The Evap System

minibike132 · November 7, 2009

I am bored and I have a sick kid on my hands so I'm strapped to the house today. I always see issues with the evaporative system here so I thought I would write a once and for all description of the system, what is supposed to be happening, how we test and fix it and what you can do at home if you are having issues. Most people outside of the shop don't understand the system I hope this sheds some light. It's gonna be a long one, grab a cup of coffee or a beer.

The evaporative emission (EVAP) control system limits fuel vapors from escaping into the atmosphere. Fuel tank vapors are allowed to move from the fuel tank, due to pressure in the tank, through the vapor pipe, into the EVAP canister. Carbon in the canister absorbs and stores the fuel vapors. Excess pressure is vented through the vent line and EVAP vent solenoid valve to the atmosphere. The EVAP canister stores the fuel vapors until the engine is able to use them. At an appropriate time, the control module will command the EVAP purge solenoid valve ON, allowing engine vacuum to be applied to the EVAP canister. With the EVAP vent solenoid valve OFF, fresh air is drawn through the vent solenoid valve and the vent line to the EVAP canister. Fresh air is drawn through the canister, pulling fuel vapors from the carbon. The air/fuel vapor mixture continues through the EVAP purge pipe and EVAP purge solenoid valve into the intake manifold to be consumed during normal combustion. The control module uses several tests to determine if the EVAP system is leaking.

Below are general diagrams of pretty much every evap system in place on GM vehicles excluding those vehicles made by other manufacturers (Pontiac vibe), those systems are basically the same but have some small differences. Pic 1 is the system at rest. Pic 2 is where the vapor goes while fueling. Pic 3 is the system purging to burn off the vapor.

When you fuel your vehicle the vapor needs some place to go. There used to be a vent tube that pushed that straight into the atmosphere. We don't wan't to do that so we push the vapor through the charcoal canister where the fuel vapor is trapped, oxygen should be passed out through the vent line to the vent valve and into the atmosphere.

The vent valve is normally open, it is commanded on (closed) by the PCM to close the system for testing.

Engine vacuum is constantly applied to one side of the purge solenoid. The other side is connected to the evap purge line. When the PCM wants to test the system for leaks the purge solenoid is opened and the vent valve is closed to pull a vacuum on the system. A pressure sensor in the top of the fuel tank senses the amount of vacuum in the system. If the system never draws a vacuum this indicates a restriction or an open system and a code is stored and check engine lamp turned on. Once the proper amount of vacuum is achieved the purge solenoid is turned off closing the system.

Pic 4 s the system drawing a vacuum to get ready for testing. Pic 5 s the valve position during actual testing.

The PCM monitors for large amounts of vacuum decay if there is a large amount of decay a leak is indicated and again the lamp is turned on and a code is set. At this point in older systems a small leak test is performed keeping a careful watch on the vacuum level, if a small leak is detected a different code will set. Newer trucks test for small leaks a little differently. It checks pressure in the system after the vehicle is turned off and uses complex algorithms based on temperature and time to detirmine a leak.

Problems that can happen. First (the reason for me writing this) you can be fueling your vehicle and the pump constantly shuts off making it take forever to fill the vehicle. There is a restriction somewhere between the fuel filler neck and the vent valve. Usually the restriction is the vent valve itself. To check this in the shop we would connect a smoke machine to the vehicle and fill the evap system with smoke. With the valve being normally open smoke should pour out of the valve. If it doesn't we would remove the vent line directly at the valve. If smoke comes out the restriction is proven to be the vent, if not we keep moving up the system till the restriction is found.

If you are experiencing the issue of slow fuel fill you can check the vent valve by pulling the vent line off at the valve and trying to fill it. If it fills normally there you have it, you need a new valve. Why can't I just leave it off you might ask, because it will cause a check engine light and it isn't legal.

If a leak is detected and a code stored and you bring your vehicle in these are the basic tests we do. They involve expensive equipment and there is no way around them at home. Scanners that you may have can check codes but scan tools that can control outputs are expensive and unless you do this for a living you probably don't have one that will.

