Darn near blew the gas cap across the lot - Team Camaro Tech
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post #1 of 11 (permalink) Old May 30th, 06, 02:01 PM Thread Starter
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Darn near blew the gas cap across the lot

This weekend with temps of 87 and 94, I went to fill up my camaro and instead of a slight pffttt when I backed the cap half way off, it did more of a PFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFTTTTTTTTTTTTT
with great pressure I felt on my hand. Its a vented cap but I thought it might be defective so I bought a new one and after driving around the next day, it was still holding alot of pressure in the tank. I also have been smelling unburned fuel at idle at stop lights etc, telling me that pressure is forcing fuel into my Qjet and not burning it all.
My vent line from the carb is all hooked up too.
I now orderd a new Stant locking cap #11492 that Advanced Auto said was the number called for in their catalog for my car. I pick it up tomorrow but this will be my 3rd cap now too. Anyone else have any other ideas on how or why this pressure is building up?

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post #2 of 11 (permalink) Old Jun 1st, 06, 06:52 AM Thread Starter
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Re: Darn near blew the gas cap across the lot

bump

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post #3 of 11 (permalink) Old Jun 1st, 06, 07:37 AM
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Re: Darn near blew the gas cap across the lot

Jim - I've gotten the pressure bit from time to time and remember it all the way back to my 1st '69 back in the mid 70's. As for the raw fuel smell I can create that problem by not putting my cap on properly. When I have done that I have been in a hurry and it causes the interior to really stink of raw gas. Going under the hood I found no raw gas smells and started looking around the tank and lines for a leak. For some reason I removed the gas cap and found it wasn't seated properly. Replaced the cap properly and the smell went away really quickly...

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post #4 of 11 (permalink) Old Jun 1st, 06, 09:03 AM
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Re: Darn near blew the gas cap across the lot

Nothing is wrong with your gas cap. The vented cap only allows air in, but no fumes out. A check valve, so to speak.

The exhaust system heat and engine heat driven to the rear by the forward movement is helping to heat up the tank and fuel inside. The cap is keeping the fumes inside the tank like it is suppose to do.

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post #5 of 11 (permalink) Old Jun 1st, 06, 09:35 AM Thread Starter
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Re: Darn near blew the gas cap across the lot

hmmm ok, thought maybe it was excessive. thanks guys.

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post #6 of 11 (permalink) Old Jun 1st, 06, 09:37 AM
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Re: Darn near blew the gas cap across the lot

What is that vented fuel filter supposed to do?
should the pressure relieve thru there?





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post #7 of 11 (permalink) Old Jun 1st, 06, 09:45 AM Thread Starter
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Re: Darn near blew the gas cap across the lot

the fuel filter vent line goes right back into the tank with no openings or 'vents' other than the tank that I know of. Its supposed to return 'unused' fuel vapors to the tank so they dont overload the carb.

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post #8 of 11 (permalink) Old Jun 1st, 06, 09:45 AM
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Re: Darn near blew the gas cap across the lot

Quote:
Originally Posted by Everett#2390
Nothing is wrong with your gas cap. The vented cap only allows air in, but no fumes out. A check valve, so to speak.

The exhaust system heat and engine heat driven to the rear by the forward movement is helping to heat up the tank and fuel inside. The cap is keeping the fumes inside the tank like it is suppose to do.
I have had the "raw gas smell" inside (when driving) as well. I removed my gas cap and noticed that the one way check valve concept may have not been true (at least on the cap I have). My cap may be defective, but it is new. This is a 67 fuel cap. The the vent seems to have a valve in the center. When I shake the cap back and forth though, this little valve clatters back and forth kind of like a PCV valve. It seems to me, it SHOULD operate more like what is quoted above...allow air in, but not out.

I can smell fumes from around the gas cap no matter how I put the cap on...especially taking the car from a cool gargage to hot drive way.

Is this cap just simply defective?
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post #9 of 11 (permalink) Old Jun 1st, 06, 10:27 AM
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Re: Darn near blew the gas cap across the lot

Starting in CA with 1970 cars, and continuing with all 1971 cars, gas caps are both pressure relief/vented caps.

These cars had the tank & bowl vents hosed/tubed to a charcoal cannister. The cannister purge valve was controlled by ported vacuum from the carb.

Might try a '71 or later gas cap?

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post #10 of 11 (permalink) Old Jun 1st, 06, 12:28 PM
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Re: Darn near blew the gas cap across the lot

My 69 SS does the same thing. When I get gas, I have to remember to remove the cap real slow......

Sometimes I think there is way too-much pressure in there to be normal......
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post #11 of 11 (permalink) Old Jun 1st, 06, 01:17 PM
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Re: Darn near blew the gas cap across the lot

I have had the same problem. There are several post
on this subject. One even suggests that todays
fuel has more alcohol and evaporates quicker and
build pressure. I have had the pressure build so
that it will actually force fuel out the o-ring on
the sending unit. I also have a 69 Nova that I
have had to double up the clamps on the fuel
line because of the pressure. When I stored my
Camaro last year I drove it about 7 miles. The
temp was in the 50's. When I checked my cap
there was alot of pressure so I don't buy into
the theory that it is caused by heat from the exhaust.
It will also build pressure just sitting. I believe it
does have something to do with evapo and temp change.


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