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Vapor Lock In Old Truck

I have a 1978 Ford F150, purchased used, lots of miles. Runs well, but when I start it and drive for 15-20 minutes and shut if off, if I try to restart after another 10 minutes or so, it won’t. So, I let it sit for 20-30, and then it will restart. Not every time, but getting more frequent. Does this sound like vapor lock, and what are my possible options for prevention, and is there a way to get it restarted more quickly, so I’m not late for work, etc.

Try starting it as if it were flooded. Hold the gas pedal to the floor and crank it. If it turns over faster and faster and then starts, it was flooded. The carburetor is leaking internally, fuel leaking down into the engine. Usually caused by a failed plastic float or a defective needle & seat. You can remove the air cleaner and simply look down the throat of the carb after you shut it off. If it’s full of vapor or you see it dripping, that’s the problem.

It does sound like vapor lock.
If this turns out to be the problem there are a few things you can do to help alleviate this.

If the fuel pump/carburetor fuel line is the original steel one you can try cutting some of it off and replacing it with a rubber line. You might consider using fuel injection hose; it’s pricier but much more durable.

You might also note if the carburetor has a fiber block underneath the carburetor. Some Fords had fiber spacers about a 1/4" thick and others do not. You could also consider the addition of a thicker phenolic block which would help even more. These blocks are available at AutoZone, O’Reillys, etc. and are about an inch thick. The original intent is to change the engine torque characteristic a bit but can help on vapor lock problems by insulating the carb body from engine heat somewhat. Hope that helps.

To determine if it’s vapor lock, carry a bottle of water in the vehicle with you. The next time the engine doesn’t start, get out and pour this water along the fuel line to cool it down. Now try starting the engine. If the engine starts right up, you’ve confirmed it’s a vapor lock problem.

Among the other suggestions, wrapping the fuel line with a insulating material sometimes fixes this problem.


Another problem with the early Electronic Ignition Fords was the Ignition Control Module. The Module had some issues with Heat Sink from the engine. When moving, the Module stayed cool but when parked, the Engine heat would soak into the Module and cause this condition. It would always restart after a cool down.

15 minutes of driving seldom causes vapor lock or heat problems, especially this time of year…The OP could check for ignition spark the next time this happens…Simply remove the coil wire from the distributor cap, hold the end about 1/2" away from any metal surface and have someone crank the engine while you check for spark jumping off the end of the coil wire…

If you try the spark plug wire check, hold the wire with an insulated tool of some kind. Otherwise, you’ll get a pretty good ZAP! of 40,000 volts+.

Already replaced the coil, still happens. Plug wires, etc. seem in good shape too.
Will try insulating fuel line, see if that helps. Water bottle a good idea, in case of emergency, will try that next time.
Thanks everyone.

Raplacing the coil wasn’t the suggestion. The suggestion was to remove the coil wire to check for spark. You’ve apparently still not done that, so you still don’t know if your problem is fuel or spark related.

There are MANY other things that can cause a no-spark condition besides a failed coil.

It sounds like vapor lock or a bad ignition module. The module is a block of aluminum sitting at the left rear corner of the engine compartment up high. It as some bumps on it that resemble fins. It is cheap to just replace it. They will ask if it has a blue or black grommet. If you turn it over, you may see that it is blue and not black. The top gets dirty and always looks black. Then if changing that doesn’t work, try for vapor lock. You relocate the fuel line a little to prevent vapor lock. You shift it away from anything hot or you insulate it with a sliced new fuel hose or some heatproof gasket material.

Caddyman has the best suggestion about checking for vapor lock. After shutting it off wait a few minutes and look down the carb throat.

Some of the Asian vehicles used carbs that had sight glasses on the side. When those cars were vapor locking one could look at the sight glass and actually see the gasoline in the float bowl boiling just like a coffee pot.