Car/truck comes into my bay with a leak code. I hook up the Tech-2 and command the evap vent solenoid closed and listen for an audible click. If I hear the valve click I know it is getting what it needs electrically, if not I go to the valve and check for power. Our valves use a constant power, you can check for that. Then I check the other side for ground while commanding the valve on, the ground is controlled by the PCM and is what turns the valve on and off. There is no way for you to command that ground, you can manually apply a ground to the ground side to see if the valve is working but that will not take the PCM or wiring into account.

Let's assume that the valve clicks. I start the vehicle, command the vent closed and the purge open. If no vacuum is achieved I tighten the cap a few clicks and see if it draws a vacuum. If it does then the customer didn't tighten the cap enough. Problem solved, educate the customer on tightening the cap. At the same time I educate them about fueling with the key on. If the key is on the system may try to test itself, if you have the cap off it will fail and turn on the light. Don't fill the vehicle with the key on or engine running.

If vacuum is achieved I close the system off with the Tech-2 and watch for decay. If the vacuum level does not stay constant there is a leak, time to go looking for it. We hook a machine up to the system that will inject smoke. We close the system off with the Tech-2 and start looking for smoke. With the larger leaks the smoke will be very visible and we repair the leak. With smaller leaks the smoke may not be visible, this moves us to the final test, the nitrogen flag test.

The smoke machine has the ability to inject nitrogen into the system which is at a higher pressure then the smoke. It is useful for finding small leaks and verifying that the system is working as designed after repairs. The machine has a cylinder on it with a small ball inside. While injecting nitrogen into the system the ball will float high in the tube indicating that there is flow. Once the system is full the ball will drop down indicating less or no flow. If the ball does not drop below a calibrated level there is still a leak and we start disconnecting and capping off the system to pinpoint the leak. If the ball does drop below the calibrated level there is no flow and hence no leaks.

One final thing to touch on is fueling the vehicle. When the pump shuts off the tank is full (unless there is a vent restriction). If you continue to top off the tank it will pour raw fuel into the canister. The canister is designed for vapor, not raw fuel. Saturating the canister causes a restriction and you will either set codes for a restriction, cause a rich condition, or make it so that the pump constantly shuts off while fueling. Do not top the vehicle off, the extra $0.50 you get into the tank isn't worth the $100 canister.

I hope this isn't too confusing I tried to make it as simple as possible. Every answer about the evap system should be right here, I don't think I'm going to answer any more of them. Oh and I know my paint skills suck so if ya don't like the diagram's, too damn bad .

CMAC79 · November 11, 2009

I am bored and I have a sick kid on my hands so I'm strapped to the house today. I always see issues with the evaporative system here so I thought I would write a once and for all description of the system, what is supposed to be happening, how we test and fix it and what you can do at home if you are having issues. Most people outside of the shop don't understand the system I hope this sheds some light. It's gonna be a long one, grab a cup of coffee or a beer.

The evaporative emission (EVAP) control system limits fuel vapors from escaping into the atmosphere. Fuel tank vapors are allowed to move from the fuel tank, due to pressure in the tank, through the vapor pipe, into the EVAP canister. Carbon in the canister absorbs and stores the fuel vapors. Excess pressure is vented through the vent line and EVAP vent solenoid valve to the atmosphere. The EVAP canister stores the fuel vapors until the engine is able to use them. At an appropriate time, the control module will command the EVAP purge solenoid valve ON, allowing engine vacuum to be applied to the EVAP canister. With the EVAP vent solenoid valve OFF, fresh air is drawn through the vent solenoid valve and the vent line to the EVAP canister. Fresh air is drawn through the canister, pulling fuel vapors from the carbon. The air/fuel vapor mixture continues through the EVAP purge pipe and EVAP purge solenoid valve into the intake manifold to be consumed during normal combustion. The control module uses several tests to determine if the EVAP system is leaking.

Below are general diagrams of pretty much every evap system in place on GM vehicles excluding those vehicles made by other manufacturers (Pontiac vibe), those systems are basically the same but have some small differences. Pic 1 is the system at rest. Pic 2 is where the vapor goes while fueling. Pic 3 is the system purging to burn off the vapor.

When you fuel your vehicle the vapor needs some place to go. There used to be a vent tube that pushed that straight into the atmosphere. We don't wan't to do that so we push the vapor through the charcoal canister where the fuel vapor is trapped, oxygen should be passed out through the vent line to the vent valve and into the atmosphere.

