So it took me a bit to get this far, thought I would share
1984 Bronco, 351
boss said was dying on hill, especially when summer, would return in an hour and ran fine. have had 5 years, problem started 2 years ago, no changes to vehicle
checked plugs- fine
checked distributor- fine
found bad coil wire- hmmmm smoking gun to problems? replaced
ran a lot better
113F after 30 miles on road, onto dirt then 4x4 low .. coughed couple times in wash
onto the hill (climb from 1100 to 4800 ft) died half way, no restart
shot carb with either, not even temp start... hmmm
pulled fuel line from carb (factory holly 4bbl) no fuel
cranked, no fuel
had to go with clients in other vehicle up hill
returned 62.117 minutes later, shot with either, started right up
drove up rest of hill, parked, had dinner, drove home no problem... hmmmm
because no fire with either, suspected ignition
changed ignition mod
changed distributor
114F, flat city street, alone w/ac on full blast
after 9 miles, pulled into parking lot, died
no restart
forgot tools
waited 45 min, remembered had swiss army knife
no fuel in clear filter had installed just before pump
disconnected, sucked on hose (yuk!)
reconnected hose, drove home w/o AC, no problem
read threads about vapor lock
key ingredients:
extreme heat
engine working hard (more heat, low air flow)
altitude
questionable fuel filter (small stone type, minor sediment is all takes to clog), low fuel flow allows to boil; when I removed it, filter before pump full, stone inside factory steel housing on carb was absolutely dry, so was hose to it.
was not (to best of my knowlege) change from before, but noted 1' of steel factory fuel line to pump awfully close to exhaust manifold... will wrap/insulate
removed stone filter, installed large clear filter (G2) right before carb
taking on tour tomorrow.... will post results

 

It could be a combo of a weak fuel pump and vaporlock.

You could try insulating it like you said.

 

Remember vapor lock is not likely to occur on the pressure side of the fuel pump. It occurs on the suction side of the pump. That's why most modern vehicles run the electric pumps rearward in the vehicle, so the pump is pressurizing the long distance from the rear to the front. I would look for a modified exhaust system. It could be too near the fuel lines along the frame if it has been re-routed.

 

Trying to rationalize how mechanical fuel pump can be "weak"... either diaphragm is good or bad. Blocked filter reduced flow, allowing to boil ... seems was always vapor in line when failed

 

"It occurs on the suction side of the pump."
That's what was happening... verified by empty clear filter I placed just prior to pump
All fuel lines and exhaust are stock. Am thinking that partially occluded filter was like pinch in garden hose, slowing flow. Other than that, nothing (fuel line routing, exhaust, etc) had been changed in last 5 years. Problem started 2 years ago, only happened summer hot (afternoon tour) days, in low range (slow speed, less airflow) on (specific) hill. When cooled, was fine up rest of hill, then for drive home (which included climb on pavement out of valley... more airflow, cooler ambient temp)

 

Not saying your fuel pump is your cause, but it can pump less pressure than it's rated for when it wears out. Other things can cause low fuel pressure. Worn fuel pump essentric, clogged filters, clogged sump filter in tank, bent or restricted lines, poor venting of tank, etc...

 

If nothing is wrong with your fuel components, and you think your situation is special, then just go buy a cheap electric fuel pump and install it as near to the tank as you can get it. Just forget the factory mech pump and route around it.

 

These guys bring up a good point. If the tank is not venting as it should, it will create a vacuum inside the tank, and the pump will not be able to pull any fuel thru the line.
A simple way to test for this. When the conditions are right for the problem to occur, loosen/remove the fuel cap. If the problem goes away, you've got a tank venting issue. If it still happens, you've got some kind of issue in the supply line, could be a small hole, or clogged screen in the tank.
Heck, it could even a combo of a restricted tank vent and a hole/leak somewhere in the supply line.
A fuel pump is designed to push fuel, and relies on the siphon effect to "suck" fuel from the tank. It doesn't take much restriction to cause the supply to the pump to become unavailable at the pump.

 

update: replaced small stone in carborator filter with larger plastic external filter
filters

drove around block several times with ac on high
left ideling with ac on .... returned 7 min 23.114 sec to find dead, no fuel in filters
would not restart
sprayed fuel pump with hose
after couple cranks started right up
ideling in driveway with ac off

 

Has the evaporative emissions been disconnected or bypassed, or plugged in anyway? This is how the carb, and fuel tank vents normally. Through the charcoal canister. Fuel vapor is trapped by the activated charcoal in the canister when the truck is off, and lets fresh air into the system through the activated charcoal when it runs. Make sure the system is working correctly.

Try removing the fuel cap as suggested, this might reveal something.

Do a pressure test on the fuel pump.

 

Removing fuel cap made no difference (tried that already, sorry did not mention)
Took on tour, 110F... was fine till in wash (4x4 low) , stopped for 3 minutes, died, refused to restart; no fuel in filter before carb or before pump
doused pump with water (had 5gal jug)
reprimed line (sucked on other side of filter till fuel in it)
started
drove through rest of trail keeping at 5-10mph then 13 miles blacktop to destination

 

This is not a common problem with these trucks. I can almost guarantee something is not original on your truck as far as the fuel lines are concerned. I bet there is a cheap piece of hose somewhere getting hot and sucking shut, a piece of hose that has been put in place of the original steel line, the original steel line is not routed and clamped as it was from the factory. You are just going to have to get under there and mess with it till you figure it out.

 

all lines from tank to pump are factory
from pump to carb had 17" of fuel hose, seemed good condition, had replaced anyway
key factor seems to be temp of fuel pump... letting cool for 30min to hour or pouring water (on nothing but fuel pump) solves / allows restart

 

just a tip i picked up in las vegas (110 deg. today) check the sticker on the pump at the gas station for the ethanol content. in my area the chevron and shell stations say the fuel contains "up to 10%" the cheapie places dont seem to have a sticker but ive heard it can be as much as 25%. ethanol has a lower boiling point than gasoline so it is more prone to vaporlock. an old mechanic told me to put an electric fuel pump near the tank that you can run in conjunction with the mechanical pump. this keeps the suction side of the fuel line under pressure instead of a vacuum. the higher the pressure the higher the boiling point (just like your antifreeze). also i have been around these dentsides and bumpsides my entire life and have never seen a fuel tank under a vacuum. as you burn fuel out of the tank the vapor left in the tank expands and more than makes up for the volume loss on a non vented tank.

 

bit the bullet and put an electric in, abut 3ft forward from tank, on frame, lower than tank... also removed thermostat (should have done that a while ago)...
shakedown run today, ran w/AC on full, 15 min parked, 10 min street, 20 miles on freeway... cool as cucumber, ran great. Turned off, let set 5 min 23.118 sec, restarted like a charm. Let idle 10 min, stopped 5min 23.119 sec, restarted like a charm
Outside temp at 14:30 = 117

am thinking combination hot engine, and ethanol (did not think of that, although I do not think station uses mixed gas... will look next time... outside Phoenix)

Bottom line= no more problem (for now)

Thanks for all input

Cowboy Steve
Carefree Adventures, Cave Creek AZ