GTV6 Gas Tank Noise

[jpc823]

Recently the gas tank on my GTV6 has started to make a noise when I shut the engine off after driving the car. The sound is like a piece of metal that flexes back to it's orignal shape from a distorted shape. It's almost like there's a pressure buildup that finally releases after the car is turned off. Has anyone had a similar problem or any ideas on what's causing this???
Thanks

[Balocco211]

After you shut off engine, what happens if you open the gas cap?

[jpc823]

I can hear the pressure releasing, but still get a bang from the gas tank.

[jpc823]

I've also noticed that the bang I get from the tank is louder when there is less gas in the tank. With a full tank the sound is more muted.

[mjr]

tank over pressurisation. empty tank, remove vent valves, and clean

[jpc823]

Thanks for your feedback, will let you know the results when I'm able to clean the valves.

[macahan]

I have the same on mine..... Where is the ventilation placed?

[Balocco211]

This .pdf of Group 4 should help. On my 1982 I don't have the canister (#4).

Check valve is (#9) located under rear deck lid cover behind and above rear seats.

Problems usually occur when oily residue accumulates in areas 3, 4, and 6; occasionally blocked check valve.

Greg Gordon has an outstanding a tech bulletin on this:
L-Jetronic

Here are 1987 Tech notes from the late great Fred Di Matteo (Velocissima-AONE)

And for the 1983 and before models; EMM.pdf attached, section D-21 from Group 4, probably best shows the check valve in question #3 (notice no charcoal canaster; in 1983 and before from oil separator to firewall then hardline (#2 - on vehicle interior) to rear of alfetta where check valve placed in line prior to vent line and expansion tank)

[macahan]

Thanks Balocco211

[jpc823]

Clogged vent valves was my problem. After cleaning no further problem

[fast goat]

My car does not have a vent valve (removed by PO?).
Does anyone know of a suitable replacement?
Thanks for your help..