Gas Cap & Mileage Problem

[Special_K]

I lost my gas cap recently...probably forgot to put it on after filling up. So I drive around without a gas cap for a week or two. No big deal right?

Apparently not. I noticed two things. (1) My gas mileage by almost half and (2) each time I got out of my Jeep, I noticed the strong smell of gas even close to the front as though the engine were spewing it or something.

I'm slow but after a couple of days I attributed the smell to the gas cap. The drop in gas mileage further confirmed that something was fishy. So I go the local pep boys, buy a cap for $8.99 and viola...both problems seem to have been solved.

But WHY? Why does the absence of measly little gas cap cause so many problems? Did I do any lasting damage while my gas cap was MIA?

Curious Ken

[bluejeep]

Originally Posted by Special_K

I lost my gas cap recently...probably forgot to put it on after filling up. So I drive around without a gas cap for a week or two. No big deal right?

Apparently not. I noticed two things. (1) My gas mileage by almost half and (2) each time I got out of my Jeep, I noticed the strong smell of gas even close to the front as though the engine were spewing it or something.

I'm slow but after a couple of days I attributed the smell to the gas cap. The drop in gas mileage further confirmed that something was fishy. So I go the local pep boys, buy a cap for $8.99 and viola...both problems seem to have been solved.

But WHY? Why does the absence of measly little gas cap cause so many problems? Did I do any lasting damage while my gas cap was MIA?

Curious Ken

gas Evaporates

[currupt4130]

yes... that smell was the smell of your money floating away in the form of gas vapors

[Craig]

Not sure how much truth there is to this but I've heard running without a gas cap will affect your fuel return line....something to do with the pressure in the tank. I do know that some of the newer vehicles will not run properly without a gas cap on because of the pressure in the tank but I don't know if this applies to jeeps.

[Sparky-Watts]

Originally Posted by Craig

Not sure how much truth there is to this but I've heard running without a gas cap will affect your fuel return line....something to do with the pressure in the tank. I do know that some of the newer vehicles will not run properly without a gas cap on because of the pressure in the tank but I don't know if this applies to jeeps.

Yup, on the newer Jeeps (and other vehicles) a loose, missing, or cracked gas cap will throw an engine code and turn on the "check engine" light.

[bluejeep]

+1 on the Check engine light

[Captaintek]

Not having a gas cap can cause the check engine light to light up as well as just "topping off" when you fill up. I have had to have mine cleared out a couple of times, so I never "top off" any more as well as make sure I hear three clicks when I put the cap back on. Most new car's fuel systems are pressurized and must have the tank sealed for emissions.

[Jeepstud31b]

Another one for the check engine light. YOUR REALLY LUCKY some vehicles like my partner's Mercedes, the engine will stop and you have to have it towed to have the blasted thing reset. As long as you didnt do it for a prolong amount of time, you should be fine.