EVAP, this is an abbreviation you first heard from your mechanic when the fuel cap from your car suddenly went missing. What exactly does it mean? Here is what you need to know about EVAP systems.
You know that a car is a complicated machine but you probably don’t realize just how complicated it is. It is actually made up of several hundred systems which all work together to make the car operate as it should. Some are essential to basic working, others increase convenience and safety and others are aimed at protecting the environment. This is why it is so important to maintain your car in good condition. All these systems are interrelated and quite often a problem in one system causes one in another.
The EVAP (Evaporative Emission Control) system is one whose function is to protect the environment from emission of harmful gases.
Automotive Emissions
The mention of automotive emissions probably brings to mind dirty, sometimes smoke-like air emitted by cars through exhaust pipes. Internal combustion processes do produce harmful gases like carbon dioxide and nitrogen oxides. However, these are not the only type of emissions from our cars.
There is another type which is referred to as evaporative emissions. Where do they come from? The gasoline you put in your fuel tank contains more than 150 chemicals such as toluene, benzene and sometimes lead which are very harmful to us and the environment. They are added to fuel to enable it burn cleanly, increase engine efficiency and reduce air pollution. As the gas flows through fuel lines to keep the car running, it continues to evaporate, albeit slowly. This evaporation releases volatile organic compounds into the air which cause smog, general air pollution and contribute to climate change.
Continuous Emissions
The worst part about evaporative emissions is that evaporation continues as long as there is fuel in the tank. If there is the slightest leak in the system, pollution continues 24 hours a day whether the car is being driven or not.
About 20% of all hydrocarbon emissions from a vehicle can be attributed to evaporative emission. This is why it is a major concern for the Environmental Protection Agency, in our quest for a carbon zero society.
Government Mandate
During the 1950’s and 1960’s several federal, state and local governments in the US carried out studies on air pollution. Their aim was to find ways to mitigate worrying pollution levels. Research on various sources of air pollution found that a significant portion of pollution was as a result of automobiles. This led to a government mandate requiring all vehicle manufacturers to include systems to not only collect but also store and dispose of harmful vapors to prevent air pollution.
Since 1971, all vehicles made and sold in the US have to have fully sealed systems which do not vent directly into the atmosphere. Other governments around the world including most European countries and Japan soon followed suit.
EVAP System Flow
In a nutshell, the purpose of the EVAP system is to prevent harmful vapors from fuel from entering the atmosphere. It is not particularly complicated in design but is somewhat complex in its operation. It is a completely closed system and has to remain as such in order to function optimally.
The process begins at the fuel tank when you fill it up with liquid fuel at the gas station. The fuel immediately begins to vaporize. The vapor flows past a liquid, vapor separator which ensures only fuel in gaseous form gets into the system. It flows through the fuel tank vent line and into the charcoal canister which stores excess vapors. Whatever amount is required by the engine flows through the canister, past the purge valve and into the carburetor where it is mixed with air. The air and fuel mixture is then sent into the engine for combustion.
EVAP systems are made up of several parts which all work together to achieve the overall goal.
Components of an EVAP System
Primary components
- Fuel Tank: This one is a basic component. It is the container your fuel is stored in when you fill up at the gas station. Be careful not to overfill the tank when you are filling up because this leaves no room for it to expand when it gets warm. This creates unnecessary pressure on the tank’s walls which can damage the engine.
- Gas Cap: It is sometimes referred to as a fuel filler cap. This is a small but terribly important part of any car. It serves to prevent dirt and debris from getting into the tank and provides a reliable seal to prevent evaporation of fuel. Always tighten the gas cap until you hear a click.
If the cap is not tightened properly, the car’s computer detects a leak and illuminates the check engine light. You will probably also pick the strong smell of fuel in and around the car when the cap is loose.
- EVAP canister: This component, also known as the charcoal canister is not only one of the most important parts of an EVAP system but also one of the least understood. Here is how it works. Imagine what would happen if you put a bucket full of fuel in a room. You would be able to smell the gas from a long way off because it emits hydrocarbons (raw fuel vapors.) These naturally react with nitrogen oxide to form ozone which is harmful to you and the environment. When you put fuel in your gas tank, it releases vapors but you never get to smell them because of the charcoal canister.
The canister is a sealed container full of activated carbon or activated charcoal. This kind of carbon is processed to give it an incredibly high surface area, disproportionate to its size. It works as a sponge for absorbing fuel vapors. A single gram of activated carbon could have a surface area of between 500m² and 1,500m².
