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It was really hot out today (100 degrees) and i think i experienced vapor lock. I was driving and all of a sudden the motor died, and i pulled off the road. I opened the hood and it looked all good, 7 PSI of pressure, so i pumped the gas, it spit a couple times and it fired again. Has not happened again. What happened and how do i prevent it?


Eric
 

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gas line gets so hot the gas turns to vapor (gas).

the stock EFI system prevents this with the return line, so cool gas is being pumped from the tank continually.

james
 

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Vapor lock occurs in hot engines that won't re-start after a brief shut-down. (e.g. Stop at an ATM) Vapor lock comes from heat soaking the fuel line and carb where you essentially "boil" fuel out of the float bowl (What's left has lower volatility) and/or vaporise fuel in front of the engine mounted fuel pump (Thus rendering it ineffective for sucking fuel out of the tank.)

Typical cures for vapor lock involve re-routing fuel lines to avoid over heating them, using an electric fuel pump mounted closer to the fuel tank, or simply waiting for the engine to cool.

I have not heard of anyone getting vapor lock in a running engine nor in an EFI motor (In a running engine, the gas doesn't have time to boil before it hits the combustion chamber and with EFI, fuel delivery is being pushed from tank mounted pumps, so there is always positive pressure.)

The symptom you described could also be the early signs of a fuel filter getting plugged.

Good luck!
 

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I had vapor lock once in my 351C powered Midstates Cobra on a very hot day while I waiting for a light to change. The engine just died and wouldn't start until it cooled down a bit. Sucks when you're sitting at a very busy intersection.

I put some heat wrap around the fuel line where it passed closest to the header and never had the problem again.

Cheers
Jim
 

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I had an engine that was prone to this problem. I wrapped the fuel line with aluminum foil as an inexpensive fix. It works fairly well. Getting a heat wrap or using an electric fuel pump are better long term solutions. Scogtt
 

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Electric may not help. If the gas is turning to vapor between the regulator and the carb, it won't make a difference.

If you can, re-route the lines so they'll run cooler or cover them up. Otherwise, run a return line, that's the best way of curing the problem.

-steve in nj-
 

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i had a car that would vapor lock when running on hot days.

i have 2 240z's, the race car has dual electric pumps, the stock 1973 240z came with both an electric and a mechanical pump- the electric was added to prevent vapor lock. the 70-72 just had the mech pump. you can add an electric pump to prevent this problem.

james
 

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Originally posted by CobRat:
Vapor lock occurs in hot engines that won't re-start after a brief shut-down. (e.g. Stop at an ATM) Vapor lock comes from heat soaking the fuel line and carb where you essentially "boil" fuel out of the float bowl (What's left has lower volatility) and/or vaporise fuel in front of the engine mounted fuel pump (Thus rendering it ineffective for sucking fuel out of the tank.)

Typical cures for vapor lock involve re-routing fuel lines to avoid over heating them, using an electric fuel pump mounted closer to the fuel tank, or simply waiting for the engine to cool.

I have not heard of anyone getting vapor lock in a running engine nor in an EFI motor (In a running engine, the gas doesn't have time to boil before it hits the combustion chamber and with EFI, fuel delivery is being pushed from tank mounted pumps, so there is always positive pressure.)

The symptom you described could also be the early signs of a fuel filter getting plugged.

Good luck!
Cobrat, you have blended two very similar Heat related problems.. Fuel bowl boiling and Vaper lock.


Pure Vapor Locking.. . . . Imagine Freeze dried coffe.... If you put a liquid under a low pressure situation it will boil at a lower temp. If you put a liquid under pressure, (like a radiator pressure cap) it will boil at a higher temp.

the reason OLD cars with a mechanical fuel pump were so proned to vaper lock were several.. the main reason being the fuel was being PULLED (sucked) to the fuel pump, ie lower pressure and the fuel would boil at a lower temp... when it boils, you get a whole lot more vaper than you had gasoline. so the fuel pump PUMPS, PUMPS and PUMPs Vapor while you run out of gas at the carb.

Insulating the fuel line helps a great deal and often solves the problem

with an electric fuel pump, placed back at the gastank, you are pushing fuel, not sucking it. SO, the gasoline is under pressure causing a higher boiling temp which reduces vapor locking. PLUS, if you do get some boiling in the line, the pump quickly pushes the vapor bubble thru the carb.

As said by several folks, True Vapor locking with modern EFI cars with return lines is a pretty rare thing.....

earl
 

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Earl,

Agree. I included the fuel bowl boiling cause, as it is commonly included in a typical vapor lock diagnosis.

FWIW, I liked your freeze dried coffee example. All I could think of were those vacuum glass vials filled with alcohol that boiled when you held them in your hand, and figured that few would remember them.

Regards.
 
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