Finding and Fixing an Evaporative Emissions Leak

What Is The EVAP System? 

Your vehicle’s EVAP system (evaporative emissions control systems) are in place to prevent fuel tank vapors from escaping into the atmosphere. All cars and trucks are required to have an EVAP system to protect the environment from harmful fuel vapors and you from breathing them in.



Before you go, check out these commonly asked questions.

Is It Safe to Drive with an EVAP Leak?

It is safe to drive with an evaporative emission (EVAP) leak. The only potential danger is that the engine may run rich and damage the catalytic converter.

How Long Can You Drive with an EVAP Leak?

It is recommended not to drive your car more than 30-50 miles with an EVAP leak.

How Do I Know If My EVAP Canister Is Bad?

Here are just a few common symptoms that may indicate that your EVAP canister is bad:

  • The check engine light is on
  • Poor fuel economy
  • The service light is on for the fuel system
  • There are problems with the vehicle’s performance or drivability in general

How Long Does a Charcoal Canister Last?

The lifespan of a charcoal canister varies depending on the make and model of the vehicle. However, it is usually not very long, around 5-7 years.

Does EVAP Leak Affect Gas Mileage?

Yes, a car’s fuel economy may be lower in vehicles with a defective EVAP system.

How Do I Reset the Check Engine Light and Code?

There is no universal way to reset the check engine light and code. However, most carmakers have their own specific procedures. You will need to refer to your owner’s manual or contact the dealership for more information.

What Are EVAP Leak Symptoms?

There are a few different symptoms that can indicate an EVAP leak in your vehicle. If you notice any of the following, it’s important to have your car checked by a mechanic as soon as possible:

  • Your “check engine” light is illuminated
  • There is a gasoline smell coming from your car
  • Your car is stalling or having difficulty starting
  • Your mileage is lower than usual

How Does The EVAP System Work?

The evaporative emission control (EVAP) system captures gasoline fumes and other emissions. When the fuel evaporates inside the gas tank, the excess vapors are transferred to the charcoal canister. They’re stored there until they can safely be transferred back to the engine to be burned with the normal air-fuel mixture.

When that’s ready to happen, a valve creates a vacuum that draws the vapors into the engine. Fresh air is also drawn in through the vents and valves to mix with the vapors for better combustion. These systems can be controlled mechanically, or like on newer cars, through the engine’s computer.

If the fuel tank was sealed tight, the fuel pump could create enough negative pressure to collapse it.

So, on older EVAP systems, the tank is vented by a spring-loaded valve inside the gas cap. While, On newer vehicles, it is vented through the EVAP canister.

It’s difficult to wrap your head around how a typical EVAP system works. But the good news is that the system’s functions can be broken down into three primary operations: storing fuel vapors, purging fuel vapors, and self-monitoring.

The EVAP system has three primary operations:

  • Storing fuel vapors.
  • Purging fuel vapors.
  • Self-monitoring.

#1. Storing Fuel Vapors

The vapor canister is the focal point of the EVAP system. When the engine is off, fuel vapors from the gas tank are stored in the canister. The canister contains activated charcoal, which traps the vapors until the engine is running, and conditions are correct for a vapor purge.

Normally, the vapor canister is open to the atmosphere to allow fresh air to enter. The canister is only closed when the EVAP monitor is run.

#2. Purging Fuel Vapors

In modern vehicles, the PCM determines when to start a canister purge. When the module deems conditions to be correct, it commands a solenoid to open the purge valve.

Opening the purge valve creates a vacuum that pulls fresh air through the vapor canister. The fresh air picks up the fuel vapors and delivers them to the engine to be burned during the normal combustion process.

#3. Monitoring For Leaks And Proper System Operation

As was mentioned, all vehicles built after 1999 have enhanced EVAP systems that can perform self-tests for both leaks and proper system operation. This test sequence is referred to as the EVAP monitor.

The monitoring strategy will vary, depending on the type of vehicle. When the conditions are correct, the control module closes the vent valve and opens the purge valve, creating a vacuum in the system.

The control module then monitors the fuel tank pressure (FTP) sensor to verify the system can reach a specified amount of vacuum.

If the vacuum is lower than specified, the module assumes there is a large leak somewhere in the system and stores a DTC in its memory.

Which Are The Available EVAP Systems?

There are different evaporative emissions control systems. These systems can be classified into five different categories:

  • Diurnal: This usually represents gasoline that evaporates because of the rise in ambient temperature.
  • Running losses: Represent gasoline that vaporizes because of the heat of the engine and exhaust system during normal operation.
  • Resting losses: Natural permeation that occurs from the fuel delivery system while not operating under ambient conditions.
  • Hot Soak: Vaporization of fuel because of the keep heat of the engine after the engine is turned off.
  • Refueling: Represents the fuel vapors that escape from the tank by the displacement of liquid fuel.

Can You Repair EVAP Leaks Yourself?

