Vapor canister leaking gas

[AuroraGirl]

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.

definitely would throw a new cap on while in there

 

[Rusty Nail]

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.

This is not a valid test at all.
I think Charlie has ignored me but I dont want him or his truck to catch fire, somebody help.


Not only were time of day but ambient conditions were different, AND you introduced a number of OTHER variables that modify the original configuration, therefor invalidating the "test".

Raw fuel CANNOT overfill up the return line. Physically impossible and that is the wrong. tree. to be barking at.
(Air pressure+gravity)
Pesky physics.

More research is necessary.
There is no such thing as a non vented gas cap for a Chevrolet truck.

Dont listen to that guy, he has no. clue. what he is talking about and following his advice could KILL YOU.

"took the path of least resistance"?
That is *******. RETARDED.

Dude, the weight of the entire atmosphere offers mighty resistance.
Ask a science teacher OR an auto mechanic.

PUBLIC IGNORE!

Wrong / bad / dangerous repair advice.

 

[AuroraGirl]

This is not a valid test at all.
I think Charlie has ignored me but I dont want him or his truck to catch fire, somebody help.


Not only were time of day but ambient conditions were different, AND you introduced a number of OTHER variables that modify the original configuration, therefor invalidating the "test".

Raw fuel CANNOT overfill up the return line. Physically impossible and that is the wrong. tree. to be barking at.
(Air pressure+gravity)
Pesky physics.

More research is necessary.
There is no such thing as a non vented gas cap for a Chevrolet truck.

Dont listen to that guy, he has no. clue. what he is talking about and following his advice could KILL YOU.

"took the path of least resistance"?
That is *******. RETARDED.

Dude, the weight of the entire atmosphere offers mighty resistance.
Ask a science teacher OR an auto mechanic.

PUBLIC IGNORE!

Wrong / bad / dangerous repair advice.

So what you are saying is a gas cap can fit 73-87 and would cause no pressure issues inside the tank if it didnt have the third line in the sending unit?

 

[AuroraGirl]

This is not a valid test at all.
I think Charlie has ignored me but I dont want him or his truck to catch fire, somebody help.


Not only were time of day but ambient conditions were different, AND you introduced a number of OTHER variables that modify the original configuration, therefor invalidating the "test".

Raw fuel CANNOT overfill up the return line. Physically impossible and that is the wrong. tree. to be barking at.
(Air pressure+gravity)
Pesky physics.

More research is necessary.
There is no such thing as a non vented gas cap for a Chevrolet truck.

Dont listen to that guy, he has no. clue. what he is talking about and following his advice could KILL YOU.

"took the path of least resistance"?
That is *******. RETARDED.

Dude, the weight of the entire atmosphere offers mighty resistance.
Ask a science teacher OR an auto mechanic.

PUBLIC IGNORE!

Wrong / bad / dangerous repair advice.

https://www.gmsquarebody.com/thread...n-all-emissions-components-are-deleted.27060/
my forum thread from september about the subject, i had responses. I just feel like we are misunderstanding eachother, generally.

 

[Rusty Nail]

I'm sure you're probably right, it shouldn't matter at all that I was talking to Charlie even though this is his thread and I addressed him by name.

I misunderstood, you're right.

 

[Charlie]

Update

After several fill ups with no more leaking, installed new canister and new cap. No more leaking. Maybe it is good even after several fill ups since new parts installed.

 

[75gmck25]

Did the new canister look just like the old one? My ‘75 GMC 350 had the canister purge connected to a dedicated port on the carburetor, and there is no separate purge valve. This is different from newer designs that use a separate purge valve, usually controlled by engine vacuum.

I have the 74/75 shop manual, and it says the ‘75 Quadrajet carburetor purges the canister automatically, but it provides no further explanation. I found references that suggested connecting the purge line to the PCV hose when I swapped carburetors. It works, but I’m not sure it’s the right solution.

Bruce

 

[Charlie]

Did the new canister look just like the old one? My ‘75 GMC 350 had the canister purge connected to a dedicated port on the carburetor, and there is no separate purge valve. This is different from newer designs that use a separate purge valve, usually controlled by engine vacuum.

I have the 74/75 shop manual, and it says the ‘75 Quadrajet carburetor purges the canister automatically, but it provides no further explanation. I found references that suggested connecting the purge line to the PCV hose when I swapped carburetors. It works, but I’m not sure it’s the right solution.

Bruce

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

This is the one I got. About the same size, has purge line to carb. with ported vaccuum, and one line to tank. I don't think it needs to be manifold vaccuum.

https://www.ebay.com/itm/Fuel-Tank-...e=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649

This is the cap I got. Fits perfect.

 

[Bextreme04]

This is not a valid test at all.
I think Charlie has ignored me but I dont want him or his truck to catch fire, somebody help.


Not only were time of day but ambient conditions were different, AND you introduced a number of OTHER variables that modify the original configuration, therefor invalidating the "test".

Raw fuel CANNOT overfill up the return line. Physically impossible and that is the wrong. tree. to be barking at.
(Air pressure+gravity)
Pesky physics.

More research is necessary.
There is no such thing as a non vented gas cap for a Chevrolet truck.

Dont listen to that guy, he has no. clue. what he is talking about and following his advice could KILL YOU.

"took the path of least resistance"?
That is *******. RETARDED.

Dude, the weight of the entire atmosphere offers mighty resistance.
Ask a science teacher OR an auto mechanic.

PUBLIC IGNORE!

Wrong / bad / dangerous repair advice.

I could be wrong here, but while I agree with you that generally all gas caps are vented, they are usually vented to allow air pressure to vent INTO the tank. The older pre-smog and heavy duty trucks that do not have evap canisters(like my 80' K25) have a fuel tank pressure vent on top of the sending unit that just goes to the same style one way pressure relief valve that you find on diffs and t-cases. On emissions controlled vehicles, that goes to a vapor canister and has the one way valve on the inside of the sending unit rather than the outside.

The reason for the gas cap to allow pressure to vent INTO the tank, is to allow the fuel pump to suck gas out without building up negative pressure in the tank(you MUST have this kind of vent or the pump will eventually stop pumping) I have had bad cap vents restrict the fuel pump and cause the engine to starve for fuel at highway speed.

The vent on the sending unit keeps vapor that could build up inside the tank when the fuel heats up from pressurizing the tank. There are various systems between the years, but generally there is at least a one-way check valve that keeps liquid from going out, and on my non-emissions 4x4, a one-way keeping liquid from going in.

I suppose in my case the vent on the tank allows vapor to equalize in both directions without liquid going in either direction, so a vented cap is not necessary at all. I would imagine that it wouldn't take much for that silly plastic one way cap to get stuck closed and block off the way for the tank to negative vent though, so maybe that's why the caps are vented.

 

[Rusty Nail]

Charlie gots all the good threads.