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.

[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.

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.

have a look at superhawkes diagram above.

that third port on the fuel pump is not a line where fuel comes back into the pump.
its a port where fuel escapes the pump if excessive pressure builds in the injector supply line out of the pump. that can happen if your fuel pressure regulator sticks shut and does not release pressure and fuel flow out at the design pressure. which in the case of an L jet is 35psi (+/-).

how do i know. i have fully pulled 3 of those pumps apart and rebuilt them.
if you want tech info on the pump to see how the head works with the three ports its in the topic i linked to. port 3 or R is a spring loaded sliding pin that gets pushed by fuel pressure out. it blows open at around 60-70psi. its flow is in one direction. out.

remember the fuel returning from the injector circuit after the fprv is not under pressure. its under pressure before the fprv not after. it flows back to the fuel tank. it physically cannot force its way through that third port on the pump. on the other side of the port inside the pump the fuel is at pressure. 35psi. fuel in the return line to tank is not under pressure. its flowing. the laws of physics prevent it from making its way back into the fuel pump. there is not circuit there. the circuit is the bigger one back through tank.

any fuel in the injector line after the pump and before the fprv should remain under pressure after hot shut down in an engine with a properly function fprv and a fuel pump in good condition. in fact for up to 24 hrs. i've tested mine. same car as one being discussed here. if that line is losing pressure straight away its time to lose the fuel pump. so the injector line does not boil or vaporise fuel after hot shutdown. pressurized fuel is immune to the problem. its the unpressurized fuel before the fuel pump that is a problem in vapor lock conditions.

i am with superhawke on this one.
i doubt this is a vapor lock problem.

the car cut out on the exit ramp off the freeway.
i've never had vapor lock do that. only happens after you shut down and walk away from the car for a time. and i;ve never had it last 3 hours.
so i think its something else.

travelling at freeway speed, even on a hot day, that hot air being released from heater exchanger valve is being instantly sucked out by wind passing under the car. esp if it has the little flaps on the undeside of the body still there. so its not suffering heat problems while the car is moving. esp at higher velocities. maybe stuck it traffic heat can build up there while its running, but not when its moving. no heat comes off the magnesium fan casting while the engine is running. if you don't believe me put your hand on it. it will be stone cold. the heat builds up in the casting after shut down. so there is no heat radiating at the fuel pump etc from it while the car is running at speed.

i've suggested checking the AFM plug and the electrical loom up to it - along with all its connections at the other end. into the ECU and also i think the dual relay.
the symptoms are identical to a AFM plug thats not connecting.
not saying it is that, but its something that ought to be eliminated.

the fact that its staring the next day and running fine from stone cold suggests to me some kind of heat or vibration related issue may the cause. but again not saying it is.

the L jets are a bit different how the fuel pump works.
cranking activates the fuel pump. which pressurizes the circuit.
however as soon as the engine fires there is a hand off to the AFM to run the fuel pump.
the reason this can happen is that the draw from the engine moves the flap and it hits the fuel pump contact switch and takes over running the pump and pressurizing the injector lines. if for any reason that hand off does not happen it won't keep running.

it wil fire off that bit of pressure it gets during cranking, but almost immediately cease running. this does not happen in D jet cars or carb cars. most if not all have the fuel pump running all the time the ignition is on. the L jet car is designed to switch the fuel pump off if the engine stops running. its a safety feature. say in the event of a crash.

sounds like art's car is very original. from what i have seen in photos of it - its vey intact. i'd say all the pump wiring is factory and unmeddled with.

anyone who has owned an L jet knows the embarrassment of not plugging that AFM back in after working on the car and then wondering why it won't run. Starbears done it.
i've done it. interestingly the guys with L jet who have wired in the fuel pump to run all the time get a different phenomina. the engine will start and it will run at elevated idle. the idle just keeps creeping up. and it will run for a while before cutting out. mine is still original. it just fires, runs very briefly and then cuts out.

from what art is describing - its sounds identical.
certainly worth investigating.