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Symptoms of Vapor Lock in Cars: All You Need to Know 

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Symptoms of vapor lock in cars include engine stalling and difficulty starting, engine running rough, potential misfiring, and engine acceleration and hesitation. 

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Your vehicle's fuel system needs to operate under a certain pressure. When the fuel turns into vapor or when a vapor lock occurs, the fuel pressure gets distrurbed and you'll have many troubles with the engine performance. 

This article provides you with all you need to know about the definition of the vapor lock, the different causes of vapor lock, the symptoms of vapor lock in cars, and other details to help you prevent the situation and fix it without needing a professional mechanic. 

What is a vapor lock? 

Vapor lock indicates that the fuel is vaporized in the fuel line or the carburetor. This vapor causes some backpressure that impacts the overall operation of the fuel system. The fuel must be maintained in liquid status all the time for it to run and get to the engine at the right time with the right frequency.

One of the common reasons for vapor lock is that the current fuel system has a lower boiling point than older fuel systems. Thus, you'll see that fuel turns to vapor quicker than it should in older cars. 

What causes vapor lock in cars? 

Vapor lock occurred mostly in fuel-injected vehicles that rely on a certain type of fuel called the volatile fuel. This fuel has a lower boiling point to help it mix faster and provide a better combustion without waiting for extended timeframes. However, this characteristic of the volatile fuel caused it to vaporize in the fuel lines and cause vapor lock. 

  • Modern engines 

With the advanced technology and invention of electric fuel pumps, modern vehicles do not have major problems with vapor lock because the pump usually operates at the lowest point of the fuel tank, which allows the fuel to be at low temperatures. 

You also won't see the vapor lock as much in modern vehicles if equipped with a certain electric cooling fan. The cooling fan continuously drops the temperature of the fuel to prevent any vaporization or forming of vapor bubbles within the fuel lines. Thus, you'll see that fuel lines will not suffer from any vapor lock under ambient temperatures. 

Keep in mind that it's not rare to see vapor lock in modern vehicles here. This is because some vehicles might rely on certain old blended fuels that could reach higher temperatures resulting in vaporizing the fuel.

Furthermore, some certain high altitude areas could decrease the fuel boiling point resulting and a higher potential of vapor lock. 

  • Older engines 

On the other hand, in vehicles equipped with mechanical fuel pumps, you'll see that vapor lock occurs more frequently. This is because the pump sits in a location higher than the fuel tank resulting in pressure issues. Furthermore, the fuel pump is continuously exposed to extreme temperatures from the engine that could reach up to 100 Fahrenheit. Thus, its temperature can escape and help the fuel vaporize faster than another vehicle with an electric fuel pump. 

Once the fuel vaporizes in the fuel line, it forms common fuel vapor bubbles that impact the fuel system's overall pressure. Keep in mind that the fuel system has to operate under a certain pressure level, and this pressure has to go one way, not the other. Thus, when these bubbles form, they cause backpressure preventing the fuel from reaching the engine, which could impact the combustion process and the engine's behavior. 

What happens when your car has a vapor lock? 

As we indicated earlier, your vehicle's fuel system must operate under a certain pressure level. Therefore, when the fuel vaporizes, you'll have different fluid characteristics, and this characteristic caused backpressure that prevents the fuel from reaching the engine. Thus, you'll see all sorts of engine troubles related to the engine running lean.

Remember that your engine must have a certain mixture of air and fuel. Thus, when the fuel itself comes with its air, the engine will not receive the right mixture and will not operate properly. 

Therefore, when your car has a vapor lock, you'll see troubles with the engine's power, especially when you accelerate. 

At what temperature does vapor lock occur? 

There's no one specific temperature for the vapor lock to occur. It all depends on the type of fuel and other factors. Certain fuels might have a lower boiling point, especially those blended with ethanol. Thus, you'll see that these fuels might suffer from vapor lock faster than other types of fuels.

Also, your geographic area's altitude might impact the fuel boiling temperature. Thus, you'll see that vapor lock might occur for the same vehicle in certain areas but not the other. 

In general, vapor lock occurs at very extreme temperatures that could reach 100 Fahrenheit, if not higher. However, there are some instances where vapor lock might also occur under lower temperatures as well. Thus, even if you are fuel system did not reach the 100 Fahrenheit temperature threshold, this does not necessarily mean that you won't experience vapor lock. 

