I agree that every ones experience is different and that there is more than one right way and many wrong ways but I do agree with you not to agree on any issue with anyone unless they agree with me in which case I will not disagree unless I want to but then I may agree to disagree which in fact may be wrong unless it is right.
Agreed?

 

Whoa! My head just exploded Spike.

 

I had my tank replaced with the recall, before I had no hiss now i do, the gas cap has a valve to let air in but I think it wont open til there is a certain amount of vacuum in the tank. so depending on temp there may or may not be any vacuum in the tank as gas expands and contracts.

 

Pre-AEl may hiss. Post will not. That is what AEL is. It takes the excess fuel vapor pressure and burns it through the combustion. That way it isn't vented out when you fill up.

You get full fuel pressure at the rail from the pump. That full pressure is bled off by the regulator which is controlled by vacuum. The more vacuum you have, the more it closes, the more fuel pressure you have.

I have never had a hiss.

 

Guys,

My 97 is a Pre AEL and it definitely has a hiss.

The following is imprinted on my new Stant brand Gas Cap from Advanced Auto for the 4.0 Pre-ael discovery " Caution: Rotate Cap one-half turn to vent. After Hissing Stops, Continue Turning To Remove"

I did not say the theory of operation is that it works by presurrizing the tank as some of your responses regarding aircraft would imply.

My does definitely hiss, which I believe is a side effect of the gasoline expanding with temperature increases coupled with the fuel pump operating pressure and the unit basically being a closed loop system with no apparent pressure bleed off mechanism that I am aware of. I don't see how it could do anything other than develop some pressure differential above ambient.

The warning on my gas cap reinforces my belief. It would be interesting to crank it up measure, the fuel pressure at the under the hood valve, then shut it off and open the cap to see if that drops the pressure at the check point.

I don't think the strong hiss on mine is a problem. I would be more concerned if it did not hiss as before when the top of the old fuel pump assembly was severely cracked.

 

Would you get steady pressure there with the Gas Cap Off?

 

Enlighten me, which aircraft have you maintained? The late 60's thru late 70's supersonic US Navy Recon Aircraft I worked on had pressurized fuel sytems.

 

5 years powerplants/flightline mechanic AV-8B Harrier (pressurized fuel storage, something like 5 psi if I remember right)
Other than that:
A couple of F-18's, EA-6B's, Cessna's, Cirrus (not sure what the plural of this is), Columbia Lancairs before Cessna bought them and renamed them Corvalis, King Airs, a Piper Malibu, fixed a fuel leak on Bobby Unser's plane once, Learjets (chased fuel leaks on the oldest models, am now helping to develop the newest).
That's the condensed list. I'm an A&P mechanic, licensed pilot, and am in the final semesters of completing my Aerospace Engineering degree.
So no, I'm not just talking out of my a$$.

 

And you are saying those fuel systems are not pressurized?

OK if you are so smart, why ain't you rich?

Regardless my Disco fuel system hisses when I remove the cap and it would have to be holding pressure to do so.

 

I really wasn't trying to insult your intelligence or anything Danny. It's just that in my experience and opinion, I don't think that a pressure differential large enough to cause a big hiss when the tank is open is a sign of normal operation for a fuel system as simple as you would expect to find in a truck. That's all.