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Evaporative Emission System Leak Causes and Testing Strategies

I think you will agree with me that an evaporative emission system leak (EVAP leak) is just one of the many issues hiding behind your vehicle’s check engine light. More common on older vehicles, EVAP leak repair is also one of the most do-it-yourself tasks with simple fixes, provided nothing else is wrong with the engine.

The following guide exists to show you what an EVAP leak looks like so you can fix it and quickly get back on the road.

 OBD2 EVAP Monitor detects vapor leaks as small as 0.020 to 0.040 inches in diameter.
Illustration of OBD2 EVAP System monitor
Credit:  bharathautos.com

Evap Leak: What Is It?

When you receive a problem that is related to the evaporative leak, your car’s evaporative emission system (EVAP) is probably face a fault. The EVAP is there to prevent the fumes (released from the gasoline tank) from polluting the environment and the air inside the vehicle.

Gasoline naturally evaporates into vapor. Thus, there is always some fumes in the tank whether this is fuel in it. The vapors also constantly pollute the air even if you are not driving. It happens day and night, accounting for up to 20 percent of your vehicle’s emitted air pollution.

Normally, an EVAP leak is harmless. You can drive with one. Though, you will want to fix it as soon as possible as it might be a symptom of something worse.

Causes Of An Evap Leak

Evap leaks develop from several issues. While none of them are serious in of themselves, they might be signs of something else going on inside your engine.

 They might be signs of something else going on inside your engine.
They might be signs of something else going on inside your engine.

A loose gas cap is the most common of these issues. You have a leak if the cap is missing or not fully closed all the way. If your check engine light lights up, your first instinct should be to check the cap.

If your cap is on tight, and you still get the light, you have more work to do. You might need to check the hose system, vent O-rings, the leak detection pump, among others,

Here is a complete list of the most common EVAP leak causes:

  • Missing or loose gas cap
  • Broken cap or the wrong cap for the vehicle
  • Punctured vapor hose or tube
  • Broken or worn-out O-ring seal
  • Any other leak that altered the flow of emissions

You may also get an Evap leak if your vehicle is old or poorly maintained to allow pressure to build up in its systems.

How To Check The Evap Leak

The check engine light is typically your first sign of an EVAP leak. For instance, if it turns on while you drive but not when you have your vehicle in the parking, you might have a leak. You may notice a strange smell or temperature change inside your vehicle if there is a leak.

Because you can fix most EVAP leaks by fixing the gas cap, that should always be the first thing you do when you have a lit check engine light. If this simple fix works, you can relax knowing that you only had a simple leak, and you can go on with your day.

However, if fixing the cap does not turn off the light, then you must bring your vehicle to a mechanic. Your mechanic can run an OBD test to find the real cause of your leak.

How To Test Evap Leaks Manually

If you cannot get to a mechanic immediately, there are a few tests you can do to find the leak. While EVAP leaks are difficult to find without special equipment, these tests are simple enough you can do them in your garage.

Vacuum Test

EVAP systems output to the intake manifold, giving you an easy way to test for leaks with a simple engine vacuum gauge.
EVAP systems output to the intake manifold, giving you an easy way to test for leaks with a simple engine vacuum gauge.

EVAP systems output to the intake manifold, giving you an easy way to test for leaks with a simple engine vacuum gauge. If the gauge reports the presence of a vacuum while the engine is on, you might have a leak in the purge valve.

Hand Vacuum Pump

You can use a hand pump to check for leaks even with the engine off. With the power off, you should have a closed purge valve and an open vent valve.

Smoke Test

Smoke tests just send smoke into the EVAP system to see if it escapes through a valve, a seal, a hose, or a tube. This is the best accurate method to manually test your vehicle for EVAP leaks. However, it is also the most expensive as smoke machines can cost as much as $600.


Smoke tests just send smoke into the EVAP system to see.

Bubble Test

If you do not have a smoke machine, you can try this method. You pressurize the system with an air-mattress inflator, or so-called a shop-vac outlet, taking precautions to not overprescribe it. Once you reach a good enough pressure, spray the system down with soapy water and look for bubbles or foam build-up. You can even use windshield washer fluid for this.

How To Diagnose The Evap Leak With A Scan Tool

When the OBD PCM detects an issue it saves a trouble code for it.  A good OBD2 code reader can get this code out of the computer’s memory The code identifies where the problem is and what type of problem it could be.

If the problem system is EVAP, you will see one of the following codes:

  • P1444 – indicating Purge Flow Sensor Circuit having Low Input
  • P1440 – indicating that Purge Valve Stuck is Open
  • P0440 – indicating a System Malfunction
  • P0441 – indicating an Incorrect Purge Flow
  • P0456 – indicating a Leak is Detected (very small leak)
  • P0442 – indicating a Leak is Detected (small leak)
  • P0455 – indicating a Leak is Detected (gross leak)
  • P0457 – indicating a Leak is Detected (fuel cap is loose/off)
  • P1442 – indicating a Leak is Detected
  • P1443 – indicating Valve Malfunction
  • P1455 – indicating a Leak is Detected (Gross Leak or No Flow)
  • P2421 – indicating Vent Valve Stuck is Open
  • P2450 – indicating Switching Valve Performance/Stuck is Open

What To Do With An Evap Leak?

Once you identify that you have an Evap leak, you have a few troubleshooting options.

The Easy Stuff

As mentioned before, if you have a leak, check the cap. Loose gas caps lead to Evap leaks. So, you should try it first, making sure the cap is as tight and seals against the filler neck. If that does not fix the issue, you should then check the cap’s O ring seal. The O ring degrades with age, leading to gaps between the cap and the neck.

If the problem is not the cap, you must go under the hood and look at the vacuum feed line. The line can decay from the engine heat and vibration which can leave tears or cracks,

Little More Difficult

If the problem still does not go away, you will have to remove the vacuum control solenoid valve. The valve should be closed when not powered, otherwise, it should be opened.

If the solenoid is not the problem, you must go under your vehicle as the rest of the VAP system is near the tank.

You will find the purge valve in the charcoal canister along with the vapor and vacuum lines which can decay through normal wear and tear.

The Bottom Line

While you can drive with an evaporative emission system leak, an EVAP leak might be a sign of a more severe problem. Therefore, you want to repair them as soon as possible. Most of the time, the fix will be as simple as tightening the gas cap or a valve. However, there will be times when you will need professional equipment to diagnose and seal a leak. When that happens, you can use a scan tool to read the code so you can quickly repair the issue and keep the air you and your family fresh and clean.

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