Fuel vapor pressure is a measure of how easily fuel sample evaporates. Any additives used in gasoline contain aromatics. Aromatics are light hydrocarbons distilled off the top of a crude oil sample. They are effective at increasing the research octane of a fuel sample but can cause vapor lock (bubbles in the fuel line) on a very hot day If you have an inconsistent running engine an you suspect a vapor lock, use a piece of clear fuel line to look for bubbles, indicating that the fuel is vaporizing. One negative side effect of aromatics is that they create additional combustion products such as carbon and varnish. If your engine requires high- octane fuel, to prevent detonation, de-carbon the engine more frequently with an internal engine cleaner to prevent ring sticking due to excessive varnish buildup.