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2005 volvo 240t5---check engine light and gas cap aka fuel tank cap---delayed affect

The dreaded light came on and after checking the internet I bought an after market gas cap at the local chain store—and it worked! The light went out—for about a week. So I bought a factory cap from a volvo parts house, but the light did not go out—until about five days later! Now three days hence it is still out. I figure it went on in the first place due to a seven year old gas cap, and the fact the light has gone out twice when a new cap is used allows me to feel there are not other possible problems. However, is a delayed reaction a common occurence? Did the factory fuel tank cap actually need to get “broken-in” over a five day period? Appreciate your comments. Tnx. “Rakey”

Gas caps, when they don’t seal properly, will throw a code and turn your light on. The old one stopped sealing. The replacement may not have been any good. It usually pays to buy certain things OEM so you know it will work.
You do turn the thing so it clicks three times, right?

When a code is set, some time is needed to cycle the ECU and reset the code. This sounds like the system is doing exactly what it is supposed to do.

To add to what jtsanders said, the ECU has to see a specific number of drive cycles without the failure code appearing before it will turn off the light. However, I don’t think it was the gas cap to begin with. If you add gas after the handle clicks off the first time, you eventually saturate your canister. At first, this is a self clearing problem, until the canister goes bad. Good news, stop overfilling now and the problem wont come back.

What happens when you add more gas is that the gas expands in the tank. It comes out of the ground at about 60°F and when it warms up to ambient temp in the summer, it gets pushed down the vent tubes to the canister. The canister can tolerate this for a long time, but eventually it saturates. Different vehicles have different tolerances for this. BMW’s seem to be more sensitive to this than other vehicles. Some vehicles have so much headroom in the tank that they will never see this unless the driver is in the habit of “burping” the tank.

are you sure the code was for the gas cap, specifically?

To all who commented on the check engine light and my gas cap, thank you very much for the education! Had no idea the gas cap was such a sensitive item. For those that wondered, I do click at least three times, and I haven’t topped off my tank since the seventies gas crunch (seems we all did it then so must have been before high tech fuel tank caps). Thanks again! “Rakey”