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What Vapor Lock Is and How to Fix It

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Vapor lock may prevent your engine from starting.

Vapor lock may prevent your engine from starting.

What Is Vapor Lock?

There are several symptoms that may point to vapor lock.

When Your Engine Is Warm, It May:

  • run rough,
  • experience rough idle,
  • lack power,
  • hesitate on acceleration,
  • surge,
  • stall and start after cooling for an hour or two,
  • be hard to start,
  • not start, or
  • perform poorly.

Several Factors Can Contribute to Vapor Lock:

  • High underhood temperatures
  • Fuel lines next to or touching hot engine components
  • Lose of fuel delivery system pressure or flow (volume); this may be caused by a restricted fuel filter or insufficient voltage to the fuel pump
  • Fuel pump next to hot operating engine
  • Faulty check valve in an electric fuel pump assembly
  • 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.

I. Why Vapor Lock Occurs

Vapor lock occurs when fuel overheats and vaporizes in a fuel line, carburetor or fuel injector, disrupting the correct air-fuel ratio.

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, 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, creating vapor lock and causing the engine to run lean.

Still, a modern fuel-injected system can suffer from vapor lock, not only if pressure in the fuel line drops (faulty fuel pressure regulator or fuel pump, for example) and the fuel line is subjected to high temperatures, but also modern engines that may reach between 248 and 284 F (120 and 140 C) using ethanol blended fuels.

High-altitude geographic areas can contribute to vapor lock as well by lowering the fuel boiling point.

Have an assistant crank the engine while conducting the test.

Have an assistant crank the engine while conducting the test.

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 need 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.

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.
  • You can also find fuel injector wrappers that reflect heat away.
  • 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

Question: Why does the engine die when I'm driving down the road in the summer?

Answer: Although vapor lock could play a role, on a modern vehicle this is now hard to happen. Still, there could be many reasons for a stall-while-driving condition. This other post can help you diagnose the problem:

Question: I am having difficulty filling my gas tank. The gas seems to fill the feeder tube, then shut off the nozzle. Is this a vapor lock?

Answer: If there's a section of hose in the filler tube, it's probably collapsed and blocking proper fuel flow. It's possible the EVAP (evaporative emission control system) has a restriction. A faulty vent valve.

Question: If the engine doesn't turn over at all, just makes a clicking noise, is it vapor lock?

Answer: If you don't hear the starter motor running at all while trying to start the engine, the problem could be in the starter motor itself, the solenoid or the circuit. The fact that you can hear a click, means the voltage is reaching the starter solenoid, but, either the solenoid is not transferring this voltage to the starter motor, the starter motor is not working properly, or the circuit is not sending all the voltage necessary to run the starter motor. Start by checking the voltage drop for the starter motor circuit:

If everything works fine, have the starter motor checked. Usually, a local auto parts store will diagnose it for you. Hope this helps.

© 2019 Dan Ferrell