The vent valve is normally open, it is commanded on (closed) by the PCM to close the system for testing.

Engine vacuum is constantly applied to one side of the purge solenoid. The other side is connected to the evap purge line. When the PCM wants to test the system for leaks the purge solenoid is opened and the vent valve is closed to pull a vacuum on the system. A pressure sensor in the top of the fuel tank senses the amount of vacuum in the system. If the system never draws a vacuum this indicates a restriction or an open system and a code is stored and check engine lamp turned on. Once the proper amount of vacuum is achieved the purge solenoid is turned off closing the system.

Pic 4 s the system drawing a vacuum to get ready for testing. Pic 5 s the valve position during actual testing.

The PCM monitors for large amounts of vacuum decay if there is a large amount of decay a leak is indicated and again the lamp is turned on and a code is set. At this point in older systems a small leak test is performed keeping a careful watch on the vacuum level, if a small leak is detected a different code will set. Newer trucks test for small leaks a little differently. It checks pressure in the system after the vehicle is turned off and uses complex algorithms based on temperature and time to detirmine a leak.

Problems that can happen. First (the reason for me writing this) you can be fueling your vehicle and the pump constantly shuts off making it take forever to fill the vehicle. There is a restriction somewhere between the fuel filler neck and the vent valve. Usually the restriction is the vent valve itself. To check this in the shop we would connect a smoke machine to the vehicle and fill the evap system with smoke. With the valve being normally open smoke should pour out of the valve. If it doesn't we would remove the vent line directly at the valve. If smoke comes out the restriction is proven to be the vent, if not we keep moving up the system till the restriction is found.

If you are experiencing the issue of slow fuel fill you can check the vent valve by pulling the vent line off at the valve and trying to fill it. If it fills normally there you have it, you need a new valve. Why can't I just leave it off you might ask, because it will cause a check engine light and it isn't legal.

If a leak is detected and a code stored and you bring your vehicle in these are the basic tests we do. They involve expensive equipment and there is no way around them at home. Scanners that you may have can check codes but scan tools that can control outputs are expensive and unless you do this for a living you probably don't have one that will.

Car/truck comes into my bay with a leak code. I hook up the Tech-2 and command the evap vent solenoid closed and listen for an audible click. If I hear the valve click I know it is getting what it needs electrically, if not I go to the valve and check for power. Our valves use a constant power, you can check for that. Then I check the other side for ground while commanding the valve on, the ground is controlled by the PCM and is what turns the valve on and off. There is no way for you to command that ground, you can manually apply a ground to the ground side to see if the valve is working but that will not take the PCM or wiring into account.

Let's assume that the valve clicks. I start the vehicle, command the vent closed and the purge open. If no vacuum is achieved I tighten the cap a few clicks and see if it draws a vacuum. If it does then the customer didn't tighten the cap enough. Problem solved, educate the customer on tightening the cap. At the same time I educate them about fueling with the key on. If the key is on the system may try to test itself, if you have the cap off it will fail and turn on the light. Don't fill the vehicle with the key on or engine running.

If vacuum is achieved I close the system off with the Tech-2 and watch for decay. If the vacuum level does not stay constant there is a leak, time to go looking for it. We hook a machine up to the system that will inject smoke. We close the system off with the Tech-2 and start looking for smoke. With the larger leaks the smoke will be very visible and we repair the leak. With smaller leaks the smoke may not be visible, this moves us to the final test, the nitrogen flag test.

The smoke machine has the ability to inject nitrogen into the system which is at a higher pressure then the smoke. It is useful for finding small leaks and verifying that the system is working as designed after repairs. The machine has a cylinder on it with a small ball inside. While injecting nitrogen into the system the ball will float high in the tube indicating that there is flow. Once the system is full the ball will drop down indicating less or no flow. If the ball does not drop below a calibrated level there is still a leak and we start disconnecting and capping off the system to pinpoint the leak. If the ball does drop below the calibrated level there is no flow and hence no leaks.

One final thing to touch on is fueling the vehicle. When the pump shuts off the tank is full (unless there is a vent restriction). If you continue to top off the tank it will pour raw fuel into the canister. The canister is designed for vapor, not raw fuel. Saturating the canister causes a restriction and you will either set codes for a restriction, cause a rich condition, or make it so that the pump constantly shuts off while fueling. Do not top the vehicle off, the extra $0.50 you get into the tank isn't worth the $100 canister.

