Vapor Lock: Symptoms, Causes, and Solutions

Automotive technology has improved a great deal over the years. In the past, vintage vehicles equipped with carburetors often suffered from vapor lock—a phenomenon that’s rarely a concern with today’s fuel-injected engines.

Vapor lock can cause a variety of problems that make a vehicle difficult to start and drive. If you’ve got a classic car, it’s helpful to understand vapor lock and the associated symptoms

What is Vapor Lock?

Vapor lock is a phenomenon that occurs when liquid fuel becomes hot enough to vaporize. The vaporized fuel creates a lean condition (not enough liquid fuel) that prevents the fuel system from delivering the proper amount of gasoline to the engine.

Problems with vapor lock are most common on older vehicles that have a carburetor and a mechanical, engine-driven fuel pump. This type of setup, which operates at relatively low pressure and locates the fuel pump a long way from the gas tank, increases the likelihood of a vapor lock between the tank and pump.

What’s more, vapor lock can have a significant impact on a carbureted fuel system, as the issue can lead to the float chamber inside the carburetor being depleted of gasoline. The engine is starved of fuel as a result.

Meanwhile, modern fuel-injected engines have the fuel pump located in the gas tank. The location keeps the pump running cool and the entire fuel system under higher pressure, thereby reducing the risk of vapor lock.

Common causes of vapor lock include:

• High outside ambient temperatures
• Extreme engine operating temperatures
• Fuel lines routed too close to the exhaust system and other hot components
• Running a winter fuel blend during warm weather
• Driving at a high altitude

What are the Symptoms of Vapor Lock?

Vapor lock prevents the engine from getting the correct amount of liquid fuel. The problem can lead to a variety of noticeable symptoms, such as:

Stalling and Hard Starting

A gasoline engine needs three primary ingredients to run: a precise air-fuel mixture, good compression, and proper spark. Vapor lock can disrupt the delivery of liquid fuel to the engine, causing the vehicle to stall. The engine may also be difficult to restart once it’s warmed up.

Misfiring and Rough Running

Similarly, vapor lock can cause fuel delivery problems that lead to incomplete combustion inside of the engine. As a result, you may notice that the vehicle misfires and runs rough.

Poor Acceleration and Hesitation

An engine that is starved of liquid fuel (due to vapor lock) may exhibit poor acceleration and hesitation.

How Do You Fix a Vapor Lock?

Exactly how you address a vapor lock problem will depend on what caused the condition in the first place. For example, if the vapor lock is occurring due to engine overheating or misrouted fuel lines, you’ll need to make some repairs to fix the problem permanently.

You can, however, take some steps to relieve the fuel system from vapor lock temporarily. First, you must let the fuel cool so that it returns to a liquid state. You can get the fuel system to cool more quickly by taking steps, such as parking in the shade and opening the hood.

Once the fuel system has cooled, you can help eliminate any vapor left in the fuel system by first pressing the accelerator pedal slightly (do not press it to the floor) while cranking the engine. Once the engine starts, hold the accelerator pedal down until the vehicle runs smoothly, indicating the vapor lock is gone.

Vapor Lock Solutions

If you have an older, carbureted vehicle, there are some steps you can take to prevent vapor lock. For one, you’ll want to make sure the fuel lines are routed away from the exhaust system and other hot components. You might also consider covering the fuel lines and the carburetor in heat shields. Another option is to retrofit the vehicle with an in-tank electric fuel pump.