What Vapor Lock Is and How to Fix It

Updated on October 27, 2019
Dan Ferrell profile image

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

Vapor lock may prevent your engine from starting.
Vapor lock may prevent your engine from starting. | Source

There are several symptoms that may point to vapor lock.

When your engine is warm, it may:

  • run rough
  • lack power
  • stall and start after cooling for an hour or two
  • be hard to start
  • not start
  • perform poorly

Several factors can contribute to vapor lock:

  • High underhood temperatures
  • Fuel lines next to or touching hot engine components
  • Fuel pump next to hot operating engine
  • Clogged fuel return system
  • Hot summer weather
  • Stop-and-go traffic
  • Dirty or restricted in-tank filter
  • Faulty fuel tank vent
  • Ethanol-enhanced gasoline
  • Geographical areas with high altitude

The following sections explain why vapor lock occurs, and how to test for, fix and prevent vapor lock.

I. Why Vapor Lock Occurs
II. Vapor Lock Fix
Testing for vapor lock
III. Preventing Vapor Lock
Ambient and engine high temperatures can transfer to the mechanical fuel pump, carburetor and fuel lines, creating vapor lock.
Ambient and engine high temperatures can transfer to the mechanical fuel pump, carburetor and fuel lines, creating vapor lock. | Source

I. Why Vapor Lock Occurs

Vapor lock occurs when fuel overheats and vaporizes in a fuel line or carburetor, preventing proper fuel flow.

Although common in older vehicle models, it wasn't until the appearance of the modern fuel injection system that vapor lock became more prevalent in carbureted engines.

Fuel-injected engines use a more volatile fuel that is easier to vaporize and mix with air for a much better combustion. So this new fuel has a lower boiling point.

However, vapor lock rarely occurs in a modern engine because of the use of an in-tank, electric fuel pump. This allows the pump to operate at a low point in the tank, submerged in fuel, and under a lower temperature than the older mechanical pump located in the engine compartment. Also, fuel in a modern fuel delivery system fuel is pressurized.

Furthermore, modern engines are equipped with an electric cooling fan, making it even harder for vapor bubbles to form in the fuel lines under high ambient or operating temperatures.

In contrast, most carbureted engines use a mechanical fuel pump to pull fuel out of the tank to feed the engine. This creates a vacuum in the fuel line (lower atmospheric pressure) that makes fuel more prone to vaporization under engine high temperatures.

Besides, this mechanical fuel pump sits next to the engine, allowing hot ambient, underhood and engine temperatures (usually above 100 F or 37.8 C) to transfer to fuel lines and carburetor.

The heat vaporizes the nonpressurized fuel in the line or carburetor. This vapor expands and creates bubbles that displace fuel, reducing or preventing flow altogether.

Still, a modern fuel-injected system can suffer from vapor lock if pressure in the fuel line drops (faulty fuel pressure regulator, for example) and the fuel line is subjected to high temperatures.

Have an assistant crank the engine while conducting the test.
Have an assistant crank the engine while conducting the test. | Source

II. Vapor Lock Fix

Diagnosing a fuel system for vapor lock problems is not a clear-cut process, but you can search for clues that may point to it. For example, ask yourself the following questions:

  • Does your engine uses a carburetor?
  • Does the engine hesitate or stumble once it warms up?
  • Does it only happen under hot weather conditions when driving in heavy traffic?
  • Does the warmed engine stalls and won't start until it cools?

Although rare, a fuel injection system can show the same symptoms if fuel vapor finds its way through the delivery system. However, similar symptoms can appear when dealing with a faulty ignition coil, ignition control module or fuel pump. In this case, though, you'll notice the same symptoms even in cold weather. A component with a broken coil or electrical circuit may only to reach operating temperature to fail.

Testing for Vapor Lock:

If your engine show these symptoms, try the following test the next time your warmed engine stalls or is hard to start when warm:

  • Pull your vehicle to the side of the road, in a safe place.
  • Open the hood.
  • Remove the air cleaner assembly to gain access to the throttle plate or valve on your carburetor or throttle body.
  • Have an assistant crank the engine or use a remote starter switch.

    If you are using a remote starter switch:
    • connect one of the switch leads to the battery positive terminal and the other one to the ignition switch terminal on the remote relay or starter solenoid.
    • Set the transmission to Neutral (manual) or Park (automatic).
    • Engage the emergency brakes.
    • Turn the ignition switch to the Run position.
    • Crank the engine using the remote switch.
  • As the engine cranks, spray a shot of starting fluid through the throttle valve.

If the engine seems to catch while spraying the starting fluid, there's a good chance the fuel delivery system is vapor locked.

You can do a similar test using a small plastic bag with ice:

  • After your warmed engine stalls, pull to the side of the road.
  • Open the hood.
  • Place a bag of ice on the fuel line between the fuel pump and carburetor and the one that connects to the fuel pump to bring down the fuel line's temperature and allow vapor fuel to condense.
  • After a few minutes, try starting the engine.

If the engine starts, there's a good chance vapor lock is blocking fuel flow.

An in-line electric fuel pump can prevent vapor lock.
An in-line electric fuel pump can prevent vapor lock. | Source

III. Preventing Vapor Lock

There are several potential solutions to prevent fuel lines from overheating and solve vapor lock issues in a carbureted or, when necessary, in a fuel injected engine. For example:

  • Check that fuel pump and fuel lines, including the one between the pump and carburetor, are not too close or touching hot engine components.
  • Make sure the fuel pump and fuel filter are not restricted.
  • If your fuel system has a return line, make sure it's not plugged.
  • Heat shield fuel lines, pump and carburetor with special metal plates, or insulating tubing.
  • Install a thick composition spacer block or carburetor heat insulator to isolate intake manifold heat from the carburetor.
  • In a carbureted engine, install an in-line, low-pressure, electric fuel pump near the fuel tank.
  • Install an electric fuel pump near the fuel tank with a vapor discharge (return) valve (this requires a return line).
  • Replace the old cooling fan with a modern electrical cooling fan, if possible connected to the engine temperature sensor.

Sometimes, all you need is a carbureotr heat insulator as you can see in the video below. If necessary, consult with a local shop that may suggest possible solutions to vapor lock for your specific vehicle make and model.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

Questions & Answers

© 2019 Dan Ferrell


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    • Dan Ferrell profile imageAUTHOR

      Dan Ferrell 

      12 days ago

      Try insulating the fuel lines that run close to the exhaust. There's too much heat. That might be causing trouble. Hope this helps.

    • profile image


      12 days ago

      I have a 10 gallon fuel cell in my trunk directly above my main tank that gravity feeds into my main tank could this cause vapor lock we vented the tanks as well my fuel lines run along the inside of my frame close to the exhaust is 3" below fuel lines any idea's please


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