I hope this isn't too confusing I tried to make it as simple as possible. Every answer about the evap system should be right here, I don't think I'm going to answer any more of them. Oh and I know my paint skills suck so if ya don't like the diagram's, too damn bad .

Its sound's like you have done this a few times. That was very informative. I replaced the vent valve solenoid with ACdelco part 214-2082. I could live with the check engine light. I could not live with the hard starts after I filled up. I guess I'll have to do what you said and stop after the pump stops the first time. Unless you no a way to stop the tank from over flowing into the canister. It's hard to believe that that could actually be designed to let that happen. This 2004 is the newest vehicle I have owned. I normally fill up till its almost spilling out and never had a problem.

zaac · November 15, 2009

Thanks for the info. I'm gonna have to read it a few more times but I'm hoping you'll answer just a few more questions. I didn't see any explanation on how the fuel cap works. On my 2000 Silverado there is a one way valve in the cap operated with a spring. It will let pressure in but not out. If there is a vent valve to let air in why does the cap also let air in? Is it just a back-up in case the vent valve gets stuck closed? When the key is turned on and it does its vacuum test why doesn't air enter through the cap and cause the test to fail? Is the spring in the one way valve strong enough to overcome the amount of vacuum in the tank during the test? When the engine is running what amount of vacuum/pressure is in the tank? I know these are anal questions but I've always wondered about it and since you have such a good understanding I figured I'd jump on the opportunity. Sure hope you read this. Thank...Jack

zaac · November 16, 2009

This morning I removed the fuel cap before starting and drove around all day. The engine light did not come on. wtf? The engine light does work and the computer has set one of the evap codes before(about every 2 months). This don't make sense to me. I thought it did a self test every cold start.

minibike132 · November 17, 2009

Thanks for the info. I'm gonna have to read it a few more times but I'm hoping you'll answer just a few more questions. I didn't see any explanation on how the fuel cap works. On my 2000 Silverado there is a one way valve in the cap operated with a spring. It will let pressure in but not out. If there is a vent valve to let air in why does the cap also let air in? Is it just a back-up in case the vent valve gets stuck closed? Back up for any restriction in the vent system. When the key is turned on and it does its vacuum test why doesn't air enter through the cap and cause the test to fail? It is designed to test during vehicle running, for some reason it will test key on engine off sometimes. There would be no vacuum during key on engine off, the vacuum is provided by the engine. Is the spring in the one way valve strong enough to overcome the amount of vacuum in the tank during the test? Yes unless the cap is bad, that is one way to fail the test under vacuum but pass when tested with pressure. When the engine is running what amount of vacuum/pressure is in the tank? During normal engine running not much, normal operation should have the valve open. During testing it is drawn into a vacuum, I've never caught the PCM commanding a test so I don't know for sure how much but not more then 5in Hg of vacuum. I know these are anal questions but I've always wondered about it and since you have such a good understanding I figured I'd jump on the opportunity. Sure hope you read this. Thank...Jack

minibike132 · November 17, 2009

This morning I removed the fuel cap before starting and drove around all day. The engine light did not come on. wtf? The engine light does work and the computer has set one of the evap codes before(about every 2 months). This don't make sense to me. I thought it did a self test every cold start.

The test does not run every time the vehicle is run. Conditions must be right and even if they are right it may not run.

• No active manifold absolute pressure (MAP) DTCs

• No active intake air temperature (IAT) DTCs

• No active throttle position (TP) DTCs

• No active heated oxygen sensor (HO2S) DTCs

• No active vehicle speed sensor (VSS) DTCs

• No active system voltage DTCs

• The ignition voltage is between 10 volts and 18 volts.

• The barometric pressure is more than 75 kPa.

• The fuel level is between 15 percent and 85 percent of full capacity.

• The engine coolant temperature (ECT) is between 4°C (39°F) and 30°C (86°F).

• The intake air temperature (IAT) is between 4°C (39°F) and 30°C (86°F).