In most cases, repairing an EVAP leak requires the expertise and specialized tools of a trained mechanic. If you suspect that you may have an EVAP leak in your vehicle, it is best to seek diagnosis and repair from a trusted professional.

However, there are some steps you can take to prepare for your appointment with a mechanic, including gathering any relevant maintenance records or receipts as well as having your car inspected by another trusted mechanic prior to arriving at your appointment.

With proper care and attention to detail, though, you can help reduce your chances of experiencing another EVAP leak in the future and keep yourself and those around you safe from potential harm caused by leaks in this important system.

EVAP Leak Causes

Strangely enough, a very common leak cause is a ga

Strangely enough, a very common leak cause is a gas cap that wasn’t properly secured. This typically happens right after filling up the car. Many people head to the mechanic when the Check Engine Light comes on only to realize that the gas cap hadn’t been tightened correctly or if you didn’t shut off your car engine while you were refueling.

If the gas cap is properly secured, then there could be a more serious problem. It could be due to a leak in one of the hoses or a defective O-ring seal. There might also be a malfunctioning leak detection pump or faulty purge valve. 

The best and most efficient way to find a leak is to use an EVAP smoke machine.

Causes Of EVAP Leaks

There are several things that can cause an EVAP Leak:

  • A bad seal on the gas cap – By not being be tightened properly and /or the gas cap itself not sealing.
  • A bad or saturated charcoal canister – The canister gets saturated by over-gassing and allowing the gas to run down into the charcoal canister, damaging it.
  • There are also two purge solenoids. One is in the engine compartment, and the other is usually connected close to the charcoal canister at the rear of the vehicle. They control when the gas vapors are released back into the motor to be burned, and when one goes bad they either can be stuck open or closed and when stuck open, it allows a constant vacuum to run.

All of these will cause a check engine light to come on and all of them are apart of the EVAP system. This can be a very complicated diagnosis so be sure to bring your BMW into our shop to fix the issue as soon as you can.

How Much Will This Cost To Fix?

Depending on where the leak is in the system and whether or not there is another damage, you can expect to pay up to $600 or so to fix a leak in your vehicle’s EVAP system. If you have an OBD2 code reader at home, you can diagnose the problem yourself, but it’s best to leave the fixing to the professionals.

Why There Can Be Leaks

A leak in the EVAP system can occur almost anywhere. It can be because of damage to the hoses that convey the gasoline fumes to the engine. As everyone knows, hoses are not exactly immune from damage. Over time, they can become brittle and form cracks where fumes can escape. It is also possible that one of the EVAP hoses has been disconnected, allowing the vapors to escape.

Another potential cause of a leak in the EVAP system is leaks in the charcoal canister itself. Again, these can degrade over time because of corrosion or exposure to impact forces. The canister itself can get cracked allowing vapors to hiss through the small opening. The canister can get severely damaged that your only recourse is to have it replaced.

Other causes of EVAP leaks can include faulty vent control valve or the purge valve. These valves can get stuck in their respective openings, too. When stuck, they will not be able to function normally and will not be able to purge the stored gas vapors into the engine’s intake valve. It is also possible that there’s a problem in the leak detection pump or any other leak in the car’s fuel system.

But the most common – and most often overlooked – cause of EVAP leaks is a damaged or loose gas cap. As always, this device can be subject to wear and tear with the passing of time. Or you simply failed to tighten it well or to close it all the way such that vapors can still escape.

How to Repair EVAP Leaks

How to Repair EVAP Leaks

Something as Simple as a Cracked O-Ring or Seal May Be the Source of an EVAP Leak.

Finding EVAP system leaks is arguably the most difficult part of this project. Repairing EVAP leaks, though, can vary in complexity and expense, depending on which part of the EVAP system is leaking. Remove and replace is the usual repair procedure.

  • Valve prices can vary, depending on if they are available separately from other components. Standalone valves, such as the EVAP Purge Valve and some Canister Vent Valves, are usually $25 to $100 and may take just a few minutes to replace.
  • Some Canister Vent Valves are only available as part of the Charcoal Canister, which can range from $300 to over $500.
  • O-Ring Seals are located in many parts of the EVAP system and usually cost less than $2. Simply remove the old O-ring with a pick tool, spray clean the area with carburetor cleaner and allow to dry. Use spray silicone lubricant on the new O-ring and sealing surface, then reinstall.
  • EVAP system tubes, hoses, and clamps can vary in price and complexity, because they are often routed in difficult-to-access areas. Replacing these requires patience, but getting a good seal usually isn’t difficult. Use spray silicone lubricant to ease installation and prevent O-ring binding and rollover.
  • Gas cap prices usually range from $10 to $50, and gas cap O-ring prices usually range from $5 to $20. It only takes a few seconds to replace either one.

EVAP system testing and repair is not for the faint of heart, but it can be done. Because of the complexity of the system, it is often recommended to leave it to the professionals. When you’re done repairing the EVAP system, be sure to reset the DTCs


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