Vapor canister leaking gas

Filled up after work today. Started off from station, ran a little rough but settled out quick. Got home and checked under hood and canister dripping gas. Not sure why. Used the same station and pump I have used before. Searched net and can't find any new ones or an updated replacement. Have had truck 5 years but not this before.


What to do now?

https://www.summitracing.com/parts/smp-cp1022/year/1974/make/chevrolet


you need to figure out why yours failed. either its not purging bad purge line? or getting raw fuel from the tank severe overfill? hooked up wrong?

C10MixMaster said:

https://www.summitracing.com/parts/smp-cp1022/year/1974/make/chevrolet


you need to figure out why yours failed. either its not purging bad purge line? or getting raw fuel from the tank severe overfill? hooked up wrong?

Click to expand...

It is hooked up correctly and has been since I got it in 2015. Hoses have been replaced since 2015. I suppose the pump that I have used before somehow filled more this time for some reason. I did see that canister at Summit but does not look like the one I have. There is one there that says AIR. Is that just to vent the canister? Cause I don't know. The cap does not look to be vented either. May not need to be since it has the canister. This is new to me, never had this before.

Selector valve failed on mine and pumped fuel from drivers side tank to the passenger side and when it was full into the canister and then into the carb vent. Barely ran trying to flood. Got it home and opened the passenger side cap and gas poured out.
Either you really over filled the tank or something more dangerous like mine.

bedwards said:

Selector valve failed on mine and pumped fuel from drivers side tank to the passenger side and when it was full into the canister and then into the carb vent. Barely ran trying to flood. Got it home and opened the passenger side cap and gas poured out.
Either you really over filled the tank or something more dangerous like mine.

Click to expand...

I only have one tank on passenger side. Could have over filled, not sure. I removed cap and did hear air puff. Is the canister scrap now?

Possibly but only from the perspective of the canister got contaminated with fuel. If it was me, I’d try to air it out. If you can dry it out without it spitting out any of the charcoal, I’d put it back on and try, and it may very well work fine. I’m cheap, though.

First, I’d see if the canister is plugged. You can check that by simply sucking air in the direction that vacuum would suck on the canister. It should be free of obstruction. You can also pump up the vacuum control valve with a Mityvac to see if it holds vacuum and to see if it’s blows through clear when the valve is opened. I’m assuming it’s the one that sits on the canister and has a big RP written on it. If there isn’t one, ignore that step, and just focus on the canister. Finally, there’s a little cage or an igloo looking thing (depending on who made the sender and possibly how old it is) with a float ball on the underside of the sending unit. That’s supposed to prevent liquid gas from being drawn from the tank, but that cage isn’t completely debris proof. If enough trash got in it to stop the checkball from seating, then it could suck liquid gasoline through, especially after a fill up and even more easily after an overfill.

You’ve also got a canister filter and the cap, and while I’m leaning towards fuel getting by the sender, I’d check the easy stuff first. And hell, it might just be an old sender, and you have to be careful not to put too much gas in the tank from now on. If you kept running it, and the leaking subsides and altogether stops, I think you have your answer.

If canister and subsequently fuel tank vent is not working correctly, that will usually make the gas pump not work right.. such as it turning the pump off numerous times and otherwise not cooperating nicely. but i have never heard of it making it overfill. Id lean towards this checkball.

Charlie some charcoal canisters have replaceable filters in the bottom of them.
They are two dollars at O'Reilly's.
Some of you may recall i've said this before - it's a real thing.


Remove it with 2x 10mm bolts and look at the bottom of it.
To the best of my knowledge, if yours does not have a serviceable filter, chances are it is full.
I do NOT conceive that some other malfunction would cause a backup of LIQUID FUEL underhood.
That is NOT GOOD.
Proceed with EXTREME CAUTION and keep an extinguisher handy while messing with it until fixed.