• The start up ECT and IAT are within 9°C (16°F) of each other.

zaac · November 17, 2009

This morning I removed the fuel cap before starting and drove around all day. The engine light did not come on. wtf? The engine light does work and the computer has set one of the evap codes before(about every 2 months). This don't make sense to me. I thought it did a self test every cold start.

The test does not run every time the vehicle is run. Conditions must be right and even if they are right it may not run.

• No active manifold absolute pressure (MAP) DTCs

• No active intake air temperature (IAT) DTCs

• No active throttle position (TP) DTCs

• No active heated oxygen sensor (HO2S) DTCs

• No active vehicle speed sensor (VSS) DTCs

• No active system voltage DTCs

• The ignition voltage is between 10 volts and 18 volts.

• The barometric pressure is more than 75 kPa.

• The fuel level is between 15 percent and 85 percent of full capacity.

• The engine coolant temperature (ECT) is between 4°C (39°F) and 30°C (86°F).

• The intake air temperature (IAT) is between 4°C (39°F) and 30°C (86°F).

• The start up ECT and IAT are within 9°C (16°F) of each other.

Thanks Mike. That answeres any questions I had about the evap. I'm sure I met all the variables over the last 2 days so i guess the pcm can go several days between tests. I had no idea.

minibike132 · November 17, 2009

This morning I removed the fuel cap before starting and drove around all day. The engine light did not come on. wtf? The engine light does work and the computer has set one of the evap codes before(about every 2 months). This don't make sense to me. I thought it did a self test every cold start.

The test does not run every time the vehicle is run. Conditions must be right and even if they are right it may not run.

• No active manifold absolute pressure (MAP) DTCs

• No active intake air temperature (IAT) DTCs

• No active throttle position (TP) DTCs

• No active heated oxygen sensor (HO2S) DTCs

• No active vehicle speed sensor (VSS) DTCs

• No active system voltage DTCs

• The ignition voltage is between 10 volts and 18 volts.

• The barometric pressure is more than 75 kPa.

• The fuel level is between 15 percent and 85 percent of full capacity.

• The engine coolant temperature (ECT) is between 4°C (39°F) and 30°C (86°F).

• The intake air temperature (IAT) is between 4°C (39°F) and 30°C (86°F).

• The start up ECT and IAT are within 9°C (16°F) of each other.

Thanks Mike. That answeres any questions I had about the evap. I'm sure I met all the variables over the last 2 days so i guess the pcm can go several days between tests. I had no idea.

It may take several days of perfect conditions for the test to run. Around here (Chicagoland) with the Iat and Ect needing to be above 39 and below 86 it may not run between November and April or July to September depending on the time of day you start the vehicle. I have waited weeks for the tests to run and pass to get them through emissions and had to put them in the shop to keep the temps correct. Even then it has taken up to a week of sitting in the shop and starting periodically on a few vehicles to get the tests to run and pass.

zaac · November 17, 2009

It may take several days of perfect conditions for the test to run. Around here (Chicagoland) with the Iat and Ect needing to be above 39 and below 86 it may not run between November and April or July to September depending on the time of day you start the vehicle. I have waited weeks for the tests to run and pass to get them through emissions and had to put them in the shop to keep the temps correct. Even then it has taken up to a week of sitting in the shop and starting periodically on a few vehicles to get the tests to run and pass.

Do you find it strange that the engineers designed it like this? I sure do. I guess it's not a critical thing but I would think it could have been easy for them to get it to run a test more often. Would have made your job easier.

Daverj · November 24, 2009

Excellent information. This is my first post and I am having trouble with my EVAP system and passing the California smog test. Code P0449. I was wondering if the EVAP system was closed or is it open to the atmosphere?

zaac · November 24, 2009

Excellent information. This is my first post and I am having trouble with my EVAP system and passing the California smog test. Code P0449. I was wondering if the EVAP system was closed or is it open to the atmosphere?

Did you check out the pictures? The purpose of the evap system is to stop the gas from evaporating so in that sense I guess you could say it's closed. The vent is open but the charcaol canister absorbs the gas fumes while allowing the air through in order to allow re-fueling. Did you look up what that code is? Think it's a purge valve.

minibike132 · November 25, 2009

Excellent information. This is my first post and I am having trouble with my EVAP system and passing the California smog test. Code P0449. I was wondering if the EVAP system was closed or is it open to the atmosphere?

