

 

sounds to me like this is a good reason to drop a good ol 390 in there! heheh i like burning gas

 

Today, I:
1) changed the plugs/wires/cap/rotor
2) most of the pollution stuff is already gone, just the charcoal canister is left
3) checked the fuel line

The old spark plugs were a nice tan colour, so I don't think the problem is with the ignition system.
When I unhooked the rubber fuel line from the carb, there was no gas right there. I'm used to gas spilling when you unhook the line. But, when I unhooked the rubber line from the steel line 6" away, there was some gas there??? So, I replaced this 6" chunk of rubber line at the carb with one of those Mr. Gasket glass fuel filters. I figured that when it starts to run rough, we could get out and check to see if the filter is full of gas, like it should be.
I also noticed that the steel line from the fuel pump comes up the side of the block and then goes around the front of the valve cover, but the steel line was resting against the side of the upper rad hose (Maybe heating the gas up?) I bent the steel line away from the upper rad hose, and covered the steel line where it goes past the rad hose with a 3/8" rubber line to insulate from the heat.

After all that, I drove the truck around for 1.5 - 2 hrs. I found some hills to lug it up, and generally drove it hard, trying to make it act up. It's hot here today, 29 degrees celsius (84 F). The temp gauge never went higher than the "N" in NORMAL, and the engine ran like a top.

So...either I've found the problem (gas line against the rad hose) or the truck was just having a good day. I would still like to know why there was no gas at the carb when I pulled the line off. Tomorrow, I'll check to see if the glass filter is still full.
Sorry about the long post, I'm just trying to give all the info.

 

Yesterday, dad took the truck back to his hometown for a car show, and sure enough, after about 45 min, it started to act up. He pulled over and let it idle, got out and checked the clear gas filter, and it was empty. It filled up again though, and off he went. It did the same thing two more times during the day. I asked him about the gas lines from the tank, they're new. The next step is to change the fuel pump. Any other ideas?

 

Fuel pump would be my next step.

 

you could have trash washed up into the line from the tank, it happened to my dads f250 we took the line loose from the tank and from the pump and blew air through it til it was clear

 

Brother, i got a suggestion that rings a bell on Mr Peabody's Way Back Machine. What size line was used when replacing the lines from the tank to carb? Ford always kind of ran smallish lines. When you changed them out could they actually have been swapped out slightly smaller?

I had a similiar problem on a mid-seventies Mercury Monarch. Swapped out some fuel when i went from FOMOCO 2V to Holley 4V and accidently got some slightly smaller fuel line when i hooked up the 4bbl. Just a thought...Don Garlits always ran slightly biggish fuel line on his racers. Swore it was worth a lil time off the quarter.

 

You know....the chunk of line I replaced at the carb was 5/16". I wonder if it should be 3/8". My 460 has 3/8" line, so I'd assume 5/16" would be OK for the 300 - 6cyl???

 

The truck is still acting up. Dad has changed the fuel pump, and the coil with no change. He also went to the local FORD dealership and talked to one of the older mechanics. He told dad that it could be a pin hole in the metal line coming out of the sending unit at the tank that occasionally sucks air. What do you guys think?

 

Do you have the dual-tank setup? Mine has two fuel filters, one at the carb inlet (brass, smaller), and one at the fuel tank switch valve close to the rear tank, underneath the truck. The one underneath is an inline canister-style, and it's easily overlooked. If it's plugged, bad fuel things happen.