BACK TO VAPOR LOCKS
So let’s circle back to the vapor lock issue. Vapor lock is closely related to the volatility of gasoline—that is, the gasoline’s tendency to vaporize under a given set of conditions. The unit of measurement for volatility is known as “Reid Vapor Pressure” or RVP—vapor pressure of gasoline at 100 degrees Fahrenheit. It is actually measured by placing a specific quantity of gas in a sealed ‘bomb,” shaking it and reading the attached pressure gauge.
Normal atmospheric vapor pressure (the pressure of air above our heads) is about 14.7 lbs/sq/in. Any liquid that has a vapor pressure greater than the atmospheric pressure (remember, this is typically 14.7) will boil. The higher the RVP rating, the greater the tendency of fuel to vaporize (and vapor lock).
Fuels sold in the winter are blended to have a higher RVP than those in the summer to aid in starting. By increasing the RVP, they lower the boiling point and increase its volatility so it takes less to turn it into vapor and hence start in the cold. In the summer, the warm temperatures are already assisting in the vaporization process so the gas itself doesn’t need to be as volatile.
Winter: High RVP = Lower Boiling Point = Faster Vaporization
Summer: Low RVP = Higher Boiling Point = Slower Vaporization
Raising the winter RVP is usually done with the addition of butane that is very volatile (high RVP) and inexpensive. The highest RVP gasoline commonly sold will have an RVP of around 15.0psi (measured at 100 degrees F). If that gas were in your tank on a hot summer day when the temperature reaches 100 degrees, it will boil in the tank. On a 70 degree day, the heat in the engine compartment will reach well over 100 degrees, especially in a parade or at rest.
Vapor lock often occurs when you first stop and shut off the engine as the temperature in the engine compartment will dramatically rise with no airflow. If the gas in that truck has an RVP of 15 and the atmospheric pressure is 14.7 you will have the gas boil in the engine compartment (carburetor) and enjoy what you commonly know as “vapor lock.”
The RVP standards do vary throughout the country as the RVP in Duluth, Minnesota, needs to be significantly higher in winter than that in Jacksonville, Florida. If you live in a northern climate you probably experience the full range of RVP rated gas through the course of the year. This is very important to note.
Remember that I had vapor lock at the Memorial Day parade even though I thought I had the problem licked? My inspection on this truck was due in early spring, so while it was out, I filled up the gas tank.
I live in a relatively rural area, so the gas station probably gets filled every couple weeks. Depending on the volume of business a gas station does, I could have easily filled the truck with Class D fuel with a boiling point of just 131 degrees. Do you think the engine compartment could reach 131 degrees on an 87 degree day moving along at parade speed of about 3 mph? You bet! Add to this, ethanol raises the RVP by an additional 1 psi (from 13.5 to 14.5). There is no question boiling can occur. In general, it can be said that ethanol will increase the likelihood of vapor lock by 10% just because it raises the RVP so much.
It was the perfect storm of low boiling points and hot engines. Like taking the cap off of a very hot radiator, and having it violently boil and steam, boiling is most likely going to occur where there is a sudden drop in pressure. This can happen just prior to the fuel pump or as the fuel enters the carburetor bowl.
At one of these points, the slight pressure created by the fuel pump within the fuel line will drop to zero, immediately lowering the boiling point. As the fuel boils, it turns to a frothing combination of boiling gas and vapor. This vaporish, boiling brew will not flow and is highly resistant to pumping, so normal flow will cease until it cools back to just liquid.
These issues are pretty much restricted to carbureted engines. Newer engines with fuel injectors are generally not susceptible to vapor lock.
