| Author |
Message |
Lawrence Coppari (Lawrence)
Junior Member
Username: Lawrence
Post Number: 84
Registered: 4-2002
| | Posted on Wednesday, July 03, 2002 - 7:33 am: | |
Positive pressure is normal. The pressure should be a little higher when the gas level is lower because the gasoline will be warmer. The fuel pump supplies more gasoline than the engine uses. The fuel goes through the engine compartment and warms. What is not used to run the engine is routed back to the tank. The gasoline in the tank is initially cool. The returning gasoline that is warm due to its trip through the engine compartment gradually warms the fuel in the tank by mixing with it. If the tank has less fuel in it, the warming happens faster because there is less to warm.
Gasoline, like all liquids, evaporates. Gasoline is highly volatile so it really likes to evaporate. When the vapor space in the top of the tank reaches a certain vapor pressure due to evaporating fuel, evaporation stops. But it'll continue evaporating until that point (vapor pressure) is reached. The equilibrium vapor pressure is dependent on the temperature of the liquid gasoline. The higher the gasoline temperature, the higher the equilibrium vapor pressure. This is reason for positive pressure in your tank. It is normal.
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F-J'87EuroTR (Ferrarijoe)
Junior Member
Username: Ferrarijoe
Post Number: 245
Registered: 5-2001
| | Posted on Sunday, June 23, 2002 - 5:22 am: | |
OK,
It sounds like a normal situation to have this pressure according to all the good input. I won't worry about it in the future.
Thanks James for detailing the reason for this happening.
Joe |
James Selevan (Jselevan)
New member
Username: Jselevan
Post Number: 15
Registered: 6-2002
| | Posted on Saturday, June 22, 2002 - 7:44 pm: | |
Joe - positive pressure is good. One does not want a relative vacuum. Negative pressure will lead to an interesting event - the collapse of your fuel tanks.
I recently changed my charcoal canisters. There is the ventilation valve near the canisters (in your case - no canisters). In addition, there is a rollover valve near the passenger-side rear window (up against the firewall). The pressure will be greatest when the tank is not full - petrol is volatile, hence evaporation occurs readily from the surface. When full, the surface area is diminished. However, once the level decreases below the inward tapering of the tank, the surface area remains constant until empty. The gas PRESSURE will NOT be increased because there is more empty volume - the VOLUME of gas will be greater, however. Thus, the lower the level of gas, the LONGER the whoosh, but the pressure should be the same whether 3/4 full or 1/8 full.
I was impressed with the MINIMAL differential in resistance to flow in the ventilation valve when I gently blew air (using my mouth) in both directions. I did this to check the integrity of the valve. Very little difference when gently blowing in either end.
Hope this helps.
Jim Selevan |
Steve Magnusson (91tr)
Member
Username: 91tr
Post Number: 803
Registered: 1-2001
| | Posted on Saturday, June 22, 2002 - 7:28 pm: | |
Joe -- normal to have a positive pressure after shutoff. I believe the valve is there to allow air into the tanks if necessary as the fuel is removed to prevent a vacuum condition -- but I think that takes some special circumstances and the pressure in the tank is usually positive. |
Edward Gault (Irfgt)
Intermediate Member
Username: Irfgt
Post Number: 1509
Registered: 2-2001
| | Posted on Saturday, June 22, 2002 - 5:44 pm: | |
My 308 does the same thing and it seems to do it more the lower the fuel level. I have no problems with my car so I assume it is normal. My Impala and Sebring do it also. |
F-J'87EuroTR (Ferrarijoe)
Junior Member
Username: Ferrarijoe
Post Number: 243
Registered: 5-2001
| | Posted on Saturday, June 22, 2002 - 4:29 pm: | |
When I remove the filler cap to fill up the tank, every time there is a lot of + pressure (blows out) released from the tanks. The cap will literally blow out of my hand if I do not have a good grip on it when I remove it.
I have a Euro-spec TR and do not have the charcoal carbon canisters. The small hose from the filler neck routes behind the motor on the firewall and heads down by the right rear wheel well to a tank ventilation valve and a separate silencer then into the atmosphere in that order.
I have to assume the ventilator valve should vent one way (out). Is having the + pressure normal and if not, can it hamper the fuel delivery system?
Can anybody give me some input on this?
Thanks, Joe
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