5 Ways To Test Gas Cap for Leaks

You don’t always have to scrutinize to be able to detect a leaking gas cap. If your gas cap starts to leak the signs are obvious. In a way, it’s a good thing because these signs enable you to know when to change your gas cap.

Driving a car with a failing gas cap is extremely dangerous, you will be toying with your life and that of others. A leaking gas cap spells one thing and that is your lid is weak and needs to be changed immediately. But if you fail to notice this, it’s at your own risk.

How To Test a Gas Cap for Leaks

To test a leaking gas cap you must first observe some signs that may occur, which tells whether or not your suspicions are correct. 6 ways to tell if your gas cap is leaking include:

  1. Loose Gas Cap
  2. Gas Cap Light
  3. EVAP Test Signals
  4. Petrol Smell in Car
  5. Gas Tank Discoloration
  6. Smoke Test

1. Loose Gas Cap

This is the most common sign of a gas leak. It is the first change that occurs. You will notice the cap no longer fits. When you lock your gas cap do you hear a clicking sound while screwing in an attempt to tighten it?

In some vehicles, it clicks twice, while in some, thrice, depending on the brand. The moment you hear this sound, it means the cap has been tightened, if not then it means the cap is worn out.

This is something that happens if the cap has been long in use, it wears and tears, and has to be changed. Considering how important this piece is, preventing unwanted particles from getting into the tank, keeping your car in good condition, it shouldn’t take you long to get it replaced.

How To Fix a Loose Gas Cap

Noticed your gas cap is loose? Fixing it isn’t rocket science. You can fix it without the help of a mechanic. To fix, expose the gas cap by opening the tank cover which is next to the compact box.

If the gas cap appears tight, take it out and examine the seal that laps firmly to the cap. If the seal is broken, it won’t sit firmly on the gas cap, allowing gas to leak out. You should get a replacement gas cap seal.

Check the surface of the gasket on the body of the tank filler. If the area where the gas cap is attached to the body is damaged or weak, it prevents it from sealing completely. You will need to get a new tank filler.

If there is no damage after opening the gas cap, tighten back and make sure you hear a clicking sound at least twice or thrice before you stop, this indicates that the cap is firm.

2. The Gas Cap Light Comes On

If you notice your Gas cap light is on, it shows that there is a problem. This light doesn’t just come on for no cause. It is prompted by something and that is a leaking gas cap.

This often happens when the cap is loose or broken. It means that the car computer has detected a leak in the system. You may have noticed this in your car. If you have, then you shouldn’t take it calmly, it is a sign that your gas cap has some leakages and need to be attended to ASAP!

3. EVAP Test Signals

This is probably the surest way to test a gasp cap. The EECS( Evaporation Emission Control System) helps detect any leakage of gas through the EVAP test. When you have a leaking gas, these test sends a signal (PO455 & PO457) to the emission system.

EVAP tests don’t run immediately, only a long period when the car’s engine is not running. But if you can’t wait this long for the self-test, then try using a gas cap pressure tester

This is another way of testing for gas leakage. How is it done? Most times the gas gets stuck in the vapor hose. To get this done, take a needle and a cloth or nylon, something to seal the mouth of the tank.

Check for a change in pressure after few minutes using the pressure tester. You can refer to this guide for more in-depth information on using a gas cap pressure tester.

4. Check For The Smell Of Petrol in Your Car

Now this one here is really disturbing. Think of the pollution it’s causing in your car, the repulsive smell of fuel that’s upsetting your stomach. If you had ever experienced this, you’d know what I mean.

This smell is traced to a leaking gas cap. Fuel escapes from the loose ends of the cap. When the gas cap cover lifespan is exhausted they wear out, losing the ability to hold in fuel.

This situation is critical and should be treated as such. Loose petrol can easily attract fire, exposing you to danger. To avert this, get a replacement. It’s pretty cheap to replace. You can select the best fit for your car and have it delivered to you.

5. Check For Discoloration Around The Gas Tank Cover

If you have a broken gas cap, fuel escapes from the tank leaving a strange color at the opening. This is one of the major symptoms of a leaking gas cap. The fume gradually sips from the tank through the loose cap and deposits a colored print on its surface.

Gas cap and lasts for a distance of 50,000 miles or more if handled properly. It is also dependent on the brand of the vehicle. Some don’t get to last this long because of the unscrewing and screwing process it goes through each time you fuel your car. You can’t control the number of times you refuel but you can change the gas cap if it wears out.

6. Using A Smoke Machine

Have you tried out other methods and had no positive results? This is an alternative way of testing for gas cap leaks. It can be very dangerous for amateurs and should be done in an open area.

For this, you need oil, pressure, colored dye, and a smoke machine. If you’re new to this, we recommend the AUTOOL Automotive Smoke Machine. Using this, pump the EVAP system continuously. The coloured dye will enable you to detect the area where the gas leaks from.


The gas cap is a vital part of a car. As tiny as it looks, it is responsible for the maintenance of the fuel system, making sure your car functions properly. It stands as a wall between the gas tank and dirt particles.

Without it, debris will be ingested into the gas tank, polluting the fuel and diminishing the car’s performance. The gas cap should be checked regularly to know when it’s failing and needs replacing.

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