A common problem with older cars is vapor lock, which can often prevent a car from restarting after a hot soak. That is, difficulty in restarting a car with a fully-warm engine after it has been shut down for a short time (like after refueling). To clarify, Renton Technical College's automotive glossary defined hot soak to be:
- Condition that may occur when an engine is stopped for a period of time after a hot hard run, causing the gasoline to evaporate out of the carburetor.
- A period of time where localized combustion heat dissipates through the engine.
- Allowing a vehicle to sit at a high temperature until all internal and external components stabilize.
- Federal test procedure for new vehicle certification of overall emission output. This test is normally done in a sealed chamber and all emissions coming from the vehicle are measured. These include exhaust, evaporative. tires, paint, and interior component emissions, in other words, all emissions given off from a vehicle.
The vapor pressure of a liquid depends on its temperature and gasoline requires a low boiling point in order for it to easily vaporize in the carburetor. Petroleum companies vary the volatility of their gasolines according to the seasons as a trade-off between good vaporization and vapor lock. Gasoline volatility is measured by its Reid Vapor Pressure (RVP). Modern gasolines are comprised of a variety of hydrocarbon components, each with their own boiling points.
- Modern petrol in vintage engines - BP
- Gasoline Vapor Pressure Control - Cemda
- Gasoline Volatility Information - Michigan Department of Agriculture and Rural Development Laboratory Division / Motor Fuels Quality
Ethanol-free gasoline (ie, E0) is still available but usually as a high octane (91+) fuel. See Ethanol-free gas stations in the U.S. and Canada. Try a couple of tanks of E0 to see if this improves your car's hot weather performance. Using high-octane fuel when not necessary to control engine knock (pinging) can add a significant cost to running your vehicle and there are other solutions that work as well or better at a lower long-term cost.
The fuel systems in older cars commonly use a mechanical fuel pump mounted on the engine that is driven by a lever actuated by an eccentric cam on the camshaft. Unless the fuel line is in close proximity to the exhaust system, gasoline gains very little heat as it moves from the gas tank to the fuel pump. From the mechanical fuel pump to the carburetor, the fuel line will absorb some heat from the hot environment surrounding the engine.
Mechanical fuel pumps are generally diaphragm pumps where the lever moves a diaphragm and an internal spring controls the mechanical pump's outlet pressure. Check valves at the pump's inlet and outlet control the direction of flow and the pressure within the pump chamber alternates between suction and pressure with every stroke. However, at low or no-flow, the temperature of the fuel in and around the fuel pump can easily increase to the point where the fuel flashes into vapor on the suction stroke. Mechanical pumps do not pump vapor very well so carburetors can experience fuel starvation until the fuel condenses back into liquid. That is why a car that won't start when hot will easily start after given some time to cool down. This process can be accelerated by hosing down the fuel pump with cool water.
The carburetor can also become hot enough to boil gasoline. Without a system to vent and recover vapor from the carburetor, this manifests itself as a progressively worse idle (along with the stink of increased hydrocarbon emissions) while idling in traffic or additional cranking to refill the carburetor after a hot soak. There is nothing to be gained from removing the evaporative emission controls (charcoal canister & associated plumbing) and will actually cost the vehicle owner in terms of wasted fuel.
The advent of fuel injection has largely eliminated vapor lock from modern vehicles. The fuel pump is located in the gas tank and gasoline is pressurized until it is injected into the engine. To minimize or prevent vapor lock in older carbureted cars, it is important to minimize the heat gain of the fuel until it leaves the carburetor as an atomized liquid.