Automotive

How the EVAP System Works

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  • Aug 21st, 2017 2:50 am
Deal Addict
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Aug 3, 2014
1261 posts
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Toronto, ON

How the EVAP System Works

I cut open a gas tank and EVAP canister to see what’s inside and how it works. I made a short video on the process, you can check it out here:



Here are a few photos of the teardown:
The EVAP canister is located behind the gas tank:
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The gas tank is held on by two straps. Boy was there a lot of rust on this Corolla! Down comes the tank:

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One bolt for the filler neck:
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Gas tank, EVAP canister removed:
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Here’s what some of those vacuum lines do:
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Vacuum lines on the gas tank:
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Here’s an overview of the EVAP system. Its basically there to vent gasoline vapors and catch it in the charcoal canister. Its held there until its released into the car’s air intake to get burned by the engine.
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EVAP float to ensure liquid gas doesn’t go into the charcoal canister:
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Here’s the gas tank chopped open:
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Vapour pressure sensor. ECU uses this for self-diagnostics. That’s why it’s so difficult to pass emissions tests!
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Fill check valve. This controls the ventilation of the gas tank while refueling.
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Its pretty cool inside, there’s a diaphragm that works with air pressure.
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It allows vapor to move in one direction only
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Chopping open the charcoal canister:
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Tiny beads of activated charcoal:
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Which gives me an idea…Barbeque anyone?
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Speedkar99 | YouTube
16 replies
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Sep 4, 2005
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Toronto
Awesome videos man. +1 Keep making them I find them crazy interesting.
Deal Fanatic
Apr 20, 2011
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ON
Mmmm, gasoline flavoured burgers...
Deal Addict
Feb 6, 2011
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What did you use to cut open the tank?
Deal Fanatic
May 4, 2014
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Toronto, ON
billford wrote: What did you use to cut open the tank?
Good quality steak knife.


(One of the pics shows an angle grinder.)
Deal Addict
Feb 6, 2011
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er34er34 wrote: Good quality steak knife.


(One of the pics shows an angle grinder.)
Thought cutting a tank open with a angle grinder would be dangerous, even though the tank looks like its partly full of water.
Jr. Member
Oct 30, 2007
133 posts
17 upvotes
Regina
Very interesting. I have a 99 Corolla and about 2 years ago the check engine light came on and my mechanic said that the evap canister needed to be replaced, but also suggested that it's an "optional" repair and costly. So I just left it as it was. With this video I get the impression that the warning light reflects a leak in the system and not necessarily a broken canister, though maybe the diagnostic code would be more specific to this too. The car still drives great with 230k on it and no maintenance at all, though fuel consumption is up - possibly due to evaporation.
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Aug 3, 2014
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Super_Chicken wrote: Awesome videos man. +1 Keep making them I find them crazy interesting.
SaugaDealz wrote: Great videos!
Thanks! I sure am learning a lot by gutting this Corolla.
billford wrote: What did you use to cut open the tank?
billford wrote: Thought cutting a tank open with a angle grinder would be dangerous, even though the tank looks like its partly full of water.
I thought it would be dangerous too. But once I rinsed the tank with water, then filled it right up with water, I was more worried that the water would spit up into the grinder and electrocute me than sparks igniting fumes.
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Speedkar99 | YouTube
Deal Addict
Nov 17, 2004
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Great details as always.

And I also see that just like most 90s Toyota's, the fuel filler neck is much newer than the rest of the car lol
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Jun 14, 2008
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Montreal
I remember trying to light up some activated charcoal out of a water filter when I was a kid, couldn't do it. It'll go red hot but wouldn't burn, back to original shape as soon as heat is removed.
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Jul 14, 2002
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merlin666 wrote: Very interesting. I have a 99 Corolla and about 2 years ago the check engine light came on and my mechanic said that the evap canister needed to be replaced, but also suggested that it's an "optional" repair and costly. So I just left it as it was. With this video I get the impression that the warning light reflects a leak in the system and not necessarily a broken canister, though maybe the diagnostic code would be more specific to this too. The car still drives great with 230k on it and no maintenance at all, though fuel consumption is up - possibly due to evaporation.
most of the times, it's either a hose or the rusted joints in between that have a minor crack so the EVAP system does not hold enough pressure to successfully complete the test.
how would you pass emissions with the engine light on though?
Deal Expert
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Apr 21, 2004
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Nice. Condenser/ evaporator next? Need to make sure no mold growth there.
Sr. Member
Apr 13, 2003
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North York
+1 from me always! Very good info.
Riaz
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Jul 4, 2006
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Oakville
Great info.

I have a 2003 Civic only 116k and the Engine light goes on. I had my mechanic check it and he indicated it was the fuel evap system error code.

He took it apart and couldn't find anything wrong as there were no leaks and it had vacuum on the hoses after he removed them.

I know Honda just wants to replace the entire system for close to $1,000. No dice.

Hope light it stays off since it was cleared. Could be a loose connector that he reinforced.
Jr. Member
Oct 30, 2007
133 posts
17 upvotes
Regina
dantey wrote: most of the times, it's either a hose or the rusted joints in between that have a minor crack so the EVAP system does not hold enough pressure to successfully complete the test.
how would you pass emissions with the engine light on though?
In SK we don't have emissions tests. Do they still do that in Ontario or other provinces?
Deal Expert
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Apr 21, 2004
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How to access and clean the evap besides spraying chemical into the vents?

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