Gas cap problem!!

[Jaysonb]

I just got done finishing the paint and putting together the cab and front end of my 71 C20. One of the many new parts I got was a nice new gas cap, I got it from GMC Pauls when I ordered the rubber kit from him. I was putting the new interior in it today and walked around the back of the truck for something, and the back of the cab below the rear glass was totally bowed out, I mean enough to leave crease marks. It took me a few minutes to figure out that the gas tank pushed out the back of the cab!!! I still have the box that the cap came in, and it says vented on the box, but must not be. Anyone heard of such a thing? I checked out the tank real good, and it looks ok, and isn't leaking, but man was there allot of pressure in there. The good news is that the creases on the back of the cab wont be noticeable with the box on.

[68gmsee]

Can you post a pic? That sounds either impossible or extremely dangerous.

You can check to see if the vent is free to move in and out with a small object that will fit in the vent hole. It should open at a certain psi but don't remember what it is.

[jondancer]

that would take alot of pressure. please post pics

[Jaysonb]

I spent a good while with a rubber mallet trying to pop everything back in place, so it was tough to get a good picture. About half way up the inset sections, it creased right next to every vertical section. It was tough to get a picture of the creases, but I think I got a good enough picture of one to show you what I mean. New gas cap next to old, the old one is obviously on the truck now.

[cdowns]

71 C20 came from factory with non vented gascap// venting was by the charcoal cannister into carb// the original gas tank had a hose that ran up into a high loop in the passenger side door post area then hose went foward to core support under the battery location to charcoal cannister

in always vented 71-72 tanks by takin that hose to charcoal cannister off and running on the frame rail back in back of the cab

[GASoline71]

Originally Posted by cdowns

71 in always vented 71-72 tanks by takin that hose to charcoal cannister off and running on the frame rail back in back of the cab

Same here...

Gary

[Jaysonb]

I'm sure that the truck was all stock when I got it. There was no sign of any vent line on the tank, an there was no charcoal canister? Here is a pic of the engine bay when I got it.

[cdowns]

take your hand to the rear passenger side door post , reach up around back to find where the high loop for the vapor cannister was// the gas tank should have an outlet high up toward the passenger side

they all came that way in 71 and 72

[Jaysonb]

There is only one outlet on the fuel tank. I just put peal-n-seal up the door posts, and found nothing like that. Does the new gas cap in the picture look correct?

[cdowns]

no the factory correct gas cap is non vented and the factory stock is as i described

look in GM factory service manual and also GM factory assembly manual to see what was in fact factory original on these trucks

[davepl]

So if the 70 tank is unvented, where do expansion fumes go? My 69 cars are vented, so I've never owned any kind of non-vented.

When you say non-vented, do you perhaps mean no vent OTHER than the cap, or completely and truly non-vented? Because the latter would seem to be prone to what the OP had happen.

My cap is below, looks vented to me. On a 40 year old truck, even a non-molested one, there's always a chance it got replaced and possible with the wrong one.

Can I test it somehow? I don't want to stick my mouth on it, I've siphoned enough gas (of my own, not theft) to be done with that.

[71swb4x4]

Originally Posted by davepl

So if the 70 tank is unvented, where do expansion fumes go?

If you have an unvented tank they go out the vented cap.


I believe this is how the 67-72 trucks work, but I am not 100% certain.
67-70 1/2 tons had unvented tanks, they vented through the cap.
70-72 (partial 70 year) the tanks were vented in 1/2 tons and vented through the charcoal canister, they did not have vented caps.
3/4 ton and larger trucks ran vented caps (no charcoal canisters) through 72.

California might change things a little, I am not sure.

When I took the tank out of my 71 the lines were way up high on the top corner of the passenger side of the tank, if I remember right. I never knew they were there until I pulled the seat out and started pulling the tank out.

[SS Tim]

Here are the service manual evap line drawings of the 70 and the 71-72 systems for pickups and chassis cabs. Just another example of production evolution in these trucks.

[PowerdbyChevy79]

Originally Posted by 71swb4x4

If you have an unvented tank they go out the vented cap.


I believe this is how the 67-72 trucks work, but I am not 100% certain.
67-70 1/2 tons had unvented tanks, they vented through the cap.
70-72 (partial 70 year) the tanks were vented in 1/2 tons and vented through the charcoal canister, they did not have vented caps.
3/4 ton and larger trucks ran vented caps (no charcoal canisters) through 72.

California might change things a little, I am not sure.

When I took the tank out of my 71 the lines were way up high on the top corner of the passenger side of the tank, if I remember right. I never knew they were there until I pulled the seat out and started pulling the tank out.

yup what he said..

[davepl]

Mine is indeed a 3/4 ton.

[FirstOwner69]

As I understand it...
The function of a vented cap on the trucks before 71 was to let air into the tank to replace fuel consumed and prevent a vacuum and subsequent fuel starvation to the engine. It was not designed to let air out of the tank. If it did, fuel would slosh out of the tank on turns when full. Think of it as a check valve of sorts. That said, I don't understand how enough pressure could have built up to cause your tank to expand enough to cause the damage you describe. However, the 71/72 design should allow for pressure build-up and vacuum relief even when using a non-vented cap.

