Day trip with missing gas cap

We lost the gas cap on my son’s car tonight and it’s my fault. It’s Saturday night now, and none of the 3 soon to close auto parts stores seemed to have a replacement that fit (I’m wondering if it is non-standard and needs an OEM replacement). Early tomorrow morning, he needs to take a 300 mile round trip. I was concerned about whether it could cause damage because gas caps are supposed to be properly vented. Trying to google for an answer, I found mixed answers. Most sites say it’s OK, but not ideal, and you should just stuff the opening with a rag or otherwise cover it somehow until you can replace the gas cap (as soon as possible) and no permenant harm will be done. At least one says that it can damage your engine because the gas cap is part of the evaporative emission system, which is exactly the concern I was trying to evaluate. I wouldn’t worry so much if it was just a little bit of local driving, but a 300 mile round trip before it can be replaced has me concerned. Is there any credence to the dire warnings, or will three or four hundred miles with a stop gap solution not cause damage other that maybe a temporary decrease in performance and fuel economy? If the check engine light comes on will it resolve itself with a proper gas cap replacement or will it need to be reset?

The gas cap prevents fuel vapors from escaping into the atmosphere.

While it’s not desired this happens . it’s not the end of the world.

When your son begins his trip, have him make a stop at a different parts store for a replacement gas cap.

Once installed, and he continues his trip the Check Engine light will probably turn off.


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Gas caps are no longer vented, they seal. The venting is done through the charcoal canister so that no fumes escape into the atmosphere.

Without the gas cap, the system cannot do any self checks so you will get a (CEL) check engine light. It will not harm the engine or the car but it will allow fumes to escape into the air, like they used to do back before the charcoal canisters were first installed on cars.

Once you put a new gas cap on, the system you do a series of self checks over a couple of drive cycles and if they check good, the CEL will go out


Suggest to wait to begin the trip until you have a replacement gas cap. Otherwise you risk safety issues and possible problems with the tank and fuel system.

If you think the cap got lost while refilling at a gas station, this happens all the time & gas stations usually keep a box filled with customer gas caps that they found. So go back to the gas station. They may have it, without further waiting.

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At the Citgo station where I worked, we had a box of those “spare” caps, and we allowed drivers who drove-up w/o a gas cap to try their luck with the ones that we had.

I’d say it may be a good day to lend your son your car.

I remember watching a guy drive off without his gas cap on. It was a sporty car. He was pretty quick to accelerate as he turned out of the fuel station onto the main road, and a whole bunch of fuel went pouring out the side. All that fuel probably isn’t gonna help the paint.