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VAPOR LOCK

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oldguy View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oldguy Quote  Post ReplyReply Direct Link To This Post Topic: VAPOR LOCK
    Posted: Apr/26/2022 at 7:32pm
I've owned this AMX for ten years and never had a problem. Recently moved to Fla. and having trouble with vapor lock in the fuel pump. temperatures have only been in the 70s and low 80s. In NY I used to drive it on 100 degree days with no trouble. Now after a short drive, shutting the engine off and letting it heat soak for a few minutes, upon restarting it'll run til the carb empties and doesn't pump fuel. replaced the fuel pump, no difference. If I cool down the pump and refill the float bowls it fires right up. Engine temp is fine 170-180 degrees as always.
I'm baffled any ideas other than putting an electric pump in the trunk.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 304-dude Quote  Post ReplyReply Direct Link To This Post Posted: Apr/26/2022 at 7:39pm
My vapor lock with my 73 back in the 80s was from a need to rebuild the carb. The gaskets and seals were over 15 years old. Now with gas being ethanol blend, those old kits don't hold up like they use to. I assume once every 10 years is doing good, with a daily driver.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tufcj Quote  Post ReplyReply Direct Link To This Post Posted: Apr/26/2022 at 9:28pm
It's mostly today's fuels, particularly fuel with ethanol added.  Fuels are way different than they were when our cars were built.   Ethanol has a much lower boiling point, as do other additives.  That's what vapor lock is, fuel boiling in the line, turning to vapor.  The fuel pump can't pump vapor.  Fuel under suction (think between the tank and pump) also has a lower boiling point than fuel under pressure.  That's why new cars have the pump in the tank.   Fuel is under pressure in the entire system.  You can try to find ethanol-free fuel, or put a low pressure pump in or near the tank.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote troutwilly Quote  Post ReplyReply Direct Link To This Post Posted: Apr/26/2022 at 9:52pm
So many questions, where to begin.
- Is the fuel in the tank from NY?  If yes, is it possible the fuels in the north might be different than in the south and therefore more susceptible to vapor lock?
- Is the carburetor in good shape?
- Are the (rubber) fuel lines in good shape?
- Do the fuel lines run too close to heat sources?
- Is the pump diaphragm in good shape?
- And lastly (for now), have you checked to make sure there is spark when it shuts down?  I had a car I swore was vapor locking.  It turned out to the coil failing when hot and working when cooled.
Bill O.
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oldguy View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote oldguy Quote  Post ReplyReply Direct Link To This Post Posted: Apr/26/2022 at 11:18pm
no it's fresh fla fuel
750 quick fuel carb in excellent shape 
rubber lines are all recent
stock routing on the lines not close to heat source
just replaced the pump
msd ignition good spark
the carb has sight glass in the fuel bowls, carb has gas and the engine fires and runs til the carb empties
have a clear fuel filter, the pump ins't pumping, after it cools down the pump works fine
this is the first time I've seen vapor lock at the pump not the carb.
what I really don't understand is how this became so critical so quickly
is the gas formulation any different between NY and Fla ?
 
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Johnny66 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Johnny66 Quote  Post ReplyReply Direct Link To This Post Posted: Apr/27/2022 at 4:48am
I thought it was happening to me, but it turned out to be the screen on the sending unit. It was half plugged, half off. It would run but gas would only seep into filter and the engine would stall and sputter after a short drive.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Boris Badanov Quote  Post ReplyReply Direct Link To This Post Posted: Apr/27/2022 at 5:32am
Got headers?
headers build big under hood temps and cause vapor lock.

Still have a heat crossover stove in the intake.
Another big cause of vapor lock.
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troutwilly View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote troutwilly Quote  Post ReplyReply Direct Link To This Post Posted: Apr/27/2022 at 10:06pm
2 more thoughts:
- It could be the pickup sock in the tank as Johnny66 suggests, getting plugged up by crud in the tank.  As you run it gets more plugged, then when it stops some fuel may flow back to the tank and clear it off making it ready to run again for a short time.  If you can, empty the tank as much as possible and run one of those cameras down the neck and have a look at what's in there.
- Is the tank building up a vacuum as you run?  Try it again until it shuts down and break open the fuel filler cap and see if you can tell if air is rushing in or out.  Might also look to see if the tank is expanding (pressure) or collapsing (vacuum).  Could be the vent is plugged.


Edited by troutwilly - Apr/27/2022 at 10:08pm
Bill O.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BassBoat Quote  Post ReplyReply Direct Link To This Post Posted: Apr/30/2022 at 1:41pm
Ethanol content of the fuel might be different.  Here in the midwest some stations have "reformulated" fuel and my old cars will barely run on it.  Some locales are required to run it, some are allowed to run it.  Try and buy gas in less populated areas as they are less likely to choose to run the reformulated fuel.  I have the best lucK with Amoco Ultra from the BP station.  There is one place near me that sells 91 octane ethanol free and my AMX loves it.  It is a hardware store and they sell the ethanol free for power equipment.  Maybe you can find something like that.  I used to wrap fuel line with aluminum foil to insulate it.  Since ethanol adulteration of fuel is going to get worse before it gets better, a long term solution might be a return line.  Inconvenient but it will solve the problem.  Good luck.


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Post Options Post Options   Thanks (0) Thanks(0)   Quote amx007 Quote  Post ReplyReply Direct Link To This Post Posted: Apr/30/2022 at 2:06pm
Install a-cool can , it will help a lot.
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