Activated carbon in the enclosed system stores excess fuel vapors so that they don’t escape into the environment. When they are required, the ECM (Engine Control Module) commands the vent valves and canister purge to open. As air is being pulled into the engine through the charcoal canister, vaporized fuel is flushed out and burned in the engine. This process significantly reduces amounts of harmful hydrocarbons released into the air. Instead all you get is harmless carbon dioxide and water vapor released through the exhaust.
- Fuel tank pressure sensor: This part of the EVAP system is either mounted on top of the tank or placed inside the tank. It detects leaks in the system by reading pressure levels. Leaks generally cause a drop in pressure.
The sensor is connected to the vehicle’s computer so when it detects a leak or if the sensor is faulty, the check engine light comes on and a DTC is recorded. From this, a technician can read and interpret the code and trace the problem to a leak in the EVAP system.
Secondary components
- Vent Control Valve: This is the pipe through which vapors flow from the fuel tank into the EVAP canister.
- Purge valve: It controls the amount of fuel vapors which pass from the EVAP canister into the engine intake manifold. It ensures only the precise amounts required get into the engine.
- Liquid-vapor separator: Liquid fuel should not pass into the EVAP canister. The separator does exactly as its name suggests. It separates liquid and gaseous fuel, only letting vapor pass through it.
- Vent hoses: Hollow tubes through which fuel vapors flow from one component of the EVAP system to another.
Charcoal Canister Problems
Although the system is made up of many parts, problems will often be related to the canister or the sensor. Let’s take a closer look at canister issues. These are some symptoms of problems with the canister.
Check engine light comes on: The ECM regularly checks the integrity of the EVAP system by running a pressure test. If something is found to be out or order, DTCs (diagnostic trouble codes) are recorded.
The charcoal canister could be cracked, therefore causing gas leaks. Another common problem is faulty valves which disrupt proper flow of gases in the system. In this case, you may have to replace the canister if valves are not available to be purchased separately.
Problems refueling: Refueling problems are most often caused by faulty valves but they can also be as a result of a saturated charcoal canister. Saturation happens when you regularly overfill your tank or ‘top off’ after the fuel pump nozzle has clicked off.
When the canister gets saturated, air cannot flow through it as easily. This then backs up the system and clicks off the fuel nozzle before the tank is full. A saturated canister also causes problems with excessive fuel odor because it cannot absorb any more fuel vapor and wet fuel cannot be removed during purge cycles.
Pinging sounds from the engine: When the charcoal canister fails to dispose of carbon emissions effectively, it affects the ignition of the air/fuel mixture in the combustion chamber. The result is an unfamiliar ‘pinging’ sound coming from the engine. These sounds could be caused by several other faults so it is important to check if any of the other signs listed here are present.
High Emissions: The charcoal canister prevents toxic compounds from being released into the environment. When there is something wrong with it, you can expect to have an increased rate of carbon emissions. If the fault has not been there for long, this increase may not be noticeable just by observing the exhaust fumes. However, if you live in a state which requires you to take your car for an emissions test once or twice a year, you will probably not pass the test.
Increased Fuel consumption: When carbon emissions re-enter the combustion chamber and get burned to produce power to keep the car in motion, it conserves fuel and increases fuel economy. If these emissions get released into the environment instead, you end up using more gas to cover the same distance, eventually impacting on your pocket.
How to Replace a Charcoal Canister
Replacing a charcoal canister is a fairly simple process as it involves disconnecting electrical connectors and hoses, replacing the canister and reconnecting everything back up again. What can be difficult, and sometimes beyond the skill level of the typical DIYer, is determining if the problem you are having is caused by a faulty canister. To be sure, consult a professional, if for nothing else, to diagnose and confirm your suspicions.
If you determine that a faulty canister is the cause of your car problems and opt to repair it yourself, this guide will be useful.
Canister removal requires basic hand tools like a socket set and a ratchet. If there is a lot of rust on the parts, you may need a hammer and a punch to shock a bolt or nut loose. Always wear protective clothing and safety glasses to protect your eyes from stray dust particles and dirt.
- Locate the canister. It may be near the fuel tank or under the hood. If it is necessary to lift the car, use jack stands. As long as you are doing anything which requires you to have your body under the car, never lift the car with the jack only.
- Spray the mounting bolts and nuts with penetrating oil to make it easier to loosen them. These connections probably haven’t moved for many years. Oiling helps speed things up.
- Disconnect all the vapor lines and remove any hose clamps. If you are not an expert, use a marker or masking take to mark the connection points just in case you have trouble when reconnecting everything.
- Remove the canister and check the EVAP purge for charcoal dust. If there is any, as often happens, blow the line out with compressed air. This prevents it from clogging the purge valve which will be another problem to deal with later.
- Place the new canister in place and secure it using the bolts you removed in the exact position they were in before. Apply some spray silicone to the electrical connections and vapor lines to make sure you have a good seal.
