Fuel Tank Pressure

[NIALS400]

Curious about the fuel tank pressure. I've owned my 91' for a couple of months now but have never filled the tank after driving for more than 20 minutes until today. I noticed that there is a great deal of vacuum in the tank after I removed the cap. Most of the time I don’t hear any depressurization when I remove the cap. What is normal? I know after the car sits for a couple of days and I fill there is no sound. Is it time for a new cap? Could a bad seal be the culprit to my engine miss feeling while cruising at highway speeds? After market OK or go with OEM, the cap on it now says STANT? Sorry for all the questions, just trying to learn. This car is much more sophisticated than my 82 Supra was.

[90LS400Lexus]

Not sure, but when I remove the cap on my 91' LS, it lets out a great deal of air. I have to remove it very slowly. I do not recall my old 90' LS doing this, but I may be wrong... its been 3-4 years since I owned that car.

[Lexuszeal]

Pressurization is perfectly normal it is suppose to be that way. When you are refueling be careful to remove the cap and let the air out slowly.

[eatingupblacktop]

Not sure, but when I remove the cap on my 91' LS, it lets out a great deal of air. I have to remove it very slowly. I do not recall my old 90' LS doing this, but I may be wrong... its been 3-4 years since I owned that car.

Pressurization of the fuel tank is normal for the efi to work properly. However if you notice a excessive pressure being released when you open you gas cap, then you probably need to replace the gasket in the cap. No need to get a new cap.

[SKperformance]

The faster you drive the more gas you use which causes a vacuum effect by displacement. Also the less gas in the tank causes the pressure to fluctuate as air will expand and contact unlike the liquid fuel .

[NIALS400]

Well, I got a new cap this evening and noticed the construction is quite different from what is on the car. The new one has no inner baffling. I am now suspecting the previous owner put a cap that fit that was NOT for a vacuum only system and that may be contributing to my intermittent chug, not really a miss but loss of power for a split second. The new cap specifically states VACUUM ONLY. Hope it works.

[SKperformance]

I would return the cap and get one from the lexus dealer. The caps are pretty specific as to how much vac they hold and release. Besides oem ones are like $20

[NIALS400]

What could the aftermarket do? The one I took off had an S on it which I assume was a Stant after market, this one is as well but is marked vacuum only. The closest Lexus dealer is 120 miles away, is there a cross reference to a Toyota cap? I do have a Toy dealer 10 minutes away. Thanks

[SKperformance]

Hmm , i am not sure what would cross reference . I would order one online and have it mailed to you for less than a dealer would sell it anyway.

[NIALS400]

From what I've been seeing, the Toy cross would be a 95-99 Avalon. Mainly any mid 90's with the 1MZFE 3.0L engine. I'll try my local in the A.M.

[Leadfoot]

I noticed that there is a great deal of vacuum in the tank after I removed the cap. Most of the time I don’t hear any depressurization when I remove the cap. What is normal? I know after the car sits for a couple of days and I fill there is no sound. Is it time for a new cap?

The cap is essential for holding the vacuum in the tank. Vacuum in the tank ensures the evaporative emissions from gas in the tank is collected in the charcoal canister (free hydrocarbons love carbon!) and when the car is next run the canister is purged by the intake vacuum. If you refill when the tank is very empty (i.e after a big run), you should hear a big intake hiss when the cap is popped (the air goes in not out).

Looking forward to hearing which cap is a good stock substitute.