Did you check out the pictures? The purpose of the evap system is to stop the gas from evaporating so in that sense I guess you could say it's closed. The vent is open but the charcaol canister absorbs the gas fumes while allowing the air through in order to allow re-fueling. Did you look up what that code is? Think it's a purge valve.

What he said for the closed or open question.

P0449 is a code that sets if there is an electrical issue on the vent valve circuit. Constant 12v is supplied to the valve, ground is supplied by the PCM when needed. The PCM monitors the signal, if there is an issue with any part of the circuit P0449 will set. There is no way to check this circuit actively at home. You can check for power at the valve, you can supply a ground to the ground side and see if the valve works and you can check continuity on the ground circuit. Other then that to check it correctly you need a scanner that is capable of commanding the PCM to turn the circuit on.

This is not a code that sets because of a leak or a restriction, it is a code based on an electrical issue.

voorheisb · December 15, 2009

I am bored and I have a sick kid on my hands so I'm strapped to the house today. I always see issues with the evaporative system here so I thought I would write a once and for all description of the system, what is supposed to be happening, how we test and fix it and what you can do at home if you are having issues. Most people outside of the shop don't understand the system I hope this sheds some light. It's gonna be a long one, grab a cup of coffee or a beer.

The evaporative emission (EVAP) control system limits fuel vapors from escaping into the atmosphere. Fuel tank vapors are allowed to move from the fuel tank, due to pressure in the tank, through the vapor pipe, into the EVAP canister. Carbon in the canister absorbs and stores the fuel vapors. Excess pressure is vented through the vent line and EVAP vent solenoid valve to the atmosphere. The EVAP canister stores the fuel vapors until the engine is able to use them. At an appropriate time, the control module will command the EVAP purge solenoid valve ON, allowing engine vacuum to be applied to the EVAP canister. With the EVAP vent solenoid valve OFF, fresh air is drawn through the vent solenoid valve and the vent line to the EVAP canister. Fresh air is drawn through the canister, pulling fuel vapors from the carbon. The air/fuel vapor mixture continues through the EVAP purge pipe and EVAP purge solenoid valve into the intake manifold to be consumed during normal combustion. The control module uses several tests to determine if the EVAP system is leaking.

Below are general diagrams of pretty much every evap system in place on GM vehicles excluding those vehicles made by other manufacturers (Pontiac vibe), those systems are basically the same but have some small differences. Pic 1 is the system at rest. Pic 2 is where the vapor goes while fueling. Pic 3 is the system purging to burn off the vapor.

When you fuel your vehicle the vapor needs some place to go. There used to be a vent tube that pushed that straight into the atmosphere. We don't wan't to do that so we push the vapor through the charcoal canister where the fuel vapor is trapped, oxygen should be passed out through the vent line to the vent valve and into the atmosphere.

The vent valve is normally open, it is commanded on (closed) by the PCM to close the system for testing.

Engine vacuum is constantly applied to one side of the purge solenoid. The other side is connected to the evap purge line. When the PCM wants to test the system for leaks the purge solenoid is opened and the vent valve is closed to pull a vacuum on the system. A pressure sensor in the top of the fuel tank senses the amount of vacuum in the system. If the system never draws a vacuum this indicates a restriction or an open system and a code is stored and check engine lamp turned on. Once the proper amount of vacuum is achieved the purge solenoid is turned off closing the system.

Pic 4 s the system drawing a vacuum to get ready for testing. Pic 5 s the valve position during actual testing.

The PCM monitors for large amounts of vacuum decay if there is a large amount of decay a leak is indicated and again the lamp is turned on and a code is set. At this point in older systems a small leak test is performed keeping a careful watch on the vacuum level, if a small leak is detected a different code will set. Newer trucks test for small leaks a little differently. It checks pressure in the system after the vehicle is turned off and uses complex algorithms based on temperature and time to detirmine a leak.

Problems that can happen. First (the reason for me writing this) you can be fueling your vehicle and the pump constantly shuts off making it take forever to fill the vehicle. There is a restriction somewhere between the fuel filler neck and the vent valve. Usually the restriction is the vent valve itself. To check this in the shop we would connect a smoke machine to the vehicle and fill the evap system with smoke. With the valve being normally open smoke should pour out of the valve. If it doesn't we would remove the vent line directly at the valve. If smoke comes out the restriction is proven to be the vent, if not we keep moving up the system till the restriction is found.