Fuel Tank Pressure Sensor Problems
How can you tell when the fuel tank pressure sensor in your car is failing? These are some symptoms to look out for.
Loss of engine power
If you have noticed that the car is not as powerful as it usually is, the pressure sensor could be the reason. Diminishing power may be experienced if there is an unreported leak in the system so the engine is not receiving enough fuel.
You may notice that you have to step harder and further down onto the accelerator to get the car to pick up speed. The car seems to drag itself.
Difficulty starting the engine
If you have to make several attempts at cranking up the engine, there is a problem. The car’s computer may be receiving incorrect signals from the pressure sensor. If the problem is left unresolved, the car eventually fails to start at all.
Stalling
With starting problems often comes stalling. It is attributed to a disrupted air fuel ratio which happens when there is a leak in the system.
Check engine light comes on
The check engine light comes on when the computer detects a problem in the engine. This means it could be illuminated by a wide range of problems. If you suspect there could be a problem with the sensor, have a professional scan the computer for trouble codes.
Increased fuel consumption
No one wants to pay more for fuel than they have to. If the pressure sensor whose function is to detect leaks in the EVAP system is faulty, it will not detect or report anything even when there is one. When this happens unburned vapors are released into the environment and this results in wastage of fuel.
How to Replace a Faulty Fuel Pressure Sensor
If you confirm that the fuel pressure sensor is faulty, it must be replaced. Whether you choose to do it yourself or take the car to a technician to do it for you, this is the basic process. Make sure the old sensor and the new one are alike.
Tools you will need:
- Rogue wrench
- Screw driver
- Jack stand
- 3/8 ratchet
Step 1. Make sure the car is parked in a shaded but well-lit area to allow for clear visibility. Turn the engine off.
Step 2. Disconnect the battery. It is enough to disconnect the negative cable as this breaks the circuit required for flow of electricity.
Step 3. Jack the car up then remove the wheel which is next to the fuel tank. Use the rogue wrench to get the wheel off. Once it is off, you should be able to see two pins below the inner wheelhouse. Using the screw driver, catch the pin heads firmly and remove them.
Be careful not to lose these pins.
Step 4. Using the 3/8 ratchet, remove the plastic screws on the inner wheelhouse then take out the house. Now move the wheel cover trim to either side to reveal the fuel tank pressure sensor above the wheel well.
Step 5. Disconnect the sensor by pulling the electrical wire leading to it. Pry the sensor upwards to remove it.
Step 6. Push the new sensor into the space left by the old one and make sure it is seated firmly.
Step 7. Reconnect the electrical line ensuring it clips in right.
Step 8. Reinstall the wheel house and the wheel.
EVAP System Leaks
It is imperative that the EVAP system remains closed at all times. A leak, no matter how small, immediately generates a system fault report to the vehicle’s computer. The first thing you will notice is an illuminated check engine light.
What could cause an EVAP system leak?
The most common cause of a leak in the system is a missing or lose fuel cap. It is not uncommon for drivers to forget to replace the fuel cap after filling up the tank. If the cap is not firmly in place, vapor from the tank easily escapes.
Different types of fuel caps are used on different cars. If the wrong kind of fuel cap is used, it is as good as having a loose fuel cap.
Leaks could also be caused by tiny raptures in any one of the other components of the system such as holes in the fuel tank, canister or any one of the valves or hoses.
If there is a leak in the EVAP system in your car, you will probably not notice any major changes in performance but some tell-tale signs will point to the EVAP system. These include:
- Illuminated check engine light
- Poor fuel economy
- Rough engine idling
- Constant fuel odor
- Slow and hesitant acceleration
- Stalling
EVAP System Self-Test and Fault Codes
In some vehicle models, the system can test itself to check for leaks using different methods.
Some vehicles use a vacuum or pressure sensor which is able to detect vacuum in the system and how long it is held. With this method, the engine must be running. Other vehicles use a pump to do a similar test but don’t require the engine to be running. The exact circumstances which trigger these tests vary depending on the make, model and year but will usually take into consideration factors like vehicle speed, how long the engine has been running, fuel levels and temperature of the engine.
After running these tests, if an abnormality is detected, the check engine light comes on and diagnostic trouble codes (DTCs) are stored in the vehicle’s computer. These are helpful to a repair technician because they point him towards the problematic section so little time is wasted trying to trace the exact source of the problem.
These are some common DTCs stored in relation to the EVAP system and what they indicate.
