High pressure at filler cap??

[spyysee]

Here's the scenerio. 20 or so miles, stop-n-go traffic, warm day, engine temps
between 209-227, 1/4 tank left, I pull in for gas and had trouble opening the
filler cap because of the pressure. When I did open it, man, I thought that gas was gonna erupt at any moment. I mean it was a VIOLENT release of fumes.
I've owned this car since it was born and never had that happen!
Has anyone had the same experiance? If so, why?

 

[Ken]

It's not unusual for a slight amount of pressure to build up in the tank, especially at higher ambient temperatures or elevated altitudes. However, if you have an extreme amount of pressure venting when you open remove the cap, it could possibly indicate a clogged charcoal canister, or otherwise restricted vent line to the canister. You would definitely fail an emissions test if this was the case.

 

[spyysee]

It's not unusual for a slight amount of pressure to build up in the tank, especially at higher ambient temperatures or elevated altitudes. However, if you have an extreme amount of pressure venting when you open remove the cap, it could possibly indicate a clogged charcoal canister, or otherwise restricted vent line to the canister. You would definitely fail an emissions test if this was the case.

How do I check this?

 

[Ken]

PURPOSE
The basic Evaporative Emission Control System (EECS) used on all vehicles is the charcoal canister storage method. This method transfers fuel vapor from the fuel tank to an activated carbon (charcoal) storage device (canister) to hold the vapors while the vehicle is not operating. When the engine is running, the fuel vapor is purged from the carbon element by intake air flow and consumed in the combustion process.

PURGE VALVE OPERATION
The purge vavle is an integral part of the canister. When the engine is running, vacuum is supplied to the top of the purge valve (Control Vacuum Signal) which lifts the valve diaphragm and opens the valve. The lower tube on the purge valve is connected to manifold vacuum on the throttle body.

This system has a remote mounted purge control solenoid. The ECM operates this solenoid to control vacuum to the canister purge valve. Under cold engine idle conditions, the solenoid is turned on by the ECM, which blocks vacuum to the canister purge valve and prevents purge.

The ECM turns off the solenoid valve and allows purge when:

• Engine is warm.
• After the engine has been running a specified period of time.
• Above a specified road speed.
• Above a specified throttle opening.

The control valve only opens when sufficient vacuum is supplied, thus preventing purge under conditions of low ported vacuum.

FUEL TANK PRESSURE CONTROL VALVE
The Fuel Tank Pressure Control Valve is a spring-biased vacuum diaphragm valve, normally closed. When the engine is running and the throttle is above a specified opening, vacuum is supplied to the valve and the valve is opened. Fuel vapors are now free to vent to the canister. When the engine is turned off or the throttle closes below the specified point, no vacuum is applied to the valve, and the fuel tank is vented through the restriction valve. This restriction tends to hold vapor in the tank rather than allowing the vapor into the canister.

VISUAL CHECK OF CANISTER
Cracked or damaged, replace canister.

FUNCTIONAL TEST OF CANISTER PURGE VALVE AND THE TANK PRESSURE CONTROL VALVE
Apply a short length of hose to the lower tube of purge valve and attempt to blow through it. No air should pass into the canister.

With a hand vacuum pump, apply vacuum 15"Hg (51 kPa) through the control valve tube (upper tube). The diaphragm should hold vacuum for at least 20 seconds, if not, the canister or control valve must be replaced. If the diaphragm holds vacuum, again try to blow through the hose connected to the lower tube while vacuum is still being applied. An increased flow of air should be observed. If not, the canister or control valve must be replaced.

CANISTER PURGE SOLENOID
Canister purge is controlled by a solenoid that allows manifold vacuum to purge the canister when de-energized. The ECM supplies a ground to energize the solenoid (purge "OFF").

If the diagnostic test terminal is ungrounded with the engine stopped, or the following is met with the engine running, the purge solenoid is de-energized (purge "ON).

• engine run time after start more than 1 minute
• coolant temperature above 75°C
• vehicle speed above 2 mph
• throttle off idle

 

[SSTibet]

Hissing noise from fuel filler

My problem (and I don't even know if this is a real problem) is hearing a hissing noisefrom the fuel filler cap whenever I open the cap. When the engine is warm, or cold does not matter; open the cap and hear the hisssssss.... It is a slight noise making me feel like "well there are some vapours inside the tank and when I remove the cap they are coming out, and that is what I am hearing..."

Is this normal or is it a symptom of a problem?

 

[spyysee]

would a malfunction set a code?

 

[Ken]

... a hissing noise from the fuel filler cap whenever I open the cap. Is this normal or is it a symptom of a problem?

As stated earlier:

It's not unusual for a slight amount of pressure to build up in the tank, especially at higher ambient temperatures or elevated altitudes. However, if you have an extreme amount of pressure venting when you open remove the cap, it could possibly indicate a clogged charcoal canister, or otherwise restricted vent line to the canister. You would definitely fail an emissions test if this was the case.

 

[Ken]

would a malfunction set a code?

Only if it was affecting one of the sensors, which it usually does not.

If you've ever removed the gas fill cap while the engine is running, you will notice a slight difference in idle quality, just as you would if you remove your oil fill cap while the engine's running. However, neither will set a code.

 

[SSTibet]

Thanks Ken, I guess mine is normal, but still something to watch out in the future. Are there any illustrations / books for maintenance of the charcoal canister and its supporting systems (hoses, valves, etc)?

 

[Ken]

Helm, Inc. is the only book for me.

The only maintenance required is an eyeball check every now and then to make sure the hoses are all in good condition. Other than that, there's not much to look after until things start going awry.