"Vapor lock is caused when heat from the engine or exhaust "cooks" the fuel in the lines or pump into a bubbly state-similar to the emulsion of air and fuel we discussed in the carburetor section. When the fuel pump cycles against this emulsion, the emulsion acts like a rubbery sponge, absorbing the pump energy without moving any liquid fuel and prohibiting the fuel to flow. Without liquid fuel flow, the engine will not start.
Vapor lock can-and does-occur due to design problems at the car manufacturing level. However, it most frequently occurs when the gasoline refiners and marketers put high volatility additives in the fuel which normally boil at much lower temperatures than gasoline ordinarily does. This occurs when the refiner tries to "get away with" extending the fuel with non-gasoline additives. It also occurs during "warm snaps" between winter and spring, when the high volatility additives, which are supposed to make cold starting easier, cause vapor lock problems.
If the fuel pump problem occurs only at slow speeds, or after a "heat soak" period when the engine is off, recommend changing to a different brand of fuel which is known not to contain an alcohol blend or octane booster. The problem may cure itself".
"There's a couple of things you can do to help prevent vapor lock. The first, is to understand the dynamics of what is happening. The gasoline in the fuel line gets super heated and begins to "vaporize" in the line. It literally "percolates". When this happens, the fuel pump cannot "pump" a steady stream of vapor, air & fuel. So, it "vapor locks". When you pull over to the side of the road and idle the motor you are taking all the load off the system. It only has to cool down a few degrees for the symptoms to disappear.
Now that you know what vapor lock is, how does the fuel get heated. This is something that can drive a person crazy! Sometimes, when new lines are run from the tank or if new exhaust systems are installed (particuarly headers) they come into close contact with one another. NOT! This can't happen! Vapor lock can occur. On Mopars, when the fuel outlet tube from the fuel pump is routed to the carburetor, it sort of makes a vertical bend upward on the left front side of the motor (facing the front of the engine). Sometimes, this tube is too close to the engine block and/or the upper radiator hose and/or the heater hoses and the line can become super heated.
So what do you do? Climb under the car - better yet, put it up on a rack - and determine that the line coming from the tank to the inlet side of the pump is "A-OK" and not too close to any exhaust component. Next, check the outlet line going to the carb. Ensure you're not too close to any hot water hoses and/or engine block or manifold. Next, you can insulate the fuel line (usually the outlet side only) with a wrap to ensure it doesn't get too hot. The same stuff guys use to wrap headers and/or manifolds will work great!
Very definately you want to ensure your filter is clean. The best way to keep it clean is to ensure your tank is clean. If you haven't done so already, you may want to remove your tank and have it boiled-out! You can replace all the rubber & gaskets at the same time. If your tank is "gunky" no amount of new filters is going to keep you on the road. Those in-line filters plug really easily.
Lastly, ensure your pump is good and serviceable. In extreme cases, you can install an electric pump, but this is no substitute for correcting the real problem. The other thing to look for, and I'm surprised someone didn't tell you, is to replace the fuel pump pushrod if you haven't done so. I would never recommend re-using the "old" fuel pump pushrod after an engine overhaul. They do wear and you'd be surprised just how much this can impact your fuel delivery system! I was talking with the guy who owns "Rare 440" at the reunion and he was complaining how his car was "acting up" and he thought it was vapor lock. Without knowing, I mentioned the fuel pump pushrod. He then pulled it from his pocket and was telling me 'that' was the culprit".
"Vapor lock is highly unlikely with today's fuel systems. Vapor lock occurs when the vapor pressure of the fuel is higher than the surrounding environment. In older engines with "sucking" fuel pumps at the engine the pressure in the fuel line to the tank was reduced by the sucking action of the fuel pump drawing fuel form a fuel tank nearly twenty feet away, and when heated, the gasoline actually boiled creating a vapor of gasoline which the fuel pump could not handle, thus the name "vapor lock".
Today's fuel pumps are in the fuel tank where they push the fuel under pressure to the engine systems. Excessive fuel which is not used is sent back to the fuel tank. This causes a constant flow of pressurized, cool fuel in the lines to the engine system, be it fuel injection or carburetor. As a result you would have to heat the fuel line with a propane torch to get the fuel to boil and even then it would be cooled by the flowing fuel circulating from the tank to the engine and back again to the tank. Vapor lock as we once knew it is a thing of the past".
Finally, if you have a late model vehicle:
There are (how many?) millions of good, older vehicles on the road and vapor lock is not a thing of the past to those of us who own and drive them; it is a hateful reality every time we get on the highways. We never know whether the required Ethanol label is on the gas pump, or whether the store clerk knows if the gasoline is mixed with Ethanol, or even if the station operator tells the truth when asked.
In the interest of safety and good business, a customer should know what kind of gasoline is being pumped into their vehicle and every city in every state should have some gas stations that sell non-Ethanol gasoline and a way for drivers of carburetored vehicles to find them.
Doug
