School me on TBI charcoal canisters

[nutt7]

I understand the basic idea, but:
How do I test one?
How do you know if they’re bad?
Where are the valves and how do they work? (Electric, vacuum, pressure)?
I have a few possible symptoms but don’t want to mess with it until I understand it.
Thanks!

 

[frankin5]

Basically it collects fuel vapors for the tank and uses manifold vacuum to pull
the fumes thru the charcoal and into the manifold.

 

[nutt7]

Basically it collects fuel vapors for the tank and uses manifold vacuum to pull
the fumes thru the charcoal and into the manifold.

So are there any valves or anything?

 

[frankin5]

Evap can only has two 1/4' ports. One to tank and one to TBI.

 

[Keitha]

It should not have any valves. When you shut the motor off and vacuum goes away, the charcoal absorbs gas vapors instead of them being released to the atmosphere. When you restart your motor, the vacuum then sucks the vapor out of the charcoal canister.
Not sure if any years had a valve on the TBI. My 1991 does not.

 

[frankin5]

No purge valve on the TBI motors.

 

[89 jimmy]

About the only symptom I can think of regarding the canister is a broken or cracked vacuum hose the leads from the charcoal canister, to the front bottom of the TBI, creating a vacuum leak.

 

[diesel4me]

They sometimes fill up with gas too,rendering the charcoal useless,and sensors will detect the gas and set off the CEL often..

 

[nutt7]

Thanks! While filling up my tank it will click off all the time or fuel will start seeping out while pumping or splash back when I pull the nozzle. It’s annoying and I heard a messed up canister might cause that. I also never have pressure or vacuum when opening the cap. Engine runs really well though. I occasionally get 2 CELs, one when on the highway for failed egr and one about timing when I floor it...

 

[Keitha]

Thanks! While filling up my tank it will click off all the time or fuel will start seeping out while pumping or splash back when I pull the nozzle. It’s annoying and I heard a messed up canister might cause that. I also never have pressure or vacuum when opening the cap. Engine runs really well though. I occasionally get 2 CELs, one when on the highway for failed egr and one about timing when I floor it...

I really don't see how the charcoal canister would cause either of the problems you describe. the charcoal canister It is a small system that is connected to your TBI. Your gas fill issue may be a clogged or constricted vent tube.
Checking the egr is pretty easy. It uses a module to route vacuum to the EGR valve. When vacuum is on the valve, it opens and routs exhaust to the intake. It can clog with carbon or the vacuum line could have a leak or the module is faulty. You can manually push on the egr valve to see if it sticks etc.

 

[nutt7]

I need to get down there and look around. I thought the canister was hooked up to the vent tube. I read in a few places that the canister can cause those issues but you know how the internet can be. The EGR tests fine...maybe it’s the module but it’s so infrequent that I’m not too concerned at the moment.

 

[blazinzuk]

Is your exhaust to close to your gas tank?

 

[89 jimmy]

Moving the EGR to see if it sticks is fine, but it does not definitively rule out the EGR being bad. It needs to be able to hold vacuum. Can test with a vacuum pump. If your getting an EGR code, just replace the EGR and solenoid. Fairly cheap

 

[Keitha]

I need to get down there and look around. I thought the canister was hooked up to the vent tube. I read in a few places that the canister can cause those issues but you know how the internet can be. The EGR tests fine...maybe it’s the module but it’s so infrequent that I’m not too concerned at the moment.

You are talking about the charcoal canister under your hood, right? Typically mounted front, driver-side corner of the engine bay.

 

[charliew018]

tbi cel codes are a little hard to diagnose, or they are for me sometimes. My last encounter was a cel that led me to the dist but that did not help. It was really intermittent. It ended up being a cracked hose on the egr valve that I didn't find till I replaced the manifold gaskets for a oil leak at the back of the intake on my 91 burb. If it gets hard to find it is probably cheaper to just replace the hoses before you spend money on stuff you won't need. The hoses will need replacement anyway if you start troubleshooting with a mityvac and they need replacement every ten years or so anyway. The cracked hose probably led to bad timing or the egr not working and after replacing the hoses the cel never happened again.When I replaced the dist that cost me two before I got one that moisture would not screw up and stopped my habit of buying lifetime stuff at autozone at least the electric stuff. especially injectors. I have my ramjet canister just venting to atmosphere and I don't tighten the gas cap down all the way to make sure it doesn't get pulled down in a vacuum condition. I do worry about the crummy gas getting moisture in it though. Anytime I fill up I expect some fuel to maybe puke back if I stick the nozzle all the way into the filler at first.

 

[AJMBLAZER]

Not Chevy TBI but years ago I had a ‘96 Ford Ranger with a 4.0L V6. Had some evap issues. After fiddling with all kinds of things I bought a new charcoal cannister. It was fairly light.

Old one looked fine from the exterior but was much heavier and felt dense. Definitely most of the issue.

 

[nutt7]

Is your exhaust to close to your gas tank?

No, it’s all stock.

Moving the EGR to see if it sticks is fine, but it does not definitively rule out the EGR being bad. It needs to be able to hold vacuum. Can test with a vacuum pump. If your getting an EGR code, just replace the EGR and solenoid. Fairly cheap

It doesn’t stick. Currently it’s too intermittent to worry me much. CEL only comes on on the highway...sometimes. Never on surface streets.

You are talking about the charcoal canister under your hood, right? Typically mounted front, driver-side corner of the engine bay.

Yessir! It has two hoses. One goes to the TBI, and one goes to a hard line that I assume goes all the way to the tank.

tbi cel codes are a little hard to diagnose, or they are for me sometimes. My last encounter was a cel that led me to the dist but that did not help. It was really intermittent. It ended up being a cracked hose on the egr valve that I didn't find till I replaced the manifold gaskets for a oil leak at the back of the intake on my 91 burb. If it gets hard to find it is probably cheaper to just replace the hoses before you spend money on stuff you won't need. The hoses will need replacement anyway if you start troubleshooting with a mityvac and they need replacement every ten years or so anyway. The cracked hose probably led to bad timing or the egr not working and after replacing the hoses the cel never happened again.When I replaced the dist that cost me two before I got one that moisture would not screw up and stopped my habit of buying lifetime stuff at autozone at least the electric stuff. especially injectors. I have my ramjet canister just venting to atmosphere and I don't tighten the gas cap down all the way to make sure it doesn't get pulled down in a vacuum condition. I do worry about the crummy gas getting moisture in it though. Anytime I fill up I expect some fuel to maybe puke back if I stick the nozzle all the way into the filler at first.

I noticed! The vacuum lines are certainly old. May need to look. I also get a spark control fail when I get on the throttle. Not sure if it could be vacuum or egr related.

Not Chevy TBI but years ago I had a ‘96 Ford Ranger with a 4.0L V6. Had some evap issues. After fiddling with all kinds of things I bought a new charcoal cannister. It was fairly light.

Old one looked fine from the exterior but was much heavier and felt dense. Definitely most of the issue.

I might pull the lines and see if the filter gave up the ghost. Supposedly the charcoal can break down.

 

[AJMBLAZER]

Depending on the setup of the evap system and how many previous owners liked to force in that last 27 cents worth of gas every fill up they can get liquid gas in them. The charcoal breaks down but ends up as a denser mass inside the cannister that doesn’t filter...or even let the fumes through like mine did.