I have had a vapor lock problem on my 86 GT for several years now.
I never quite fixed the problem.

Heres what I've done in the past that may have affected the problem:
New fuel pump
New fuel injectors
New Rodney SS vacuum lines
New AC-Delco gas cap (I believe its the correct one)
New AC-Delco purge canister
New AC-Delco main fuel hose assy (two-piece molded one from the Fierostore)

As I mentioned in the thread title, when I unscrew the gas cap A) there is an EVIL HISS sound
and B) the gas tank makes a very distinct "buckling" or flexing sound.
THIS IS NOT GOOD!

To stop this situation I have been leaving the gas cap "semi-loose" so that air can escape through
the filler pipe neck. But....I need to fix this before something bad happens.

Many have said there is a pinched hose somewhere but I'm not seeing it.
Is there another way to verify what is the problem?

Thanks, Kit

Check valve in the gas cap not letting air in maybe? Are you sure its the right gas cap?

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Red 1988 Pontiac Fiero 3.4L V6
Black 1985 Fiero SE

Sounds like the gas cap has failed and is holding a vacum or is not a vented cap. In either case spend 5 or 10 bucks and install a new gas cap.

88Ironduke

Fiero gas caps are non-vented, they're designed to hold some amount of pressure, a few PSI. The thermal venting is done through the charcoal cannister so that fuel vapor is captured and reused instead of allowed to drift away in the wind. There is a vent tube that goes from the tank to the cannister, if that is plugged then you'll get the symptoms you describe. Also, a plugged bottom hole on the cannister will do that. The two lines off the cannister that go to the rear two metal tubes are the vent and purge lines, they will not cause this.

In this picture you can see the silver steel line come up below the cannister and connect to a rubber line just to the left of the cannister. If you disconnect that line from the cannister and apply air pressure to it with the gas cap off there should be a free flow of air out through the filler neck.



JazzMan

[This message has been edited by JazzMan (edited 01-28-2009).]

Thanks for the very helpful pic James!

OK...I did the test you described last night.
I hooked up a bicycle pump to the hose that goes
to the solid metal (silver) line. I took off the gas cap
and pumped a few times. Seems like it was a little difficult
to pump perhaps because of some backpressure but I was
able to blow some air in there. Results were that I heard some moderate hissing
in the fuel filler neck but I couldnt pump enough to where I felt air coming back
out of the filler neck. Was I supposed to??

Thanks, Kit

There should be no resistance to air flow through that line, it connects directly to the tank and AFAIK has no valves or obstructions in it. Using a bicycle pump to pump air into it should yield the same feel on the pump as though it wasn't connected to anything at all.

There is a rubber line that connects that metal tube to the fuel tank metal line, I bet that rubber hose is kinked. You can do a visual inspection and also disconnect it to see if the resistance is in it or the metal line.

JazzMan

OK James..thanks for the info.
I will check it but probably not till the weekend due to time constraints.

Thanks, Kit