Gas Vapor Pressure ?

[Gunny87]

After reading this thread Charcoal Cannister: Needed? I'm wondering if I have a bad canister purge valve on my car cause on hot days (like today) the pressure in the fuel tank will build up to the point that it will force fuel back out the filler when I open the cap, there's a ton of pressure in the tank so obviously somethings not working.
In the aforementioned thread the diagram supplied by jrichker shows (as near as I can tell) that the pressure from gas vapor is vented through the charcoal canister via the purge valve or is there something else I'm not seeing? all I know is this is not good and I've got to get it fixed
Need advice please.
the car is an 87 GT

 

[General karthief]

How much of your purge system is present?

 

[Gunny87]

it's all there and all original

 

[General karthief]

Make sure your vac line from the front of the manifold is in good shape to the purge valve and there is a couple short rubber vac lines at the charcoal canister, they get soft over time..

 

[Gunny87]

I'll take a look at all the lines tomorrow
so if one of the lines is leaking I'm guessing that would cause the system to not vent properly?
and are they anything special or just a vacuum line that i can get from the parts store?

 

[General karthief]

It will say gas or vapor on the line.

 

[JD1964]

I'll take a look at all the lines tomorrow
so if one of the lines is leaking I'm guessing that would cause the system to not vent properly?
and are they anything special or just a vacuum line that i can get from the parts store?

Sounds to me like something is clogged or plugged. Wouldn't a leaking line simply allow pressure to vent to atmosphere?
Must be an obstruction somewhere.

Until you solve it you might want to leave the gas cap on loose so the pressure will gradually bleed off rather than build up.

 

[a91what]

If I'm not mistaken the purge valve goes between the canister and manifold. The tank should purge into the canister even if the purge valve does not open. I believe you have a blockage in the line.

 

[billison]

Have you pulled your codes?

 

[Gunny87]

so I gave it all a good once over and all components are hooked up and look to be in albeit grimy but decant condition
on a side note I didn't have a chance to look at it until after dark so I used a supper bright LED flashlight my son gave me for father's day and it work fantastic, I saw things I've never seen before, for instance my motor mounts are in bad shape, the valve covers are leaking bad at the back and so is the intake manifold, that little flashlight is amazing, but I digress.... so what I've noticed when the issue is happening there's no fuel smell in the engine compartment just at the back by the filler cap but I can hear a hissing sound coming from under the car on the passenger side and the first time I did this it had less than a half a tank and it still spewed gas out the cap when I loosened it so a lot of pressure. is there another check valve on the tank were the vent tub attaches?
No I haven't pull codes but I can and will, didn't think of that cause I never would have thought the ECU was that smart back in the late 80's? that will have to be tomorrow, any particular codes I should look for.

 

[imp]

so I gave it all a good once over and all components are hooked up and look to be in albeit grimy but decant condition
on a side note I didn't have a chance to look at it until after dark so I used a supper bright LED flashlight my son gave me for father's day and it work fantastic, I saw things I've never seen before, for instance my motor mounts are in bad shape, the valve covers are leaking bad at the back and so is the intake manifold, that little flashlight is amazing, but I digress.... so what I've noticed when the issue is happening there's no fuel smell in the engine compartment just at the back by the filler cap but I can hear a hissing sound coming from under the car on the passenger side and the first time I did this it had less than a half a tank and it still spewed gas out the cap when I loosened it so a lot of pressure. is there another check valve on the tank were the vent tub attaches?
No I haven't pull codes but I can and will, didn't think of that cause I never would have thought the ECU was that smart back in the late 80's? that will have to be tomorrow, any particular codes I should look for.

The ECU was NOT that smart back then. Too late tonight for me to go ahead, but will say "hardly possible" to comments mentioning a "blockage". Definitely one of those vapor pressure concerns which bug me! imp

 

[General karthief]

That little shiney box was actually pretty smart for its time.
Your should get a vacuum gauge, they are inexpensive, a hand operated vacuum pump would be good too, then you can test the vent line from the fuel tank. Does your car still have a factory fuel cap? Does the vacuum line from the front of the intake have vacuum?

 

[Black1987]

so I gave it all a good once over and all components are hooked up and look to be in albeit grimy but decant condition
on a side note I didn't have a chance to look at it until after dark so I used a supper bright LED flashlight my son gave me for father's day and it work fantastic, I saw things I've never seen before, for instance my motor mounts are in bad shape, the valve covers are leaking bad at the back and so is the intake manifold, that little flashlight is amazing, but I digress.... so what I've noticed when the issue is happening there's no fuel smell in the engine compartment just at the back by the filler cap but I can hear a hissing sound coming from under the car on the passenger side and the first time I did this it had less than a half a tank and it still spewed gas out the cap when I loosened it so a lot of pressure. is there another check valve on the tank were the vent tub attaches?
No I haven't pull codes but I can and will, didn't think of that cause I never would have thought the ECU was that smart back in the late 80's? that will have to be tomorrow, any particular codes I should look for.

You could disconnect the vapor line (when the car is not running) from the canister side and take a air hose and blow through the line from the tank to the engine. If its blocked towards the front of the engine, it may blow off the under the passenger quarter window/door were the rubber/plastic hose connects to the hard line from the gas tank.

Mine was pinched and found it this way.

Codes:
31 O,R,C Canister or EGR valve control system
32 O,R,C Canister or EGR valve control system
33 R,C Canister or EGR valve not operating properly

This ECM is very good at troubleshooting if you learn how it works and for its time very simple but effective. Also just know you have codes even if you do not have a hard CEL.

@jrichker

 

[a91what]

The ECU was NOT that smart back then. Too late tonight for me to go ahead, but will say "hardly possible" to comments mentioning a "blockage". Definitely one of those vapor pressure concerns which bug me! imp

A kinked or flattened hardline would count as a "blockage".. really the only option the vent is wide open from the tank thru the line all the way up to the canister.

 

[Gunny87]

So codes
KOEO 34, 81, 95
KOER 91, 34

 

[Dan02gt]

Have you thought about dropping the tank and checking the vapor valve. It could be clogged or damaged.

https://lmr.com/item/LRS-9593A/1979...YMfz2KHxHq3006R0jjGxQ7CxsyVNJQ3waAqjMEALw_wcB

 

[Gunny87]

Have you thought about dropping the tank and checking the vapor valve. It could be clogged or damaged.

https://lmr.com/item/LRS-9593A/1979...YMfz2KHxHq3006R0jjGxQ7CxsyVNJQ3waAqjMEALw_wcB

This makes sense

 

[MFE92]

I'm curious what volume fuel pump you're running.

 

[Gunny87]

I'm curious what volume fuel pump you're running.

I'm guessing just the stock volume, I pulled the tank just over a year ago to replace the leaking filler neck seal along with all the rest of the seals just to be safe and at that time I snapped a pic of the fuel pump which is a Autozone or O'Reilly replacement pump