The gas cap IS vented via a spring inside the cap, which wears out. The gas cap IS a spring-loaded service part with a lifespan and replacement interval, they are NOT DESIGNED to last FIFTY YEARS.
You can't see the vent, it's underneath the cap so rain doesnt get in your gas tank.
Makes sense?

BUT they cost seven bucks.

I suggest to look into those possibilities and be prepared to toss ten dollars worth of serviceable parts at it, because once every half a decade can't hurt.
Probably got your monies' worth.

Liquid fuel back up is very bad juju that you DO NOT want to play with.
Try taking the gas cap off, the vent spring has probably failed and that is a potentially deadly situation.

People very often disregard these pieces of the fuel system yet they ARE vital and their functions are often unknown to the end user.

It will NOT back up into the line.
No way - there are check valves.
Supposed to be...

It isnt draining.

Proceed with caution.
Safety first kids!
Good luck!

i see you are in texas. gas is cheap, and we tend to get that little extra on a fillup while prices are down. add 1. overfilled tank. 2. non vented cap. 3. hot day! and you have a liquid that expanded and took the path of least resistance, out the same hose as the engineers designed it to go-----as a vapor. set the canister upside down out in the sun for a few days. replace cap with correct one.

Rusty Nail said:

Charlie some charcoal canisters have replaceable filters in the bottom of them.
They are two dollars at O'Reilly's.
Some of you may recall i've said this before - it's a real thing.


Remove it with 2x 10mm bolts and look at the bottom of it.
To the best of my knowledge, if yours does not have a serviceable filter, chances are it is full.
I do NOT conceive that some other malfunction would cause a backup of LIQUID FUEL underhood.
That is NOT GOOD.
Proceed with EXTREME CAUTION and keep an extinguisher handy while messing with it until fixed.

The gas cap IS vented via a spring inside the cap, which wears out. The gas cap IS a spring-loaded service part with a lifespan and replacement interval, they are NOT DESIGNED to last FIFTY YEARS.
You can't see the vent, it's underneath the cap so rain doesnt get in your gas tank.
Makes sense?

BUT they cost seven bucks.

I suggest to look into those possibilities and be prepared to toss ten dollars worth of serviceable parts at it, because once every half a decade can't hurt.
Probably got your monies' worth.

Liquid fuel back up is very bad juju that you DO NOT want to play with.
Try taking the gas cap off, the vent spring has probably failed and that is a potentially deadly situation.

People very often disregard these pieces of the fuel system yet they ARE vital and their functions are often unknown to the end user.

It will NOT back up into the line.
No way - there are check valves.
Supposed to be...

It isnt draining.

Proceed with caution.
Safety first kids!
Good luck!

Click to expand...

can you link a vented cap

Yes Ma'am!

Every one you find for sale will be vented.
Gas tanks in cars and trucks are NOT sealed because altitude + air pressure and temperature..

You will find STANT brand gas and radiator caps on my vehicles because m-r-d gooduns


My '77:
https://y.com/itm/Stant-Fuel-Tank-Cap-for-1975-1978-Chevrolet-C20-Gas-Delivery-Storage-br/383405446898?fits=Year:1977|Model:C20&hash=item5944bed6f2:g:NwYAAOSwq1NeOOyi

Stant utilizes a dozen environmental chambers to recreate extreme conditions of temperature, humidity, salt spray, vibration, dust, and fuel exposure to ensure all Stant products meet the stringent requirements of the automotive marketplace. You can rely on Stant for defect-free, highly engineered products at competitive prices



My '88:

https://www.ebay.com/itm/OEM-Type-CHEVROLET-Gas-Cap-For-Fuel-Tank-Stant-10817/401210140792?fits=Year:1988|Model:Blazer|Submodel:Silverado|Engine+-+Liter_Display:5.7L|Make:Chevrolet&epid=169075276&hash=item5d69fce878:g:0TQAAOSw4shYBqNa

I would also use a GATES brand if no immediate option was available.

Hope this helps!

Not exactly 7 bucks but whatever.
Wuhan !


