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.

[emerygt350]

Yep, sounds like vapor lock to me. I have mine all in the original locations and wrapped it in high quality heat wrap. Haven't had a problem since. I can tell it still thins out when I park it on a day like you describe but it starts up and goes fine after 5 seconds of pushing new fuel. Before the heat wrap there was nothing I could do but wait.

I was racing today on black tarmac 90 degrees, started up all day under extreme conditions.

Is it design engineering? I can't remember but everyone uses it.

[Artfrombama]

Yep, sounds like vapor lock to me. I have mine all in the original locations and wrapped it in high quality heat wrap. Haven't had a problem since. I can tell it still thins out when I park it on a day like you describe but it starts up and goes fine after 5 seconds of pushing new fuel. Before the heat wrap there was nothing I could do but wait.

I was racing today on black tarmac 90 degrees, started up all day under extreme conditions.

Is it design engineering? I can't remember but everyone uses it.

@emerygt350 thanks for responding
Vapor lock came to mind right off the bat but after an hour or so of not starting I started to suspect other things.
As I mentioned, today runs normally.

[GregAmy]

With a recirculating fuel system...can a 914 **really** get vapor lock?

Dead-end fuel system with carbs, sure. But L- or D-Jet...?

Discuss.

[wonkipop]

Yep, sounds like vapor lock to me. I have mine all in the original locations and wrapped it in high quality heat wrap. Haven't had a problem since. I can tell it still thins out when I park it on a day like you describe but it starts up and goes fine after 5 seconds of pushing new fuel. Before the heat wrap there was nothing I could do but wait.

I was racing today on black tarmac 90 degrees, started up all day under extreme conditions.

Is it design engineering? I can't remember but everyone uses it.

@emerygt350 thanks for responding
Vapor lock came to mind right off the bat but after an hour or so of not starting I started to suspect other things.
As I mentioned, today runs normally.

interesting that it shut down as you exited.
not quite the usual vapor lock syndrome. that tends to happen after you stop and leave it for a bit. (i have always believed more than anything that is due to radiation from the magnesium fan shrouding. its a heat sink for all the engine heat due to metal heat flow conduction from aluminium case and iron cylinders etc).

it could be that your fuel line is collapsing somewhere along its length. possibly in the loop under the fuel tank or even in the looping into the fuel pump. coming off throttle at cruise. trying to think what the pump is doing. fuel pressure would back off and be sent through the return line via the fuel pressure regulator and return to the tank. this can upset the draw perhaps on the fuel pump. i'd say its something like that. as if the fuel supply line has collapsed. after a very long period of time the vacuum lock in the line releases and its all back to normal.

[wonkipop]

With a recirculating fuel system...can a 914 **really** get vapor lock?

Dead-end fuel system with carbs, sure. But L- or D-Jet...?

Discuss.

it can definitely boil fuel due to the heat radiation from the fan casting.
my theory has always been that where it boils is in the fuel supply line running from the exit at the rear of the tunnel to the pump. it can also boil in the plastic fuel filter pre the pump. and possibly in the pump head itself. this is in a stock set up.

with boiled fuel you get pump cavitation.

i had it from time to time with my original fuel pump years ago.
after recommissioning i had to fit a modern turbine pump to replace the original.
the problem became more frequent with that pump than it ever was 20 years ago. i wrapped them all in insulation shielding and the problem seemed to cease. though occassionally i can hear the newer pump cavitate on a hot start. but it seems to cope and settle down after a short while. the modern pump is positioned in the stock 74 location under rhs of engine bay.

[wonkipop]

another factor in USA cars is the ethanol content in your fuel.

i know from experience here with a mate when he drove his old falcon to bathurst in very hot weather he was forced to fuel with the ethanol fuel that is sold here. it just kept boiling fuel in the fuel lines. we tried everything. wrapping the fuel pump with a damp rag. etc. he never had the problem with regular proper fuel here. it makes the problems with cavitation if they are present a lot more sensitive.

but i don't think that is art's problem completely.
it really sounds like some kind of vacuum collapse in the fuel feed line.
could be triggered by coming off throttle. then getting back on throttle a bit.
flow/pressure changes in feed line. snaps or collapses the fuel line shut.

carefully look at all the loops in the flexible sections of fuel liine feeding the pump from the tank.

EDIT
and......if you can eliminate fuel supply as an issue then perhaps its an electrical component. i'd have to think about what that could be. though if its related to heating and then operating once its cooled down you would think it would have come good during that three hours you waited for AAA.

[bkrantz]

Could the pump itself be causing at least part of the problem? Either weak all the time plus a touch of vapor lock, or weak when over-heated?

[wonkipop]

Could the pump itself be causing at least part of the problem? Either weak all the time plus a touch of vapor lock, or weak when over-heated?

hmmm.
having taken one of those pumps completely apart to replace all seals right down to the electrical connection seal they are pretty rock solid. and they get cooled by the fuel passing completely through them. the shutting down while it was still on the run as he came off the ramp might point to something in the pump.
they do have a little spring loaded gizmo in the head of the pump that activates on excessive fuel pressure build up in the high pressure line to the injector ring.
its possible that might stick in someway and then the pump will just send fuel off via the release line which is the third port.

the top of the pump can come off easily without undue worry. there is an o-ring seal which you might have to replace on reassembly but its easily obtained. making sure it a nitrite seal. you can clean out that port and make sure the pressure relief is operating freely. it also has a little rubber nose on the pin which seals it normally after you switch off to hold pressure in the lines. but if it was to jamb open for any reason you certainly won't be able to hold pressure in the high pressure fuel ring.

if you take the pump top apart just be careful to note which way the roller cells lie and try and put them back in the same orientation.

but somehow i doubt it is the problem. but you never know.

