Why does this happen? Automobile related. (1 Viewer)

[Vince Maskeeper]

OK:
In many cars i've driven, when you run the car down almost out of gas, when you open the gascap there is a wooshing sound of air. I always assumed this was the result of suction in the tank- from running it so low, air rushes in cause of the amount of suction.

Well, I have a 1989 Buick that was recently serviced to replace a fuel sensor. The car hadn't been driven much in a few years (It's an old beater, I had it stored)- but now i'm driving it regularly...

Everytime I open the gascap, a LARGE volume of air comes rushing OUT OF the tank. It hisses and blows gas fumes for 20-30 seconds! I noticed this while getting gas, but since then I will just randomly open it aftern i've driven for a few minutes and it will release a ton of fumes.

In retrospect I realized the hissing i heard with other cars might have also been rushing OUT of the tank- I alsways assumed it was going in do to air suction out of the tank when fuel ran low.

Anyway- is air in the tank normal? How do cars normally deal with pressurizing the tank without air rushing out everytime you open the cap?

It also seems that when my car gets low on gas (under 1/4 tank), unless i let the air out it stalls and misses.

Any thoughts?

-V

 

[ken thompson]

You may be tightening the cap too much. It's amazing but if the cap it too tight it will efect how the car runs. Also, I'm sure there is some kind of vent, maybe in the cap itself, that lets air in as the gas is removed. This vent, and perhaps cap itself may be bad.

 

[Karl_Luph]

Vince, my car does this all the time,but only for about 2 seconds when I take off the gas cap, & I notice it more so during the warmer months too. I want to say the fuel system is a sealed system that recirculates the gas vapors that build up in the tank back into the intake manifold or be released through a charcoal canister to the outside air.I'm no expert on this so maybe a mechanic will be able to educate us on this.

 

[Todd Hochard]

The pressure in the tank is formed, due to a combination of fuel slosh, as well as the recirculating fuel from the return line at the regulator on the engine, creating vapor in the tank. As a key part of reducing evaporative emissions, gas tanks were made with sealed caps (this began in the 70s). In the later years, the charcoal canister that's in the vent path of the tank is sealed off when the car isn't running, to minimize the release of these vapors when the car is off. When the car is running, the fumes from the canister are normally directed to the intake manifold, to be burned off in the engine.

Just within the last year or two, companies have began using returnless fuel delivery systems, just to reduce this pressure buildup, and thus further reduce evaporative emissions. It's a key component of one of the SLEV/ULEV/SULEV requirements (I don't remember which level).

Regarding the stall- you need to check for proper function of the vent on the charcoal canister. If pressure gets too high in the tank, it completely messes with fuel delivery pressure.

Todd- not a pro, just a hobbyist.

 

[Kirk Gunn]

This thread reminds me of a Taurus rental car we had last year. Drove it for 3/4 of a tank when suddenly it started hesitating seriously on the highway and the engine light started flashing. Scared the crap out of us as we were on the outskirts of San Anotonio with not much around.

Made it back to the airport and swapped to another car. Drove it around and fueled up before returning it and noticed the label on the cap: "Failure to close fuel cap tightly will result in engine malfunction"....

DOH ! Who woulda thunk ?

 

[Steve Schaffer]

Cars with OBD-II (Onboard Diagnostics II) MUST have the gas cap tightenned completely, or the Check Engine (or Service Engine Soon) light will come on. The official name for this light is the MIL (Malfunction Indicator Lamp)

The OBD-II system monitors the Evaporative Emmission Control system (designed to keep gas fumes from escaping into the atmosphere) and will throw a trouble code and turn on the light if the pressure or vacuum in the system isn't correct indicating a possible leak to atmosphere.

OBD-II is on all newly designed engine "Families" built since 94, and all others starting with 96 model years.

Never leave a gas cap loose, always tighten until "clicks" are heard.

I work in a Toyota dealership service dept. and fully 90% of the cars that come in with MILs on are due to the owner not tightenning the gas cap, running the engine while refuelling, or using an improper aftermarket gascap.

 

[Jay H]

I know the loose-gapcap thing is common, but you'd think that the mfgrs would make something a little more specific besides the usual "check engine" light (CEL). They expect the CEL to cover a thousand+ OBD-II codes that you either need some device to read or go to the dealership. You'd think that if the dealerships are getting a ton of people in their service area with MILs, then you'd think they would say make the "low gas" indicator start blinking to specifically mean you have a loose gas cap or something similar to distinquish between the MIL and the ton of other CEL codes. Would save customers headaches and also the service department!

Jay

 

[LDfan]

My 99 Honda Accord has the Check Engine Light on and it won't go away. I had it scanned and it indicated a defective EGR valve which I just had replaced. Then the mechanic reset the computer but the light still came back on. My car just passed the emissions test with flying colors so I guess I'll just ignore it. Upon doing some research on the internet I've discovered that Honda Accords are notorious for having these problems.

Jeff