[Mr Chevorlet]

Guys, gas expands in the heat. Leave a plastic gas can in the sun it will expand like a balloon. I bought what was suppose to be a vented gas cap along with several others that was suppose to be vented both ways. (Not) when I dellevered my truck to the axle shop and went to pick it up later it was setting outside in the sun it started just fine but a half a mile down the road it started running real rough. I stuck my arm out the window and loosend the gas cap. And guess what, pressure came out big time. It did not help the truck run any better but I made it home. I took the gas line off at the carb. and drained all the excess fuel out of the line and there was a lot. Then I restarted the truck and it ran fine. So I took my gas cap and modified the spring in the check valve. To let pressure excape and have driven it several more times in the heat of the day with no further problems. You will not be able to buy a true vented gas cap any more., you will have modify them. I have bought several of them from several different places and not one of them would vent pressure. They might keep the tank from creating vacuum but not let pressure out. By the way the pressure building in my tank was pushing fuel past my fuel pump to the carb and flooding the carb..

[70jnktrk]

'So I took my gas cap and modified the spring in the check valve. To let pressure excape and have driven it several more times in the heat of the day with no further problems. You will not be able to buy a true vented gas cap any more., you will have modify them. I have bought several of them from several different places and not one of them would vent pressure. They might keep the tank from creating vacuum but not let pressure out.'


my truck does the same thing...runs like a top but at times there is pressure buildup in the tank..how do you get the gas cap apart to modify the check valve spring?..thx

[FirstOwner69]

To clarify what I said in post 16. My original gas cap, as well as another original I bought in the 70s, only let air into the tank... not out. Yes, sometimes, as expected, there is pressure released when I take the cap off. I have never detected any runability problems because of that. Sure, gas, or more properly the vapor above the gas, expands significantly as it heats up. However, I've never heard of that occurring to the degree noted by the O.P.

[Mr Chevorlet]

This is what one description said on one website for there gas cap.

Manufactured with OEM quality materials and safety standards. Manufactured to pass all state and federal emissions requirements. Specifically designed pressure and vacuum relief valves to prevent evaporation of fuel. A leaking fuel cap can allow up to 30 gallons of gasoline to evaporate over the period of a year.

And after taking one apart, the check valve is made to open when pressure gets to high and supposed to vent behind the rubber on the the cap. It is a normally closed check valve.I did not see how to keep the vacuum from being created. Then the cap has indentions behind the brass and rubber for the pressure to release.

To modify the cap, the little taps that hold the check valve together, bend the tabs back and remove the plastic housing and you can cut the spring for lower pressure relief then put it back together and bend the taps over. Just a suggestion do it at your own risk.

[70jnktrk]

Originally Posted by Mr Chevorlet

This is what one description said on one website for there gas cap.

Manufactured with OEM quality materials and safety standards. Manufactured to pass all state and federal emissions requirements. Specifically designed pressure and vacuum relief valves to prevent evaporation of fuel. A leaking fuel cap can allow up to 30 gallons of gasoline to evaporate over the period of a year.

And after taking one apart, the check valve is made to open when pressure gets to high and supposed to vent behind the rubber on the the cap. It is a normally closed check valve.I did not see how to keep the vacuum from being created. Then the cap has indentions behind the brass and rubber for the pressure to release.

To modify the cap, the little taps that hold the check valve together, bend the tabs back and remove the plastic housing and you can cut the spring for lower pressure relief then put it back together and bend the taps over. Just a suggestion do it at your own risk.

got it..thanks...it all makes sense ...my truck is a 70 with no emissions [vent]stuff involved...i'll take a closer look at my 'vented' cap...

[FirstOwner69]

A pressure relief valve feature does make sense. I still learn something every day.

[hotrodder]

some as*&%# stole the cap from my 67, I went to local o'reillys and got a supposed replacement, same thing as described took the cap off to get fuel the next day, and when I loosened it , darn near blew it out of my hand. found a used one at a friends house. no problem now.

[EARNHARDT#3]

The $5 one I got from classic parts vents properly. Is also allows fuel to come out when the fuel sloshes. I'd care if I had a fancy paint job.

[GSFMECH]

In post 5 cdowns describes the way they actually vent on 71-72, the vent line goes to the charcoal canister and then the canister has a vacuum line to the carb. Engine vacuum vents the tank, this was for emissions, the Feds wanted to stop the fumes from venting to the atmosphere. I had removed the canister on my truck and in hot weather mine was doing the same thing as described but only in hot weather, pressure would build up in the tank and the engine would die occasionally, especially after you shut the engine off for a few minutes when hot. When I would get back in and startup and take off down the road it would get out there a few hundred yards and die. You take the cap off and it would have pressure built up big time. My 72 also has the return line from the fuel pump to the tank which circulates fuel back to the tank (71 does not have this) so when it hot soaked the lines vapor locked, and you could not see gas in the filter. Believe it or not I found my engine was running too hot, even though the gauge (factory gauge) showed it to be OK (it was standing straight up) it was too hot and was really puzzling because it was not boiling over or anything. The radiator was looking cruddy inside and when I replaced the radiator the problem went away. Now the gauge is only reading about ¼ on the scale, who knows how hot it actually was I’ve never temporarily substituted the gauge for one that has an actual temperature reading to see what it actually ran. Anyway as cdowns stated I did the same, I took the line that originally went to the canister and have left it vented underneath the truck. Between the radiator and venting it under the truck I’ve had no more problems after that no matter how hot it is outside.