EDIT
link here to a post i made ages ago tearing down the 3 port pump.
gives you an idea of whats in there, what it does.

http://www.914world.com/bbs2/index.php?showtopic=350416

[wonkipop]

art

this is what i have got on hot start, runs then stops from an L jet diagnosis book for VW type 4.

if its not fuel supply related perhaps it might assist.
i've never had quite the problem you are describing with yours so i am not entirely sure what it could be. perhaps there might be an L jet owner who has had this one and will pipe up with a solid answer.

[Artfrombama]

@wonkipop , @bkrantz thank you for responding, I value your input.
Many why and what if’s, will examine the fuel lines and check fuel pressure tomorrow, probably investigate adding a fuel pressure gauge somewhere in the engine compartment.
One thing for sure, don’t like being stranded 50 miles from home in 93* heat.

[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.

[wonkipop]

another tip @artfrombama that came to mind - just remembered something.

if the AFM plug is left unplugged the engine will fire, run for a second or two and quit.
thats if all the fuel pump connection through the dual relay etc is factory original and the pump has not been rewired to run all the time as many have this many years down the track. basically the fuel pump will pressurize the fuel system on cranking from starter motor but as soon as it does fire the AFM takes over after the flap moves to control the pump. if its failing to take over because of no connection it will run briefly until fuel pressure drops.

i wonder if your AFM plug or harness is playing up.
dirty connection. or shook loose. or intermittent fault appears after getting warm and running for a good length of time.

might be worth playing around jiggling plug and wire loom while the engine is idling. see if anything happens.

[emerygt350]

I have found that as long as the fuel is vapor the pump is useless (as mentioned above). I suspect this is nothing more than that. Never really figured out why it takes so long for liquid to get back to the pump but I bet it has to do with the fact it's gravity fed from the tank. After the line is full of air from the firewall forward, and the pump is inoperable, it takes a long time for the gravity fed cool fuel to creep back to the pump, displacing that air or pushing it through the pump and return line.

Anyway, I would start with the simplest fix and put some heat wrap on those lines. I don't think you should cover the pump though. It does use the fuel to cool itself but I think letting it radiate its own heat is probably better for it than wrapping it in a blanket. It's also tough to reach anyway.

[emerygt350]

As a side note too, the original heat wrap was that nasty tar like material that cracks and falls off with age. I didn't have this issue in a bad way until a large section of that crumbled away. As mentioned above, that fan shroud is probably the culprit.

[ClayPerrine]

Here is what I would do to diagnose it.

Get a mechanical fuel pressure gauge. Take your engine lid off the car. Hook the pressure gauge up so the hose is long enough that you can use duct tape to tape it to the rear window, where it can be seen from the driver's seat with the rear view mirror. (Fuel in the passenger compartment is not a good idea) Then drive it in the same conditions (as close as you can get) on the same route. If it happens again, check the fuel pressure gauge. If it is vapor locked, you will not have fuel pressure.

I would consider moving the pump up front as they did in the 75 and later cars. It may not be the absolutely concourse stock location, but it would help if it is vapor locking.

[StarBear]

Had recurring vapor lock in my 74 1.8 L-Jet in the early years even with regular fuel.
Wrapped the lines in insulation, which reduced frequency of incidents but when it did happen the condition lasted longer as insulation kept heat IN!

Moved pump and filter to front, as much to address that as well as the bad fire hazard having all those lines and connections right over this hot pipes.
Funny though that yours cut off at exit while running. A clue? These folks have great ideas.

Our cars, maybe particularly the 1.8s, do not like hot, humid driving weather. At my age, now neither do I. My second solution is to keep it parked on such days.
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[Superhawk996]

There are a lot of assumptions built in to this thread.

The fact that it cut out completely and suddenly while running doesn’t sound like typical vapor lock to me.

Other possibilities:
Complete loss of spark - coil problem when hot
Wiring / grounds (less likely than coil)
Heat induced issue with ECU (rare but not impossible)

Follow Clays advice to use temporary fuel gauge to monitor fuel pressure. I’m not a fan of permanent fuel pressure gauges in the engine compartment. Especially on higher pressure Fuel injection lines. Increased fire risk / leak sources.

Have a DMM with you to trouble shoot electrical / wiring

Use basic tools like factory tool kit to remove a spark plug if in a no-run condition and verify whether or not spark is present. That little bit of troubleshooting can determine whether you have a spark / ignition problem or whether it’s a fuel delivery problem.

[Artfrombama]

This is a head scratcher for sure!

I was reading on the bird forum about a seat belt interlock under the passenger seat that was designed to keep the car from running if the belts aren’t buckled. I do remember trying to start the car right after I bought it, a buzzing sound from that area.
Would the starter work if this relay were malfunctioning?