Symptoms of vapor lock 

When your vehicle has problems with vapor lock, the car will not throw a certain error telling you that it's a vapor lock. Instead, you'll see that there are some of the following symptoms occurring suddenly without any clear explanation:

  • Engine stalling and difficulty starting 

The engine relies on a certain amount of air and fuel to operate properly. Therefore, when the vapor lock occurs, your engine deals with the issue of not receiving the right air-fuel mixture. Thus, you will immediately notice that the engine has difficulty starting and might stall in certain situations. 

  • Engine running rough and potential misfiring 

When the engine doesn't receive the right air-fuel mixture, you might deal with situations with incomplete combustion. With this situation, the engine might cause some misfiring resulting in the check engine light illuminating and running rough.

  • Issues with acceleration 

Another common symptom of vapor lock is trouble with acceleration. 

Keep in mind that all the mentioned symptoms of vapor lock might occur due to any other trouble. Therefore, to confirm the real culprits, you must consult a professional mechanic who uses certain toolsets to help determine the real root of the problem. 

Also, if you have a simple tool like an OBD2 scanner, you'll be able to scan the vehicle's internal computer and read any potential errors. Once you have the error, you can Google a translation to the error to get an idea about what is happening. 

How do you fix a vapor lock on a car?

As you noticed earlier, vapor lock causes all sorts of troubles that make the worst driving experience without causing safety issues. Therefore, when vapor lock occurs, you must take care of the problem immediately. Luckily, there are some simple procedures that you can follow at home to prevent vapor lock from happening, including:

  • Understand the problem cause 

The first step in fixing the vapor lock in cars is to understand the main cause of the problem. For example, if your problem is caused because of engine overheating, you need to address this problem first. 

  • Cooldown the fuel system 

Since vapor lock has to do with high heat impacting the fuel system, one of the quickest tips that you can use to eliminate the vapor lock immediately is cooling down the fuel system. 

What you can do is you can turn off the ignition switch and pour some cold water over the fuel pump and around the fuel lines. This way, the temperature will drop significantly, and the vaporized fuel will convert into the liquid back, illuminating the vapor lock. 

  • Depress the accelerator when turning on the ignition switch 

Once cooling down the fuel system, the next step is to carefully start the vehicle without causing too much fuel to get into the fuel lines. To do so, you can slowly turn on the ignition switch, wildly pressing on the accelerator without causing it to get to the floor.

You must keep holding the accelerator down as your engine starts running. It's OK if you notice that the engine is sputtering for a couple of minutes because you are clearing out the vapor bubbles, and the engine should work properly afterward.  

Remember that the mentioned steps are just temporary solutions for vapor lock issues. However, if you would like to eliminate the problem, you need to consult a professional mechanic who can take a closer look at your vehicle to define the real culprit and solve the issue. Sometimes the problem might be permanent trouble with the vehicle type, especially if you're driving a vehicle with a mechanical engine pump. 

How to prevent vapor lock? 

If you realize that your vehicle suffers from vapor lock continuously, the problem might be related to the vehicle's design itself, which means that you must take certain precautions to prevent vapor lock in your vehicle.

Here are some of the common suggestions by automotive experts on preventing vapor locks: 

  • Keep fuel lines away from hot areas 

Although this might take some effort and probably a lot of money, one permanent solution to preventing the prologue is to keep the fuel lines away from hot areas like the exhaust system and the engine itself. For example, you might reach out to a professional mechanic who can route the fuel lines away from these hot areas to prevent heat transfer to the fuel lines that could cause vapor lock.

  • Install an electric fuel pump

Electric fuel pumps usually put the fuel under high pressure that does not allow for any vapor lock. Thus, this might be a more effective, efficient solution for permanently getting rid of the vapor lock. 

  • Use heat Shields 

Some automotive experts recommend using certain types of heat Shields that cover areas susceptible to vapor lock around the fuel system. Thus, consult your mechanic and see if you can install any of these to prevent the prologue from happening frequently. 

  • Address carburetor vapor lock 

In vehicles equipped with carburetors, you can use a certain type of carburetor heat Shields or install a phenolic carb spacer that reduces the temperature of the carburetor and prevents vapor locks from occurring. 


Vapor lock occurs when the fuel turns into vapor in the fuel system, and this problem occurs most likely in older vehicles that are not equipped with an electric pump.

When vapor lock happens, your vehicle might have troubles with engine stalling, difficulty starting, engine running rough, potential misfiring, and troubles went acceleration. 

When dealing with vapor lock, it is recommended that you drop the temperature of the fuel system as much as possible. You can do that by pouring some cold water on the fuel lines while the ignition is turned off. If you're looking for a permanent solution for vapor lock, you might need to consider installing an electric fuel pump, using heat shields, or routing the fuel lines away from hot areas. 

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