I was forced to learn about that stuff building a race car. Thought i'd be a speed demon , get the weight off that tire, , taking the charcoal canister out to spare the 2 pounds - trying to get the tire to yank...
(weight reduction gone bad)
Which resulted in ALL KINDS OF DRAMA, including an underhood fire at speed.

If you ever hear your gas tank creaking or popping, thats a good time to buy a gas cap.

Know when you open the cap? You kind of gotta mush down on it a little bit to get it to turn sometimes?
Because it is a spring-loaded vent - the cap IS the vent.
Gots a spring in it.
Open the old one and look for yourself.

"How do you open it?"
-smack it with a hammer.

Not Unlike before @AuroraGirl , the previous aftermarket vent detailed to you in your thread, resulted from your modification to the OE fuel system but the result is the same.

Rusty Nail said:

Yes Ma'am!

Every one you find for sale will be vented.
Gas tanks in cars and trucks are NOT sealed because altitude + air pressure and temperature..

You will find STANT brand gas and radiator caps on my vehicles because m-r-d gooduns


My '77:
https://y.com/itm/Stant-Fuel-Tank-Cap-for-1975-1978-Chevrolet-C20-Gas-Delivery-Storage-br/383405446898?fits=Year:1977|Model:C20&hash=item5944bed6f2:g:NwYAAOSwq1NeOOyi

Stant utilizes a dozen environmental chambers to recreate extreme conditions of temperature, humidity, salt spray, vibration, dust, and fuel exposure to ensure all Stant products meet the stringent requirements of the automotive marketplace. You can rely on Stant for defect-free, highly engineered products at competitive prices



My '88:

https://www.ebay.com/itm/OEM-Type-CHEVROLET-Gas-Cap-For-Fuel-Tank-Stant-10817/401210140792?fits=Year:1988|Model:Blazer|Submodel:Silverado|Engine+-+Liter_Display:5.7L|Make:Chevrolet&epid=169075276&hash=item5d69fce878:g:0TQAAOSw4shYBqNa

I would also use a GATES brand if no immediate option was available.

Hope this helps!

Not exactly 7 bucks but whatever.
Wuhan !


I was forced to learn about that stuff building a race car. Thought i'd be a speed demon , get the weight off that tire, , taking the charcoal canister out to spare the 2 pounds - trying to get the tire to yank...
(weight reduction gone bad)
Which resulted in ALL KINDS OF DRAMA, including an underhood fire at speed.

If you ever hear your gas tank creaking or popping, thats a good time to buy a gas cap.

Know when you open the cap? You kind of gotta mush down on it a little bit to get it to turn sometimes?
Because it is a spring-loaded vent - the cap IS the vent.
Gots a spring in it.
Open the old one and look for yourself.

"How do you open it?"
-smack it with a hammer.

Not Unlike before @AuroraGirl , the previous aftermarket vent detailed to you in your thread, resulted from your modification to the OE fuel system but the result is the same.

Click to expand...

Yeah, this is still confusing me. If the truck has an evap canister isnt that controlling the fumes? The cap cant vent to the atmosphere in that case?

Canister from the engine (fumes ere are really extra, unburnt fuel and that amount constantly changes via throttle input)
Cap from the tank ( storage fumes)

Canister should be taking vent fumes and sending them to the engine when optimal temp or rpm so they can be combusted and NOx removed in the cats, no? IF that caps does that,its venting gas fumes to the atmosphere.

Thanks for all the input. After thinking more, I decided to test to see if problem was cap or other things. Truck sat for 24 hrs. Disconnected hoses from canister, plugged carb line, loosened cap and drove for about 40 miles. Let truck sit for 30-45 min., reconnected hoses, cap and drove around for about 5 miles or so. No leaking. Drove to work this morn.(15 miles) and no leaking. Just about assures it was an overfill of tank. I have ordered a new canister from Summit to be put on in a week or two when positive old one doesn't leak anymore. The one that is on there now is the OE so probably needs to be replaced.