P0440 – EVAP System
P1440 – Purge Valve is Stuck Open
P1444 – Low input from Purge Flow Sensor Circuit
P0441 EVAP System – Incorrect Purge Flow
P0457 EVAP System leak – no fuel cap or fuel cap loose
P1443 EVAP System – Valve Malfunction
P0455 – EVAP System – gross leak
P0456 – EVAP System – minor leak
P2421 EVAP System – Vent Valve Stuck Open
P2450 EVAP System- Switching Valve Stuck Open
Testing For EVAP System Leaks
DTCs make it a lot easier for a repair technician to locate a leak in any part of the EVAP system. An OBD2 scanner is used to read trouble codes from the vehicle’s computer and a professional interprets the codes to come up with findings. What happens if a technician does not have access to an OBD2 tool or doesn’t know how to interpret the codes?
These are some alternative methods which could be used to determine if there is a problem in the system and exactly where it is.
Vacuum test:
All vapor in the EVAP system ends up in the intake manifold. An engine vacuum gauge can be used to test the valves. To do this, first check that the engine idle vacuum is about 21 in. Hg. Then check for vacuum in the system ensuring that the engine is running and the electrical connector is disconnected. There should be no vacuum. If there is, it means the purge valve is stuck open.
Smoke test
For this method, smoke is blown into the system and the technician then observes to see if there is any smoke escaping from any part of the system. If there is, it can be concluded that this hose, valve or any other component is damaged.
Some people use colored smoke to make it easier to spot compromised parts. This method is effective but very expensive.
Hand vacuum pump:
When the system is not powered, the vent valve should be open and the purge valve closed. This pump can be used to check some valves with the engine off. The gauge shows whether the valve is able to hold pressure. You could power the valves manually and observe sealing and operation.
EVAP System Repair Cost
The two parts of the system which often require replacement are the charcoal canister and the fuel tank pressure sensor.
A new charcoal canister will cost you between $180 and $600. The exact price depends on the make and model of your car. If a professional technician does the replacement, expect to part with between $30 and $140 as labor costs. In addition to this, you will probably be required to pay some form of tax imposed by your local government.
A new fuel tank pressure sensor will cost you anywhere between $60 and $80 depending on the make and model of your car. If you take the car to a mechanic, you will also have to part with some money which will bring the total cost of the part and labor to between $250 and $350.
Here are some figures given by owners of various vehicle models for EVAP system repairs.
| Model | Parts | Labor | Total |
| Nissan Altima | $329-$409 | $35-$389 | $364-$798 |
| Ford Focus | $142-$230 | $70-$178 | $212-$408 |
| Ford F-Series | $142-$230 | $70-$178 | $212-$408 |
| Ford Fusion | $155-$529 | $79-$134 | $234-$663 |
| Honda CR-V | $188-$322 | $26-$78 | $214-$400 |
| Honda Accord | $115-$477 | $35-$78 | $150-$555 |
| Honda Civic | $144-$252 | $35-$67 | $179-$319 |
| Chevrolet Silverado | $290-$301 | $44-$67 | $334-$368 |
| Toyota Corolla | 448-$532 | $44-$67 | $492-$599 |
| Toyota Camry | $448-$532 | $44-$67 | $492-$599 |
Data courtesy of autoservicecosts.com
How to Reduce Fuel Evaporation
Even if there is no leak in your EVAP system, gas still evaporates from it, especially during warm summer months. Fortunately, there are a few ways to slow the process down.
Firstly, always make sure the fuel cap is secured tightly in place to prevent escape of vapors from the fuel tank. These caps are not particularly expensive so if you don’t have one, get one as soon as possible. Stuffing an old rug like into the mouth of the tank just won’t do. Fuel can escape right through it. It is not airtight.
Secondly, try to park your car in the shade whenever you can. This is especially useful in the summer when temperatures are very high. Although most modern cars come with advanced EVAP systems, fuel still evaporates from the tank and more of it is lost when the car is in a hot place. Parking the car in the shade keeps the entire vehicle cool and minimizes this inevitable evaporation.
Thirdly, try to make your visits to the gas station in the early morning or late evening before or after the daytime heat. When it’s hot, evaporation is more prevalent. This is why you are more likely to pick the familiar smell of fuel when you are fueling in the middle of a warm day than in the morning.
The savings made by implementing such measures may be small each time. It may be tempting not to bother with any of them at all. However, small savings accumulated over many years can be quite significant in terms of dollars saved and protection of the environment.
Conclusion
he EVAP system is one of several hundred interconnected systems in your car. Its specific function is to protect the environment from harmful vapor produced by the fuel we use. There is a lot of detail to learn about this system. If you are not keen to get into the nitty gritty details but are wondering what you need to know about EVAP systems to stay safe, keep the classic symptoms of a fault in mind. If the check engine light comes on, don’t ignore it. Take the car to a professional immediately and have him check for faults in the EVAP system.
