1fatcat

Is it normal for large amounts of pressure to woosh out of the gas cap when removing it? Everytime I fill up with gas, there is a lot of pressure released when I remove the gas cap. It's not suction, because I can feel the pressure on my hand and the cap will actually "pop up" when I get to the last thread on the cap. I always run my tank pretty low before filling up. I usually run it until the reserve light comes on before filling. Just wondering if all C4's do this or if something is wrong with mine? If it is not normal, does anyone know what would cause this? It's a 94 w/manual trans. Thanks.

EDIT: Problem has been resolved. Read through this thread to see what I did to fix mine.

17inac4

mine does the same thing, i haven't noticed an issue... i too run my tan until almost empty. it seems normal to me.

markKlein

All cars do that.

BGZQ8

My 91 does this also, but ALL cars do not.

markKlein

Yeah, I was exaggerating a little there. They started using pressurized gas caps 20 or 30 years ago. I would be surprised if anyone makes a new model that isn't pressurized. (But it wouldn't be the first time I was surprised).

ProjectC4

I thought it was under a vacuum to pull unburned hydrocarbons into the intake manifold.

BGZQ8

Mine does it too (1991), but I'm just not sure it should though.

LannyL81

My '95 did...but my '96 does not...have never figured-out what the difference was......

Cliff Harris

Mine will do that when it's hot. Never when cold.

The fuel circulates in L98s and gets hotter and hotter as it goes up to the engine, picks up heat, and then goes back to the tank. It causes the fuel to expand and build up the pressure. There is a pressure relief valve in the gas cap, but it normally won't activate until the pressure gets pretty high.

ProjectC4

I just looked at the FSM for my '84. It shows the charcoal canister under the hood connected to a fuel tank vapor line all the time (no valve). It also shows the bottom of the canister connected to atmosphere or at least it shows air being sucked in the bottom of the canister during purge. Then it shows that engine vacuum is only connected to the canister if the engine is warmed up and not idling and not under heavy acceleration.
So is the purge canister just open to atmosphere or does it take some engine vacuum to suck air into the purge canister to replace the fuel vapors being sucked out?
In any case it looks like it would be hard for the fuel tank to develop pressure except if the engine is off or idling for a long time. I think the emissions control idea is to burn all the vapors.

mtnmanut

my 84 did that when i thought i would be smart and put a locking gas cap on it. double checked fitment and second did the same thing. put the stock and original back on and has never done it since, fyi it never did it before the new cap.

PLRX

AGENT 86

The canister is always open to atmosphere. There should be no pressure build up in tank.
I have never heard a sound from mine with stock cap and correct aftermarket locking cap. The same goes for my sister's, when it was new in 86.

SunCr

Huh? Nothing is open to the atmosphere (including most gas pumps in the States). Fed Emissions has required a closed system for 35 years or more. A vacuum actuated check valve is used to keep fumes in the tank. CA emissions testing (and the 18 or so other States that have adopted CA Rules) pressurizes the system to make sure nothing escapes with the engine off. Pressure and vacuum allow fumes to travel to the Canister and back into the combustion chamber. The pressure of any gas/vapor is proportional to volume and temperature. Newer tech, like in the last 10 years, incorporates everything in the Canister. However, the OEM's advise you not to topoff these cars as raw gas could travel to the Canister and destroy it. Pump cutoffs are set accordingly (frustrating if you want a full tank and the 05 Mustang got the design so wrong that Ford had to replace/redesign tanks because you couldn't get them more than half way full). Either the owner's manual or cap itself says to remove the cap slowly. Thought this was common knowledge.

MK 82

How would a vacuum in the tank pull anything into the engine. That is the function of the vapor recovery system

SunCr

Basic physics. No such thing as a vacuum but there is negative and positives pressure and a high will always fill the low. Fuel pump creates negative pressure to pull in the liquid and then exhausts a positive pressure towards the injectors. The engine creates negative pressure on the intake stroke which then ingests the air/fuel mixture. That same negative pressure is applied to the Canister and tank solenoids to meter fumes into the plenum which is then consumed on the intake stroke. If the engine wasn't creating negative pressure, it'd never get any air/fuel in it (save for direct injection where high positive pressures squirt gas into the combustion chamber and not onto the intake valve). Since there are limitations to how much negative pressure an engine can create, turbo and supercharging are used to create positive pressure which increases airflow and so on and so forth. Eighth Grade Physical Science - it's been awhile since I took it, but I think I still understand it.

AGENT 86

I guess not that common..
I am no EVAP guru, but reading and looking at the schematics in my FSM, leads me to believe that the canister is open to atmosphere.

There is a vent at bottom of canister and I see no valve what so ever, in schematic.

When purge is called for, fresh air is pulled through canister vent(at bottom), through the charcoal and into intake.. Correct ?

With comments like this one from my FSM, " a small amount of air will pass as the canister has a constant purge hole"

What do they mean by that and how would it be able to purge if it couldn't pull in fresh air ?

I'm all ears

SunCr

Tank vacuum solenoid; left frame rail near the front tire is the check valve keeping fumes in the tank until vacuum is applied at part throttle (though it has a small bypass allowing some fumes into the canister at rest. Once vacuum is applied to the tank valve, full flow goes to the Canister which meters the stuff into the plenum when the purge solenoid is commanded by the ECM. Air inlet - at the bottom on the Vette's canister - mixes air with the fumes to keep the engine from flooding. It's a one way - in only - valve. Why there's anything said about a "drain" eludes me. Later designs places the air inlet on top, but it may be that the Vette relies on the low pressure area created by the spoiler - I really don't know.

RLG

If the solenoid fails , what happens? I have no pressure at all in my tank at any time. You can smell exhaust on me sometimes.

kimmer

At smog testing they pressurize the system to check for leaks.