Is this the dreaded 914 vapor lock?, Asking for your ideas and thoughts Options

[Artfrombama]

Drove my 914 to a PCA function yesterday morning, 93f and humid.
After driving on the interstate at 70-75mph for around 45 miles I exited and the motor died. I had enough momentum and coasted into a gas station/convenience store where I topped off the tank and tried to restart the car, no luck. The car would fire, run a few seconds and die then refused to re-start for another 10-15 minutes then the same scenario again. No tools to diagnose or repair.
While waiting on AAA for three hours (another story) I would occasionally try to start the car with the same results.

Today, car started normally, runs normally.

1974 L-jet, original three bung fuel pump, original location.
500 miles on new tank, sock, filter, ss lines.

[Chris914n6]

The 3-port recirculates fuel around the engine, instead of putting it back into the tank. So yes, vapor lock happens, because the fuel gets too hot.

You need to wrap the pump so it doesn't get hot air blown on it, from the heat exchanger/heater outlet and the engine cooling air.
It wouldn't hurt to insulate the fuel lines that cross over the engine.

[wonkipop]

The 3-port recirculates fuel around the engine, instead of putting it back into the tank. So yes, vapor lock happens, because the fuel gets too hot.

You need to wrap the pump so it doesn't get hot air blown on it, from the heat exchanger/heater outlet and the engine cooling air.
It wouldn't hurt to insulate the fuel lines that cross over the engine.

chris, thats not entirely true as i understand them to work.

there is a T junction off the return line to the tank which is coming from the fuel pressure regulator. that T branches to the third port. the regulator normally relieves the pressure first if it exceeds 35 psi (that figure of 35 is specific to the L jets). if for any reason fuel pressure builds in the injector circuit and the FPR does not relieve it the third port on the original style 3 port pump then relieves pressure. it often kicks in at around 60-70 psi. this travels down the line to the T and returns to the fuel tank.

the fuel is fully circulating back to the tank and is not restricted to circulating in the high pressure fuel line.

i have heard there is also a role that third port plays in relieving pulses in the fuel but by and large it does what i describe above.

modern dual port turbine pumps pressure relieve completely differently.
they blow the excessive pressure back down the supply line to the tank.
even some of the later versions of roller cell pumps relieved excess pressure back through the supply line. i've seen those versions often fitted to mercedes benz cars of approx same era as the 914s.

which does bring up an interesting thought i just had for @artfrombama
the L jets are interesting in that when you back off the throttle the fuel pressure relief valve is designed to release pressure and drop it from the more normal operating pressure of 35 psi to 28 psi (give or take a few psi either side of both values).
its vacuum operated off the intake plenum to to do this. they are a little different to the D jet set up which i believe operates only at 28 psi and does things differently.

a possibility may also be a fuel pressure regulator valve that is misbehaving.
i'm thinking of how its activated when you came off throttle on to the exit ramp.
did it somehow fail or stick. and then fuel pressure could not be built up in the injector ring.

dunno. its an interesting one.

and really with vapor lock (which i have experienced) it comes good inside of 3 hours.
so i don't think its all down to vapor lock. might be part of it. but doesn't feel like the whole story.

[Chris914n6]

The 3-port recirculates fuel around the engine, instead of putting it back into the tank. So yes, vapor lock happens, because the fuel gets too hot.

You need to wrap the pump so it doesn't get hot air blown on it, from the heat exchanger/heater outlet and the engine cooling air.
It wouldn't hurt to insulate the fuel lines that cross over the engine.

chris, thats not entirely true as i understand them to work.

Your understanding of fluid pumps is off. Too much typing to explain why.

OP issue could be something other than vapor lock. But it is possible that the volume of fuel consumed at speed (adding cool tank gas) exceeded the threshold for vapor lock but at idle conditions were ideal.

A pressure gauge would answer some questions the next time it happens.