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Old 09-11-2007, 03:31 PM   #1 (permalink)
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Excessive Fuel Tank Pressure

When I remove my fuel cap, there seems to be an excessive amount of pressure bleeding off. I really did not think too much about it, until I noticed that I now have a fuel leak at the front of the tank. I have not crawled under the car yet to determine where the leak is but will do so as soon as I can.

My question is what causes excessive fuel tank pressure? I assume that there might be a vent tube or a vented cap that is not allowing pressure to escape.
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Old 09-11-2007, 07:55 PM   #2 (permalink)
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Re: Excessive Fuel Tank Pressure

Vented cap usually. At least the '64 has a vented cap. Even with it sometimes when it's hot out it will release a little pressure when removing the cap but not much.

Deb
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Old 09-11-2007, 08:16 PM   #3 (permalink)
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Re: Excessive Fuel Tank Pressure

Heat causes that.... But, is it pressure or Vacuum?

If it's vacuum I'd replace my cap. The fuel pump will draw about 10 lbs of vacuum on your tank if the cap is not properly venting...

FE
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Old 09-11-2007, 08:36 PM   #4 (permalink)
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Re: Excessive Fuel Tank Pressure

My 64 has a vent tube on the drivers side of the tank.
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Old 09-11-2007, 11:17 PM   #5 (permalink)
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Re: Excessive Fuel Tank Pressure

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Originally Posted by FEandGoingBroke View Post
Heat causes that.... But, is it pressure or Vacuum?

If it's vacuum I'd replace my cap. The fuel pump will draw about 10 lbs of vacuum on your tank if the cap is not properly venting...

FE
It seems like pressure. It will do this even after sitting for many days without being driven. I will be checking it again tomorrow.

Should I change the cap or does my 69 have a vent tube that should be checked? What harm does this cause if any?
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Old 09-12-2007, 04:17 PM   #6 (permalink)
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Re: Excessive Fuel Tank Pressure

My 64 has a new vented cap and it still builds pressure,the lower the fuel gets it seems to build more pressure, which is prob. from the fuel sloshing around more.
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Old 09-12-2007, 09:26 PM   #7 (permalink)
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Re: Excessive Fuel Tank Pressure

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Originally Posted by R428Galaxie View Post
My 64 has a vent tube on the drivers side of the tank.
Mine doesn't, wonder what the difference is? I have a drain thing in one bottom corner but that's it. No probs with pressure build up though. It might be the original cap on there even. I want to get a locking one some time though.

As for the cap, maybe it's not the correct cap?

Deb
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Old 09-13-2007, 01:53 PM   #8 (permalink)
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Re: Excessive Fuel Tank Pressure

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Originally Posted by ShotRod64 View Post
Mine doesn't, wonder what the difference is? I have a drain thing in one bottom corner but that's it. No probs with pressure build up though. It might be the original cap on there even. I want to get a locking one some time though.

As for the cap, maybe it's not the correct cap?

Deb
After sitting for a full day without being driven, I checked for pressure/vacuum again. Not sure how to determine which it is, I did release the cap slowly and did hear that 'whishing' sound.

My car has a locking gas cap. The cap came with the car and so I do not know if it has the proper or any venting properties.

How can I tell if its pressure releasing or vacuum draw? Either way is it normal or should I be concerned.
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Old 09-13-2007, 03:57 PM   #9 (permalink)
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Re: Excessive Fuel Tank Pressure

Well, if you get the fumes shooting out when you release the cap it's pressure.

Also, if you let the car sit for a day or two then get the wiiisssh noise, it's very likely pressure. The fuel tank will not creat it's own vacuum just sitting there....

FE
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Old 09-13-2007, 04:31 PM   #10 (permalink)
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Re: Excessive Fuel Tank Pressure

I probably did not perform the 'testing' correctly. I did drive the car between the gas cap checks although a day had passed when I performed the second check.

I will open the cap again today as a baseline and then open it the next without running the car. Hopefully, I can determine pressure vs. vacuum.

Assuming it is pressure, is this normal or should this pressure be venting?
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Old 09-14-2007, 05:51 PM   #11 (permalink)
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Re: Excessive Fuel Tank Pressure

The late 1969's and 1970's often had pressure problems. This was caused by the emissions hardware installed at the fuel tank. They were supposed to have a non-vented cap. Off to the drivers side there ought to be some plastic parts and a 3/8 inch diameter vent line going under the body. Something plugged this up some where along the line. The fuel expansion during driving and hot weather is normal. The back pressure at cap twisting is not.

Wm.
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Old 09-17-2007, 01:19 PM   #12 (permalink)
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Re: Excessive Fuel Tank Pressure

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Originally Posted by coosbaylumber View Post
The late 1969's and 1970's often had pressure problems. This was caused by the emissions hardware installed at the fuel tank. They were supposed to have a non-vented cap. Off to the drivers side there ought to be some plastic parts and a 3/8 inch diameter vent line going under the body. Something plugged this up some where along the line. The fuel expansion during driving and hot weather is normal. The back pressure at cap twisting is not.

Wm.
Thanks coosbay. I will be under the car again tonight, so I will look for this vent line.

Does the line run all the way to the engine bay? If so, is it connected to emissions equipment such as an evap canister?
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Old 09-25-2007, 02:20 PM   #13 (permalink)
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Re: Excessive Fuel Tank Pressure

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Originally Posted by saleen428 View Post
Thanks coosbay. I will be under the car again tonight, so I will look for this vent line.

Does the line run all the way to the engine bay? If so, is it connected to emissions equipment such as an evap canister?
I got under the back of the car today. I found where my fuel leak is coming from. It is the 4 inch rubber hose that bridges the sending unit to the hard fuel line.

I did confirm that it is pressure and not vacuum that I am hearing. I tested this correctly today by relieving the pressure and then putting the cap back on and checking it again in an hour. When I removed the cap pressure was released. There was a good change in the outside temp during that hour.

What I did not find was a vent tube.
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