Interesting Evap Cannister Info
07-12-2013, 06:20 PM
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Interesting Evap Cannister Info
I recently received the Rock Auto Newsletter and it had an article about the Fuel Evap System. Because it seems we have members that have had problems in that area (and for those in the future), I thought I would post it.........
Plenty of Places for a Leak or Sensor Failure to Occur
If the Check Engine light comes on, then check first to see if the gas cap is loose or cracked. Most people may know that now, but they may not know why. How does the computer find out that the fuel tank cap is not screwed on tight and why does it care?
Since the 1970s, cars have had evaporative emissions systems that prevent gasoline vapors from escaping the fuel tank and contributing to air pollution. Vapors from the gas tank are trapped in a container called a carbon vapor canister and then sucked into the engine air intake and burned. The early evaporative systems usually seemed to last forever and be maintenance-free because the car owner had almost no way of knowing if there was a leak in the system. I can leave the fuel tank cap for my 1979 Chrysler sitting stranded on top of the pump at a gas station and still never hear any complaints from the car’s evaporative system.
Computers on newer, OBDII equipped cars receive feedback from the evaporative system and light the Check Engine light if a leak is detected. A typical system includes a vapor canister, a vapor canister purge valve, a vapor canister vent solenoid and a fuel tank pressure sensor. The vapor canister purge valve controls the flow of gas vapors into the engine. The vapor canister vent solenoid controls the flow of fresh air into the vapor canister. When the engine is running, gasoline vapors leave the canister through the purge valve and are burned up in the engine. To avoid a vacuum from being created, fresh air is allowed into the canister through the vent solenoid.
The vapor canister purge valve and vent solenoid are both normally open when the engine is running. However, periodically the computer will seal the system by closing both the purge valve and vent solenoid simultaneously. The computer will then watch the fuel tank pressure sensor to see if the air pressure in the system drops over time. If a drop in pressure is detected, then that means there is a leak and the Check Engine light goes on.
A loose gas cap could cause the leak or it could be caused by a leaking purge valve or vent solenoid. Maybe the vapor canister itself is leaking or maybe the fuel tank pressure sensor is bad and sending erroneous pressure readings to the computer.
There are plenty of places for a leak or sensor failure to occur. The OBDII trouble codes and repair manual can help. The RockAuto catalog can provide clues too. For example, if ACDelco, Airtex, Dorman, Standard Motor Products and other manufacturers all make new vapor canister purge valves for a vehicle, then that could be a sign that the original part was failure prone. Little heart icons next to a part in the catalog indicate lots of other owners have purchased this part. The hearts could be another indicator that the original part often needs to be replaced.
For some cars, some part manufacturers can be sloppy and use the terms “purge valve” and “vent solenoid” interchangeably. They both let gases into or out of the evaporative system, but they are different parts. As always, it is helpful to compare the part images in the catalog with the part you are replacing.
Tom Taylor,
RockAuto.com
Plenty of Places for a Leak or Sensor Failure to Occur
If the Check Engine light comes on, then check first to see if the gas cap is loose or cracked. Most people may know that now, but they may not know why. How does the computer find out that the fuel tank cap is not screwed on tight and why does it care?
Since the 1970s, cars have had evaporative emissions systems that prevent gasoline vapors from escaping the fuel tank and contributing to air pollution. Vapors from the gas tank are trapped in a container called a carbon vapor canister and then sucked into the engine air intake and burned. The early evaporative systems usually seemed to last forever and be maintenance-free because the car owner had almost no way of knowing if there was a leak in the system. I can leave the fuel tank cap for my 1979 Chrysler sitting stranded on top of the pump at a gas station and still never hear any complaints from the car’s evaporative system.
Computers on newer, OBDII equipped cars receive feedback from the evaporative system and light the Check Engine light if a leak is detected. A typical system includes a vapor canister, a vapor canister purge valve, a vapor canister vent solenoid and a fuel tank pressure sensor. The vapor canister purge valve controls the flow of gas vapors into the engine. The vapor canister vent solenoid controls the flow of fresh air into the vapor canister. When the engine is running, gasoline vapors leave the canister through the purge valve and are burned up in the engine. To avoid a vacuum from being created, fresh air is allowed into the canister through the vent solenoid.
The vapor canister purge valve and vent solenoid are both normally open when the engine is running. However, periodically the computer will seal the system by closing both the purge valve and vent solenoid simultaneously. The computer will then watch the fuel tank pressure sensor to see if the air pressure in the system drops over time. If a drop in pressure is detected, then that means there is a leak and the Check Engine light goes on.
A loose gas cap could cause the leak or it could be caused by a leaking purge valve or vent solenoid. Maybe the vapor canister itself is leaking or maybe the fuel tank pressure sensor is bad and sending erroneous pressure readings to the computer.
There are plenty of places for a leak or sensor failure to occur. The OBDII trouble codes and repair manual can help. The RockAuto catalog can provide clues too. For example, if ACDelco, Airtex, Dorman, Standard Motor Products and other manufacturers all make new vapor canister purge valves for a vehicle, then that could be a sign that the original part was failure prone. Little heart icons next to a part in the catalog indicate lots of other owners have purchased this part. The hearts could be another indicator that the original part often needs to be replaced.
For some cars, some part manufacturers can be sloppy and use the terms “purge valve” and “vent solenoid” interchangeably. They both let gases into or out of the evaporative system, but they are different parts. As always, it is helpful to compare the part images in the catalog with the part you are replacing.
Tom Taylor,
RockAuto.com
07-13-2013, 11:02 PM
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Join Date: 01-09-2007
Location: SE USA
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