My First Question : Why does my fuel cap release air when opened?

[A.Wilson013]

Well, I hope this is stupid.

Today while dyno tuning my 4Runner the mechanic got to a point (5k RPM) where no matter how much fuel he added, it was just staying lean. He said this may have to do with the pressurization of the fuel system being to low. He then proceeded to ask me if when I open my fuel cap, does it release air?

Why, yes, yes it does. It started doing this about 2 weeks ago, IIRC.

He was saying that this is not good.

Anyway, I am curious if this is right, or what?

Any input is greatly appreciated =)

Alex

 

[MrBobDobalina]

Did you ask him why it's not good? Just curious since he was the mechanic that worked on it and then said it wasn't good....I am not sure why that wouldn't be good/bad....Was your truck on empty when you opened it before to add gas or something? I know when I get low on gas and go to fill up it will release some gas fumes....Every car I've owned has done that tho, pretty much....Oh and you need to tell us more about the tune!!! What did they change? Did you get any HP/TQ gains from the tune??? That should be a write up with pics

 

[KellyJ]

Hm I thought that when you run low on fuel the change in pressure from a full tank to an almost empty tank is what causes the air to escape when you take the cap off. I only usually notice the cap releasing air when I'm near empty, less than 1/4 tank. Anything above that is usually quiet. When do you experience it? All the time?

 

[Mr. Leary]

Look up Charles Law and the Ideal Gas Law.

The gas tank builds up pressure because the air that is fed in to replace the fluid gets hotter during operation. Wait until the car has cooled off and you will not see nearly as much pressure.

Completely normal to build up pressure in there. Take your truck to a different mechanic.

Not trying to be a ****, but why would you be worrying about running a lean mixture at 5k RPMs? That is almost at the top of your power band and if you are running that high you are probably beating the heck out of your truck.

Don't worry about running lean, and stay away from revving your truck up that high. Its not a sports car.

EDIT: Didn't see that you have an SC. Install a 7th injector in the intake manifold and that will allow you to dump the extra fuel that you need to fix the lean running condition.

 

[the great him]

id question your mechanics "mechanic skills, no offense but yeah that is normal and expected(air to be released when opening the gas cap), check the back of your gas cap, it has a rubber seal on it, its there to create oh wait for it... but a SEAL. you have a fuel injection system, its pressurized, while yes most of the pressure is up beyond the fuel pump, some of it will return back to the tank. and that along with the thermal dynamics of the hot/cold will also create different pressures. most newer cars require there to be some pressure in the tank -if the gas cap doesnt seal(or you dont tighten it all the way) then it will throw a check engine light.

i think your lean "problem" is from either a fuel pump that cant keep up with the demand or injectors that are maybe plugged or fowled, or arent big enough, or you might even have a clogged, or clogging fuel filter... just some things id look at.

 

[A.Wilson013]

The mechanic is a very well respected performance guy around here. I just spoke with him and he agreed that when near empty, it is normal.

With my injectors I am having more fuel delivered and have the ability to control all 6 injectors, not 1. If I added another I would be way too rich, which is just as bad as too lean.

I appreciate your guys response. The problem was merely a miscommunication between the mechanic and I.

I agree that my fuel pump is the issue. The PO told me he upgraded it, but looks like it is another "upgrade" of his I will have to fix. Oh well, learning a lot, and making the upgrades myself now. Soon I will be able to say " built not bought " perhaps get a bumper sticker?

 

[Limited2K]

I am no expert, but I have read several threads concerning superchargers, and 7th injector kits. There are several guys that run with a 7th injector to even out the fuel mixture again after a S/C install. The psi of the pump you have may be plenty, but if you can't get the fuel out faster, it won't matter if you upgrade to twice the psi.

EDIT: Also, not sure if you are running upgraded or stock injectors...

 

[A.Wilson013]

EDIT: Also, not sure if you are running upgraded or stock injectors...

I am running the injectors listed in my sig. 315cc Supra Green Top's.

The 6 of these brings me more fuel then the 1 added injector sold by URD. That makes me believe that it is indeed my pump that needs attention. In retrospect, this should have been the first thing I check. I just sort of figured since I had the larger injectors, the stupid loud fuel pump, and the PO telling me he "upgraded" the pump, that it was enough.

 

[brian2sun]

URD says the 7th injector kit is best used for '99+ and the 6 injector kit is better used for '96-98s. I believe both require the upgraded fuel pump, but I have heard there is an issue using the 6 bigger injectors with the newer style MAF like you have.

Either way, if it's not pinging, I wouldn't worry about running a little lean at 5K either. You could blow your engine up pretty fast if you really get it up into that rpm range a lot.

 

[A.Wilson013]

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Either way, if it's not pinging, I wouldn't worry about running a little lean at 5K either. You could blow your engine up pretty fast if you really get it up into that rpm range a lot.

Yeah, the only time it ever goes over that is when it's getting dyno'd. Everyday driving I hardly ever even enter boost, unless there is a teenager showing off that needs embarassing =p

 

[Mr. Leary]

Well, at least the fuel pump is pretty easy to change :biggrin2::hiding: