Vapor lock

Dan Baron

Dan Baron

Senior Member
Can anyone tell me what are the simtoms of vapor lock? My 30 Town sedan every once in awhile will act like it's running out of gas. After 15 seconds or so it's fine. Is this what vapor lock is? Is insulating the gas line the best solution to the problem?
 
Sounds like vapor lock...but it could be several other issues, as well. Is it hot out when the symptoms appear...?

Insulating the gas line is a very good approach to solving a vapor lock problem. You may want to try it and see if the symptoms dissappear...
 
If it is OK after 15 seconds, you likely have restricted gas flow, not vapor lock.
If it occurs when it is less than 95 degrees you likely have restricted gas flow, not vapor lock.
If it occurs while running at road speed you likely have restricted gas flow, not vapor lock.
Make sure the gas cap vent operates properly.
Remove the fitting from the tank, clean out everything.
Make sure the fuel line is not more than 1/8" past the compression ferrule at the carb.
 
See...? There's lots of "other" problems that can look like symptoms of vapor lock. Can you give us some more information...? Things like...

...does it just slow down, or does the engine quit?
...does it happen more often while at speed?
...does it happen more often when the car's hot?

Have you performed a gasoline flow test...? (removing the gas line and allowing it to run into a container to check the stream)

Have you made sure the vent in your gas cap is open and not clogged with rust or something else?

Have you cleaned the filter in the sediment bowl? ...in the carburetor?

There's lots of possibilities here that can cause issues similar to vapor lock... :)
 
Next time stop and just turn the gasoline cap and listen to hear a hissing sound or after loosening the cap try starting it. Gas is Gas and liquid is Gasoline. I sound like a broken record .
 
It only happens when it's hot, engine quits at 50 mph but not all the time. I've driven 55 on the hyway 45 min. at a time with no problem at all, in 85' weather. I've taken the gas line apart and I have great flow. Filter screens are clear. Although I never removed the shut off valve on the tank itself to see if there is a filter there. I know that somtimes people put one there. But with good flow I don;t think that's the problem.
 
Sounds like coil -condensor problem. A bad coil will act up when hot and as soon as it cools a little will run for a while. Many vapor locks are cured by changing coil,condensor or both.
 
Sounds like it might be your gas cap. If the cap's vents are constricted, it might be producing a vacuum in the tank itself, preventing good flow to the carb. Next time it happens...follow kp's advice. Pull over, loosen gas cap a listen for a telltale "sssssss". If it hisses, it's probably the gas cap vents. This sounds like more of the possibility if it restarts right away without waiting to cool off some. Normally, a vapor locked car won't restart right away...until it cools some. If yours starts right back up...sounds like it could be a vacuum in the tank.

Once the vaccum builds up and the engine doesn't get enough fuel, of course it stops. But, once the car stops...the vaccum in the tank sucks air in through check valve in the carburetor and up through the gas line until the pressure is equalized. Then, it's easy to start again....right up until it happens all over again.

Hope this helps... :)
 
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Thanks guy's for all your expert advice. I'll definitely check the gas cap. That's something I thought coulden't be the problem, because it's always been there, and I never though to look at the vent hole. Thanks again....
 
One of the members of our club had this same problem a few weeks back during our "Vehicle Safety Check Day". Not only did he have ignition problems, he had huge amounts of rust particles in his fuel, which sometimes and momentarily plugged his fuel line and the carb jets. We ended up flushing his tank and fuel line the best we could, installed a fuel valve screen, and sediment bowl filter, plus rebuilding the carburetor and cleaning out the jets and ports. We also believe his exhaust manifold to block is leaking. No engine pans or carb filter either. Not a good scene.
 
If you also get backfiring it might be electrical. Are points just about >.18 but not quite .20? When setting timing, are points just beginning to open? Is pigtail wire tight at connection to points & not touching the plate?
 
When is the season premier of Myth Busters? By the way when it's running, it runs just fine, no backfiring, good idle smooth ride. I haven't had the time to check gas cap, but will tonight.
 
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