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#4557427 - 10/28/17 04:21 PM Evap sytem basic operation?
Rolomoto Offline


Registered: 10/10/13
Posts: 66
Loc: PA
From what I understand, the evap system works when refueling and when the car is running.
When refueling the excess air in the tank is forced through the charcoal canister, scrubbed and clean air is released through the air drain. When running, vapors in the tank are sucked into the canister and purged to the engine. My question is, if the vapors pass through the canister wouldn’t it be clean air (non-combustible) that is being purged to the engine?
Thanks for any clarification!
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#4557434 - 10/28/17 04:27 PM Re: Evap sytem basic operation? [Re: Rolomoto]
redbone3 Offline


Registered: 06/09/12
Posts: 654
Loc: Reno, NV
I don't think the system works during refueling. The pipe from the gas tank is too small for that. Plus, a purge valve has to be turned on, which isn't going to happen when the car is turned off.
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#4557469 - 10/28/17 05:13 PM Re: Evap sytem basic operation? [Re: Rolomoto]
mk378 Offline


Registered: 09/27/15
Posts: 1414
Loc: USA
It is mostly air that comes out of the canister while purging. It adds to the air the engine needs from the air cleaner. Any gasoline (hydrocarbon) vapors present should get burned in the engine though.

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#4557565 - 10/28/17 06:44 PM Re: Evap sytem basic operation? [Re: Rolomoto]
eljefino Online   content


Registered: 06/15/03
Posts: 32631
Loc: ME
When you park the tank is sealed. In the old days there was a vent to atmosphere. Now that vent goes to the charcoal canister which soaks it up. To dry out the charcoal the valves open under cruise and a trickle of air flows through the charcoal. Its dirty, VOC-laden air goes off to the combustion chamber.

OBD-II systems pressurize or depressurize the gas tank and EVAP lines to ensure they hold up. If you hear a whoosh when you open your gas cap things are good n' tight!

The gas cap has a vent, too, but it's designed to hold tighter than the EVAP one, so it's a fail-safe.

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#4557816 - 10/28/17 09:48 PM Re: Evap sytem basic operation? [Re: Rolomoto]
JTK Offline


Registered: 08/14/03
Posts: 10559
Loc: Buffalo, NY
There is typically a little filter element where fresh air is drawn in for the EVAP canister. Any other air can be drawn in through the check valve in your gas cap. The cap should allow air in, but not air out (easily).

A lot of late model EVAP system, such as those on Subarus have a pump that will run a diagnostic pressure test of the system at certain intervals, after the vehicle has been used, but then has sat a few minutes. Kind of startles you when you hear it whirring like you left something on or the fuel pump is still running.
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#4558847 - 10/29/17 10:02 PM Re: Evap sytem basic operation? [Re: Rolomoto]
NYEngineer Offline


Registered: 12/12/06
Posts: 1562
Loc: NY, NY
On my 2009 Suburban, some solenoid failed closed and it wouldn't vent the tank

It also made it difficult to get gas
I replaced it with the new, improved part and it has been working perfectly for about seven years now
I had to drive with the cap cracked loose before I repaired it

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