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The Evap System


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I recently had my vent valve replaced at a chevy dealer here in Atlanta. Two or three days later the engine light was back on and indicated a leak in the EVAP. So now I have the problem with filling the gas tank. The service guy at the dealership was trying to tell me that this problem is unrelated to the vent valve replacement. Is this true? Or are they just trying to get me to shell out more $$$? It seems way to close in time and components to be unrelated.

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  • 10 months later...
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 :) .

 

So I just replaced the evap solenoid and still having issues so I took out the main canister and tried to blow air into each of the ports. The evap canister hole blew out of the engine vacuum hole but when I tried to blow through the connection from the tank there was something restricting it and wouldn't blow out of either hole. When I took the nozzle off a huge cloud of dust blew in my face but nothing else. If you blow out through that hole should there be little to no resistance or is there a one way valve on there and that is why it is acting like it is clogged?

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  • 4 years later...

I had the problem of the pump turning off repeatedly while trying to fuel vehicle. I removed the line from the canister to the valve and now the tank will fill until it gets to 3/4 full. Then it shuts of repeatedly again. Could this be because the canister is bad or is there another problem?

 

2006 GMC 1500 Yukon XL 5.3L flex

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  • 1 year later...

I hope this thread is still active. I have an 04 Yukon xl denali. I've had the purge solenoid, charcoal canister, and vent valve replaced 3 times and still have problems fueling my truck. It will fill at normal speed but will only fill up to half a tank but no more. It also chokes out the engine after filling it and if I take off too fast or stop too fast. Sometimes it will level out but sometimes it will choke it out and die, and it takes a bit to restart it and level it out. I've explained this to the gmc dealer here ND they still only replaced the same parts and nothing more. I know the Guage is accurate. I've had the dash rebuilt and then ran it out to make sure it was accurate. Any help is appreciated. Thanks!

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