Fuel Tank cap noise (ppssshhhhh)

teng said:

I recall reading somewhere that the fuel cap makes a -ppsssshhhhh- noise when it is unscrewed. When filling my tank and unscrewing the cap I could bearly hear a -ppssshhh- noise.
Q? Does this mean that something in the fuel system is defective? maybe pressure somewhere? fuel line, sender, rail, pump... or is this normal? (it used to make a ppsssh noise)

should I be worried about this?

Click to expand...

your charcoal canister may be clogged up.

This is pretty normal on any car with a sealed fuel system. Its just releasing the gas vapor pressure buildup in the tank. Probably the reason the noies changed is the weathers cooler or the gasket under the cap is leaking so not as much pressure.

mine never made a noise, passed CA smog inspection no problem. Got a new cap (my dad ran over mine while trying to do my fuel pump =/) and now it does *shrugs* don't think it matters much.

that sound isn't positive pressure it should be a vacuume if your emissions system is working right. it's also a partial safty feature so if the gastank gets smashed it won't popopen and release gasoline enriched air(explosive!) the charcoal canister on almost ALL toyota cressidas and supras are usually the origional ones beceause they don't really change engine performance if they aren't working. it's just a metal can by the exhaust manifold. mine makes funny noises sometimes but that's because i haven't replaced it yet lol!

lol, that's normal, especially if you let it go really low before filling it back up

isn't just the buildup of vapour created by heat changes allowing the volitile gasoline to decompose and expand?

nope because your cars are desighned to make negative pressure in the fuel tank when running. anyvaors coming off the gas end up sucked into the intake system.

My 84 p-type may make negative pressure when running but has positive pressure when sitting turned off. If you do not remove gas cap and open a fuel line it will flow. This is probably due to a sealed cap, and evaportation/temp change. So both negative and positive pressure may be present depending on whether or not engine is running.

You can go to pretty much any large parts shop and have them pressure test your cap. I had a friend tell me once that if you left the gas cap off... or very loose you would get better milage... sounds like B.S. to me, any opinios?