What exactly is vapor lock? My friends say I have a problem with vapor lock. Last summer, my small block 350 Camaro had problems in the heat of late summer where it would start running poorly and eventually quit running and it has already happened again this year. If I let it cool down for a couple of hours, it starts hard but eventually runs okay. This doesn’t happen all the time, but it seems like it happens more when I drive in traffic. Any thoughts?

K.C.

Jeff Smith: Vapor lock is a term used to describe when fuel changes from a liquid to a vapor before it arrives at the carburetor. Most enthusiasts tend to focus on the pressure side of the fuel delivery system when the real problems are more likely located on the low pressure or inlet side of the pump. But first, it’s important to mention that fuel is designed to vaporize. Liquid fuel doesn’t really burn. It’s the fuel that has vaporized that supports combustion. Gasoline is a complex mix of light and heavy hydrocarbons that are designed so the light or volatile portion of the fuel vaporizes quickly, even at normal or low temperatures. This allows a cold engine to start quickly. So we’re dealing with a fuel that is designed to vaporize.

ASK-08-01Now let’s imagine we have a very warm day of 90 degrees. At this temperature, roughly 20 percent of the fuel has already converted to a vapor as measured at normal atmospheric pressure. Now let’s introduce a mechanical fuel pump into this application where the pump must “pull” fuel from the tank all the way to the engine where it can now push the fuel into the carburetor. Before we go any further, it’s important to note that pumps do not “suck” the fuel. The pump creates low pressure on its inlet side and higher atmospheric pressure pushing down on the fuel in the tank (because the tank is vented), is what pushes the fuel toward the pump. However, the fuel is still subject to lower atmospheric pressure. This reduced pressure also lowers the boiling point of any liquid and especially gasoline which means a higher percentage of fuel will vaporize.

Can you see where we’re headed?

So we have hot fuel (that has been percolating in the fuel tank exposed to heat coming off the pavement that might be 120 degrees F, and now we subject the fuel to lower pressure in the fuel line. Then, if there is any kind of restriction in the fuel line between the fuel in the tank and the pump at the front of the vehicle, this restriction will create even lower pressure in the line heading to the pump inlet. For example, an older car with an original and a fossilized sock stuck on the fuel inlet in the tank could cause such a restriction. Or perhaps the fuel line has been kinked or bent or the existing tank vent has become plugged or restricted. Any combination of these situations will force the pump to work harder and create lower pressure on the inlet side in an effort to continue to pump the fuel.

This creates a perfect scenario for vapor lock.

What really happens is that enough fuel turns to vapor that a pump designed to push liquid is now trying to pump vapor. That won’t work very well and the result is that fuel pressure drops very low as only a small portion of the fuel is still in liquid form and is not sufficient to feed the engine. So the engine stumbles and dies.

A common and successful remedy for inlet side vapor lock problems is to place an electric pump inline near the tank inlet. If you think about this, this is an excellent solution because it minimizes the distance the mechanical pump must work to move fuel under low pressure. With an electric pump, the fuel pump has now pressurized the fuel during a majority of the distance it must travel, greatly reducing its exposure to low pressure. This radically reduces the chances of the fuel vaporizing since higher pressure requires higher temperature to vaporize the fuel. This is a big reason why in-tank electric fuel pumps are used in all new car applications.

So far, we have mainly discussed the inlet side of the fuel system because this is where the majority of the problems occur. However, it is also possible that fuel can percolate in the fuel line between the pump and the carburetor–especially if the fuel line is exposed to high temperatures. We’ve seen a small block Chevy experience vapor lock when the car builder positioned the fuel line directly against a heater hose that literally pre-heated the fuel in the line. Relocating the fuel line solved that issue. Other remedies for fuel boiling in the carburetor include heat insulation plates (such as a phenolic spacer) between the carburetor and the intake manifold. A cold air induction system for the carburetor will also help since this cooler air will keep the carburetor cooler. This can be especially helpful when underhood temperatures can easily reach as much as 160 degrees F.

We’ve offered several solutions to the problem with the important information emphasizing the inlet side of the pump rather than the pressure side. This is not based on speculation, but rather on simple physics that point to the low-pressure side of the fuel delivery system.

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Ask Away, Jeff Smith, vapor lock
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Author: Jeff Smith

Jeff Smith has had a passion for cars since he began working at his grandfather's gas station at the age 10. After graduating from Iowa State University with a journalism degree in 1978, he combined his two passions: cars and writing. Smith began writing for Car Craft magazine in 1979 and became editor in 1984. In 1987, he assumed the role of editor for Hot Rod magazine before returning to his first love of writing technical stories. Since 2003, Jeff has held various positions at Car Craft (including editor), has written books on small block Chevy performance, and even cultivated an impressive collection of 1965 and 1966 Chevelles. Now he serves as a regular contributor to OnAllCylinders.