If you are experiencing the issue of slow fuel fill you can check the vent valve by pulling the vent line off at the valve and trying to fill it. If it fills normally there you have it, you need a new valve. Why can't I just leave it off you might ask, because it will cause a check engine light and it isn't legal.

If a leak is detected and a code stored and you bring your vehicle in these are the basic tests we do. They involve expensive equipment and there is no way around them at home. Scanners that you may have can check codes but scan tools that can control outputs are expensive and unless you do this for a living you probably don't have one that will.

Car/truck comes into my bay with a leak code. I hook up the Tech-2 and command the evap vent solenoid closed and listen for an audible click. If I hear the valve click I know it is getting what it needs electrically, if not I go to the valve and check for power. Our valves use a constant power, you can check for that. Then I check the other side for ground while commanding the valve on, the ground is controlled by the PCM and is what turns the valve on and off. There is no way for you to command that ground, you can manually apply a ground to the ground side to see if the valve is working but that will not take the PCM or wiring into account.

Let's assume that the valve clicks. I start the vehicle, command the vent closed and the purge open. If no vacuum is achieved I tighten the cap a few clicks and see if it draws a vacuum. If it does then the customer didn't tighten the cap enough. Problem solved, educate the customer on tightening the cap. At the same time I educate them about fueling with the key on. If the key is on the system may try to test itself, if you have the cap off it will fail and turn on the light. Don't fill the vehicle with the key on or engine running.

If vacuum is achieved I close the system off with the Tech-2 and watch for decay. If the vacuum level does not stay constant there is a leak, time to go looking for it. We hook a machine up to the system that will inject smoke. We close the system off with the Tech-2 and start looking for smoke. With the larger leaks the smoke will be very visible and we repair the leak. With smaller leaks the smoke may not be visible, this moves us to the final test, the nitrogen flag test.

The smoke machine has the ability to inject nitrogen into the system which is at a higher pressure then the smoke. It is useful for finding small leaks and verifying that the system is working as designed after repairs. The machine has a cylinder on it with a small ball inside. While injecting nitrogen into the system the ball will float high in the tube indicating that there is flow. Once the system is full the ball will drop down indicating less or no flow. If the ball does not drop below a calibrated level there is still a leak and we start disconnecting and capping off the system to pinpoint the leak. If the ball does drop below the calibrated level there is no flow and hence no leaks.

One final thing to touch on is fueling the vehicle. When the pump shuts off the tank is full (unless there is a vent restriction). If you continue to top off the tank it will pour raw fuel into the canister. The canister is designed for vapor, not raw fuel. Saturating the canister causes a restriction and you will either set codes for a restriction, cause a rich condition, or make it so that the pump constantly shuts off while fueling. Do not top the vehicle off, the extra $0.50 you get into the tank isn't worth the $100 canister.

I hope this isn't too confusing I tried to make it as simple as possible. Every answer about the evap system should be right here, I don't think I'm going to answer any more of them. Oh and I know my paint skills suck so if ya don't like the diagram's, too damn bad .

Thanks for the great write-up. I was hoping I could troubleshoot and replace the part myself. It appears as though without the Tech-2 I cannot. I have the following codes:

PO449: "EVAP Control System Vent Valve Malfunction"

PO455: "EVAP Control System Leak"

I was thinking it was the canister but wanted to get your thoughts. By the way, I am guilty of topping off. Unfortunately, I did not know better prior to this.

Again, thanks for your extensive write-up.

Nytemare · December 15, 2009

Thanks for the great write-up. I was hoping I could troubleshoot and replace the part myself. It appears as though without the Tech-2 I cannot. I have the following codes:

PO449: "EVAP Control System Vent Valve Malfunction"

PO455: "EVAP Control System Leak"

I was thinking it was the canister but wanted to get your thoughts. By the way, I am guilty of topping off. Unfortunately, I did not know better prior to this.

Again, thanks for your extensive write-up.

I think we are all guilty of topping off man...don't sweat it...just learn from it I guess.