Evap Testing

[willbur]

Greetings All-
The vapor canister has an ECM controlled vent valve. What does it vent if the sole purpose of the canister is to capture gas vapor? And, How often does the ECM test the Evap system?

 

[Kira]

The vent opens to allow purge air to be drawn through by engine vacuum. The canister is emptied thereby. I believe the charcoal in it is enabled to again absorb vapors.

I've never heard of a rate of testing in, say, cycles per hour. The test cycle does check to see if the fuel system will hold a vacuum.

 

[willbur]

The vent opens to allow purge air to be drawn through by engine vacuum. The canister is emptied thereby. I believe the charcoal in it is enabled to again absorb vapors.

I've never heard of a rate of testing in, say, cycles per hour. The test cycle does check to see if the fuel system will hold a vacuum.

Thanks- I understand. Air is drawn through the canister, vapor not vented out. Can’t believe everything one reads, which confused me. Hence, my question. Regarding the testing, each driving cycle? If you had a malfunction and repaired it, does the p0455 code stay in memory until a certain number of drive cycles/tests are completed?

 

[eljefino]

If it vents to open air it's part of the EVAP testing portion. Typically it would put a vacuum on the gas tank by opening the purge valve, measuring the vacuum on the tank coming from the intake, then venting and seeing how it reacts.

The newest cars test EVAP a couple different ways, driving and just sitting around. The really old 1996-97 era cars OTOH seem pretty numb in their diagnostics.

 

[Bladecutter]

Greetings All-
The vapor canister has an ECM controlled vent valve.
What does it vent if the sole purpose of the canister is to capture gas vapor?
And, How often does the ECM test the Evap system?

Thanks- I understand.
Air is drawn through the canister, vapor not vented out.
Can’t believe everything one reads, which confused me.
Hence, my question. Regarding the testing, each driving cycle?
If you had a malfunction and repaired it, does the p0455 code stay in memory until a certain number of drive cycles/tests are completed?

In some designs, the vapor canister IS the vent for the gas tank.
It provides 2 important functions:

1 - It takes any gas vapor, routes it to the charcoal canister, where it then gets ingested by the engine when the canister purge valve is activated, and the fumes are pulled into the intake stream.
2 - It acts as a vent for the gas tank to allow fresh air into the gas tank (usually through a small filter) as the fuel is consumed by the motor and the level decreases. Otherwise, your gas tank would implode as the fuel level goes down.

I just recently had to pull the rear seat out of my friend's 2013 Fiat 500 (over 100k miles) and give the vent valve a good whacking in order to allow her to refill the gas tank without the gas pump from shutting off every 3 seconds. She had been putting up with it for months before she mentioned it to me, and in 10 minutes, and a couple of hand tools, all was better.

 

[willbur]

In some designs, the vapor canister IS the vent for the gas tank.
It provides 2 important functions:

1 - It takes any gas vapor, routes it to the charcoal canister, where it then gets ingested by the engine when the canister purge valve is activated, and the fumes are pulled into the intake stream.
2 - It acts as a vent for the gas tank to allow fresh air into the gas tank (usually through a small filter) as the fuel is consumed by the motor and the level decreases. Otherwise, your gas tank would implode as the fuel level goes down.

I just recently had to pull the rear seat out of my friend's 2013 Fiat 500 (over 100k miles) and give the vent valve a good whacking in order to allow her to refill the gas tank without the gas pump from shutting off every 3 seconds. She had been putting up with it for months before she mentioned it to me, and in 10 minutes, and a couple of hand tools, all was better.

Thanks for helping others. Nice to hear

 

[Timmastertech]

The sole purpose of the vent valve is to allow the evap system to be sealed for leak detection. With the exception of Chryslers and a few other oddballs the canister vent is normally open. When conditions are correct for the engine control module to perform a leak test on the evaporative emissions system it will command the vent valve closed to seal the system. How the leak test is performed depends on the system.