FB Charcoal Canister vacuum restrictor?
 

Hello,

Was hoping someone could confirm if the smaller fuel inlet to the charcoal canister on a GSL-SE (not sure it matters) has what looks like a restrictor pill. Looking around online and ebay postings, it doesn't appear to be normal. Reason I ask is I have been having some symptoms of a clogged canister (fuel pump noise and fuel smell that both went away after disconnecting the CC).
https://cimg3.ibsrv.net/gimg/www.rx7...83ccf62310.jpg

Thanks!

My non -SE car has one like that as I recall.

Yes, that's how they are. Have you tried swapping in a different charcoal canister to see if the issues you are having are still present. The charcoal canister is only there to trap fuel vapors until the purge valve opens under vacuum and releases the vapors into the engine to be burned while the engine is running. I wonder if the vapors are being stored and are pressurizing, causing excess fuel system pressure. I think I would try another charcoal canister.

Thanks for the replies!

Yeah, I bought a lower mileage FC canister since FB ones were a bit expensive, but noticed the back pressure was about the same when blowing through the small tube on both, which is when I noticed the restrictor. Guess I'll try running it with the FC one to see if the issue comes back (in which case, I guess I'll cut open the FB one :fingersx:).

Mine (GS). Has that too. I had similar symptoms before and I drilled it out just ever so slightly and the issues went away. I was just experimenting since I had a second canister.

I'm not recommending you do this since these things are expensive as you pointed out.

I have no idea why a slightly larger orifice would help, but I was getting no air through mine.

Is it just an orfice or a check valve? I think positive pressure from the tank gets pushed through it, but when the purge valve opens, air should be pulled from the bottom of the canister, not from the line to the gas tank.

Following this as I have same symptoms

Good to know. I'll probably try that since I don't really see the harm in widening it a bit, plus I have the FC one as a backup. My guess is that it starts building up pressure in the tank as you drive it, as only so much air is being pushed through that restrictor. At the same pressure, a drilled-out hole should allow more air through, thus preventing as much back-pressure. Just a guess, though.


Should just be an orifice. AFAIK, the canister should never be drawing air back through it to the fuel tank, as the Cut & Check valve will pull any vacuum if necessary. Not 100% sure which way the check valve on the oil filler neck is, since its supposedly sucks the fuel vapors from the canister back in on start. I have not been able to confirm if that check valve even exists, so it may not even be a thing.
https://cimg6.ibsrv.net/gimg/www.rx7...39bf3513a8.png

There isn't a check valve between the crank case and the canister, not on my 85 at least. At idle or when the motor is off, positive crank pressure is passed to the canister and expelled through the bottom. Under light throttle, vacuum from just above the throttle plates will open the purge valve. This will suck air from the canister, into the crank case, back out of the crank case, through the purge valve, and into the intake below the throttle plates. The purge valve has an orfice to restrict how much air is pulled in.

Positive pressure from the gas tank caused by fuel warming up slowly passes through the check and cut valve and into the canister (or into the crank case if the purge valve is open). When the fuel cools down, vacuum is relieved via the fresh air port on the check and cut valve, not the canister.

Some amount of pressure in the gas tank is normal on a hot day and you'll hear/feel a rush of air outward when you remove the gas cap. The orfice on the canister port leading to the gas tank only lets so much pressure out and is likely there to prevent the purge valve from pulling a vacuum on the tank.

How to test your evap system....

Attachment 740187

There is no specifics on testing the canister but if you can suck/blow air with little resistance from the larger port while blocking off the smaller port, the charcoal isn't clogged. If you and get some amount of positive pressure to pass through the smaller port it's working fine. I don't know how much pressure it should hold on the small port, but I gather it shouldn't be free flowing. If your evap system holds pressure using the test above, that pressure is getting released into the canister at some point...if it wasn't, the hose would be popping off.