How a charcoal cannister works

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Bonnewagon

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I've seen this question a lot lately so here is an answer that should come up when you search. -Prior to whatever year the charcoal cannisters appeared, the gas tank vented to atmosphere. If you walked past a vented car on a hot summer day, you smelled gas fumes. To keep these fumes out of our air, the car makers figured out that activated charcoal (same as a fish tank filter) can absorb 100 times it's weight in gas fumes. So they ran a vent line from the gas tank to the canister and a line to the float bowl to capture and hold the fumes. When the motor starts, the fumes are drawn into the carb to be burned. That is why there is a filter on the bottom of the canister to flow fresh air over the charcoal and later cars have a solenoid to control the release. The gas cap that used to be vented, was now sealed so fumes cannot escape. This is a passive system that in no way affects the performance of the car.
 

supkylesup

Master Mechanic
Mar 7, 2010
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Toledo, OH
Thanks for the info
One question though
Does the filter on the bottom of the canister ever need to be changed?
Its a filter and filters get dirty and worn out.
So I was just wondering
 

Bonnewagon

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Yes, if you can find one. Probably Rockauto would have it. If it is clean looking, no holes, and you can suck air freely through it, it is fine.The only car I needed to change it on was a 75' Matadore. It was clogged solid and dripping gas, which was a serperate issue. More often the filter gets a hole and charcoal falls out which lowers resistance and the cannister acts like a vacuum leak. If left alone it lasts a very long time without causing trouble.
 

supkylesup

Master Mechanic
Mar 7, 2010
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Toledo, OH
oh cool thanks again

just trying to fill my young mind with random info for future reference
i dont plan on changing my filter or anything
i just wanted to know if it was a filter that could be changed
or needed to be changed often

so thanks for that
 

bill

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Jul 11, 2008
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Hey Bonnewagon,
While on this topic, what can you do to bypass the vapor line and the return line on a gbody that has an older motor without a return fuel pump? I was thinking of just capping off those lines and drilling a hole in the filler cap to allow are in to prevent a vacuum or pressure problem, depending on the ambient air temp.
Thanks!
 

dan2286

Royal Smart Person
Mar 25, 2008
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Just so everyone knows, the charcoal canister is why you don't want to overfill your gas tank. It pretty much ruins the charcoal canister if it gets raw gas in it. This pretty much applies to all new cars.
 

carmangary

G-Body Guru
Oct 13, 2009
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You can get the filter at the bottom of the canister from just most local parts stores. I got one from NAPA parts for a few dollars or so. It is just a thin spongy type of thing and is really easy to change.
 

jrm81bu

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Jul 9, 2008
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bill said:
Hey Bonnewagon,
While on this topic, what can you do to bypass the vapor line and the return line on a gbody that has an older motor without a return fuel pump? I was thinking of just capping off those lines and drilling a hole in the filler cap to allow are in to prevent a vacuum or pressure problem, depending on the ambient air temp.
Thanks!

I removed my canister and just left the line there. If anything you could attach a small rubber line to it and run it down ans out of the engine bay. And if I had had a return line, I'd have just left it there too.
 

Blake442

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Apr 24, 2007
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Like Bonnewagon mentioned, there really is no need to remove the charcoal filter system.
However, certain applications won't require them, or even have the provisions to support it.

I removed the charcoal canister system on my 442 when I installed the 455... Here's what I did.

Naturally, I removed the canister, and then started following the line down the frame rail to about as far as the brake proportioning valve. I was able to remove as far as that, and then broke off the line. At this point, the line gets tucked into a cluster of other lines, and I didn't feel like opening up the clip holding them.
I followed the line all the way back to the junction with the tank, which is hidden with a small heat shield, and disconnected the rubber hose from the vent line.
Now I was holding in my hand a section of 3/8 rubber hose going to the vent on the sending unit.
Here is where I got creative, and used a small plastic vent that is typically found on rear axles. I don't have a picture handy, but it's just a little chunk of plastic. I popped that in the end of the line, put the heat shield back in place.

Never had any problems with vapors, leaking, or smell for that matter...
 

Bonnewagon

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Bill, Blake's idea sounds like a good one but I would place the plastic vent in the rubber line at the top of the gas tank and maybe even a little higher. Gas vapors are heavier than air and would not tend to escape unless they were under pressure like on a hot day.The Jeep uses a long hose from the axle up to the top of the engine compartment with that same plastic vent so it won't get water in it while off-roading. In the bad old days the factory just ran a vent line up and away from the tank and ended it with a curl, not unlike a sanitary pipe vent. As for the return line, the main reason the factory added it was to avoid vapor lock, especially on V-8 air conditioned cars. As the gas line passes the hot-as-hell front of the motor with hot air from the radiator being blown at it, the fuel boils and creats a bubble that will not let liquid fuel pass. By redirecting the excess fuel back to the gas tank, only fresh cool fuel is sent to the carb. Lots of cars came with only a supply line so it's no big deal to remove the return line and use a non-return fuel pump. Just don't try to use a return line pump without a return line as it will flood the carb and cause trouble. Vented gas caps are hard to find and wouldn't fit our screw-in type anyway. I guess a hole is just as good.
 
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