Let me preface this by saying I recently swapped the tired 351m for a fresh 400, keeping my same carb, intake, and exhaust manifolds. I have driven this truck every day for years in all weather conditions, including stop and go traffic in the summer heat with the a/c on with no issues until now.

My truck, a 78 F150 2wd auto has developed a vapor lock problem upon installation of the new engine. It's happened a few times now, not even during the heat of the day. If the truck is at a stop and idling it will stall at some point, especially after its been parked for a little bit and allowed to heat soak before being restarted. A good example would be attempting to leave the grocery store parking lot, the engine will restart normally, but if I have to sit and wait for an opening in the traffic for more than 30 seconds the engine will stall. It may or may not restart immediately afterwards, but if it does it will not run long.
After that you can crank all you want and it will not start.

I'm positive it's a fuel issue, I can unhook the supply line to the pump and there is nothing there. An electric fuel pump just spits and sputters if hooked to that line until it can prime up again at which point fuel pumps normally.

I do have dual exhaust but there isn't really anywhere it gets terribly close to the fuel line. During these events I seem to have fuel at the tank selector valve so I don't think it's any farther back than that. Opening the hood seems to take care of the problem also, I can't replicate the problem if the hood is up, only closed.

Hopefully my ramblings are somewhat coherent, I don't really know where the issue lies. This didn't happen before the engine swap.
All I can think to do for the moment is insulate the steel line from the pump to the carb, and insulate from the pump back to the selector valve.

 

Phenolic carb spacer might help with heat soak? Maybe a carb rebuild?

https://www.summitracing.com/parts/c...waAhgREALw_wcB

Sounds like a bad mechanical fuel pump? But usually they either work or don't. Sound like you still have one on it? Maybe ditch the mech fuel pump and go elec?

Or may be a semi clogged sending unit pick up tube filter sock?

 

I already have a phenolic spacer, I forgot to mention that.
I'm not ruling out the fuel pump but I would think that the electric one would pump fuel just fine if the mechanical pump was bad, but neither seem to work.

Fuel seems to flow very readily. I've had no problems on the highway, accelerating or even towing.
I don't think I have an obstruction.

 

I would replace the mech pump, so you can rule that out.

To me if NEITHER pumps work/get fuel, then you have an obstruction in the fuel delivery system.

If you have fuel all the way and then it boils out/vapor locks do to a hot carb issue then time to insulate the line. But I bet that will really not help, but it might. One thing for sure it will do, is cool the fuel going to a hot carb.

If it WAS fine before the eng swap, then started thsi problem after the eng swap, what did you change fuel line routing wise?

Have you ever back flushed (under low air pressure) the fuel line from the rubber to steel point in the eng compartment?

 

I didn't change anything, that's what has me so puzzled.
I can blow through all the lines, nothing is in there.

It just loses fuel supply after idling hot and won't get it back until it cools.
I'm not even sure the problem is with the steel line.

Maybe this new engine is making more heat than the old one.

 

Are you running a different thermostat?

I'd still consider that the pickup is getting clogged. If you blow back through the line it will clear it till it picks it up the obstruction again (possibly parts of the disintegrated sock).

 

I am running the same thermostat from the old engine.

Again, I would think i would have problems starving for fuel going down the road, not just at idle when the engine gets hot.

 

Does it only stall in gear?

 

Question.........

Do you run headers? the way the hard fuel line runs can be very close to DS header. I ran into this with my Bronco once upon a time ago. The headers would heat soak the feed line to the mech pump and cause it to vapor lock. Heat shield fixed it.

 

It will stall in park and neutral all the same.

I'm running the same factory iron manifolds I was before.

 

There are lots of things to eliminate as mentioned... plugged up fuel pickup, loose hose clamp on suction side, cracked or collapsing fuel lines on the suction side and fuel pump are top of the list.

That said, today's fuel is not blended for carb application. It boils much easier than fuel "back in the day". Dent fuel systems are marginal with today's gas. Some little thing may have "pushed you over the edge". maybe even different fuel. An electric pump mounted low in the rear where it is cool is a robust fix.

 

Well, I started by sliding an insulating sleeve down the steel line on the front of the engine and replacing the rubber line on the pump inlet as it looked a little suspect. I tried for over an hour to make it vapor lock again but it hasn't happened yet.
I'm keeping my fingers crossed.

 

I tried everything to cool the fuel line and to be honest my solution only cost $27.00. I bought a two row cooler and mounted it in from of the radiator and problem solved!

https://www.summitracing.com/parts/d...yABEgLSP_D_BwE

 

 

From your description, it sounds like you tried the electric fuel pump up front. Electric fuel pumps push fuel, they do not pull well, and should be mounted low and near the tank.

I've put electric fuel pumps on all my trucks, and mounted them inside the frame rail, just ahead of the crossmember that holds the switching valve. No more vapor lock!

I'd suggest replacing any old hose with new EFI rated hose. If you keep the mechanical fuel pump, put 5/16 ahead of the pump and 3/8 behind the pump. That will help to keep the fuel pressurized.

Do you know if the 400 was built with a turkey pan under the intake? If not, that could contribute to your heat soak issue.