There's a very noticeable vacuum release / suction sound when I open the gas cap. Is it supposed to happen? I can't remember any such vacuum release sounds before... and it seems this started at about the same time as my elusive problem of losing power and suddenly running lean after extended drives. Am I on to something here?

It's not suction your hearing, but rather a pressure releasing, as the "cork" (gas cap) is removed.

Perfectly normal.

vacuum is bad check valve on tank. 7 bucks

A release is normal.

JamesC

Okay, but I could swear I feel the cap sticking - it's like air pressure inside the tank is lower than outside, not the other way around...

I'll have to investigate this some more.

Thinking more later.....You may actually be correct, in that is was a suction.

Since the fuel pump is "suctioning" the fuel out, it could create a suction inside the tank. In that case, then yeah...Suction results.

But there should be a "whoosh" as the gas cap comes off. If there isn't one, then something is faulty.

Nah, I don't think the tank would hold that much vaccum. Cause if you filled up your tank and ran it way down without opening the cap, there would be so much vaccum that I highly doubt that your fuel pump would be able to pump. I think it's all pressure that could be dependant on a couple things...like the temp of the fuel you pump in, air temp, and how warm the exhaust is and for how long. I'm sure that any variant in temp of the fuel and air in the tank would make it release pressure more or less each time you open it. I'm just guessing here though.

You're right that the tank level would drop and be extra vacuum. Its hard on the pump and could maybe causing his lean condition after driving for a while, throws the whole computer tuning off. In the picture Ive include is from the Haynes manual, numbered 5 the ventilator plays important role to allow air and balance the pressure when negative pressure is present. The evap system draws fumes from the tank to the intake throttle body to, theres a series of check valves on the charcoal canister and under throttle or wide open it sucks. But again that shouldn't be a problem if the car is off, because the ventilator valve should compensate the pressure.. however yours doesnt

If a gas cap shoots, that would be a case for pressure building in the tank. not vacuum. Yet your statement starts off by supporting the vacuum theory....

So are you saying there is vacuum or pressure?

i was talking about the differences. ill delete it. thanks for pointing that dumb comment out

You should have a working pressure relief valve on your gas tank that prevents that. Lots of cars have a pressure change when removing the gas cap though, i'm guessing the relief valves stick closed after a while.
It rarely (err, never?) effects how the car runs though.

If you want to be sure, leave the gas cap in your back seat for a few days and see if anything changes.

I'll give it a try when driving back to Montreal when I take the car out of storage. If it leans out again halfway home, well, at least I'll have one possibility ruled out.

I've always had a pressure release of some kind when i remove the gas cap, never noticed a difference in the way it runs, now I'm wondering if there's something wrong.

Hello Jason1313!!

Enjoy your pressure release, it's normal, there's nothing wrong!!

Hello Jonboy2312!!

You have a check valve problem like transam85dudeman said in his post. Remember the gas cap is non vented, no air in = no gas out, so, there has to be a way for the air to get in there.

For $7.00 you can't go wrong!!

Thanks!!

Just got my 1990 formula and it does the same thing i thought it was weird. Mine runs fine no lose of power or anything like that.

its should really blow out not vacuum, vacuum is a sign of a bad check valve ventalator`

Mine does blow out so I'm fine out is normal.

I have A 1991 Formula.After I've driven mine it vents off pressure with cap on sometimes. The other day I was in a drive thru with car running ,I could here it venting off pressure. I'm woundering if that's normal? Car seems to be running fine.

If it's the tank check valve does anyone know where I can get one?

Hello johnc9909!!

If your cap is the non-vented type, which it should be, it shouldn't be venting through the cap, if it is venting through the cap you have a bad cap, change it!!

Hello again johnc9909!!

If it's the tank vent valve, you should be able to get one from your local GM dealer, that's where I got mine last year!!

I noticed through further investigation of emission system it looks like someone put the vapor canister purge valve in opposite way. I blew into it the way they had it and would not allow air to pass to canister, turned it the other way and now when I blow on it it opens and allows air to pass. Makes sense to me vapor needs to pass to canister not be blocked the way they had it.Anyone have any thoughts?

Up by the vapor canister there is the vapor canister purge valve that attaches to vac hoses that run from the canister to the gas tank. If that valve is not working right, it will cause neg pressure build up in the gas tank.

My vaper purge valve was installed by someone backwards.That should solve my problem.I also got my hands on a newFuel tank vent valve, it looks like mine has some undercoating on it and could be partially clogged.