Vapor lock occurs when liquid fuel in your car's fuel system overheats, vaporizes, and forms bubbles that block the flow of gasoline to the engine. This is a common issue in older vehicles with carburetors, especially during hot weather or after the engine is turned off. It happens because the fuel, typically gasoline, is designed to be a liquid but can boil and turn into a gas if it gets too hot, creating a blockage in the fuel line or fuel pump.
The process is a matter of physics. Modern gasoline is a blend of hydrocarbons with different boiling points. When the temperature under the hood rises—due to hot weather, a malfunctioning cooling system, or heat soak from a just-shut-off engine—the lighter, more volatile components in the fuel can vaporize prematurely before reaching the combustion chamber. This vapor is compressible, unlike liquid fuel, so the fuel pump, which is designed to move liquid, can't push it through the system. This leads to a lean air-fuel mixture, causing the engine to stumble, hesitate, stall, or fail to start.
Certain conditions make vapor lock more likely:
- Vehicle Age: Carbureted engines are far more susceptible than modern fuel-injected ones. Fuel injection systems operate at much higher pressures (30-80 PSI), which helps suppress vapor formation.
- High Ambient Temperatures: Hot summer days are the prime season for vapor lock.
- Ethanol-blended Fuels: While ethanol has a higher octane rating, it can have a lower vaporization temperature than pure gasoline, potentially increasing vapor lock risk in some classic cars not designed for it.
- Component Location: A mechanical fuel pump mounted on the engine block or fuel lines running too close to exhaust manifolds are common culprits.
| Factor | Increases Vapor Lock Risk? | Reason |
|---|
| Carbureted Engine | Yes | Low-pressure system, fuel is pulled by vacuum, easier for vapor to disrupt flow. |
| Fuel-Injected Engine | Rarely | High-pressure system forces fuel to remain in a liquid state. |
| Hot Weather | Yes | High under-hood temperatures cause fuel to boil. |
| High Altitude | Yes | Lower atmospheric pressure lowers the boiling point of liquids, including fuel. |
| Ethanol Content (E10+) | Potentially | Ethanol's volatility can lower the fuel blend's overall vaporization point. |
| Faulty Coolant System | Yes | Overheating engine raises temperatures throughout the entire engine bay. |
If you suspect vapor lock, the immediate fix is to cool down the fuel system. Pop the hood to let heat escape. You can carefully pour cool water over the fuel line and fuel pump (avoid electrical components). Once things cool down, the vapor should condense back into a liquid, allowing the engine to start. For a long-term solution, consider installing heat shields around fuel lines, rerouting lines away from heat sources, or switching to a fuel with a higher vapor pressure rating if available.
