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I started getting the check gas cap message about four months ago and the car was running fine until about 2 weeks ago. Now it is idling rough in park or drive, but when i start driving or i give it a little gas in neutral it runs fine again (above about 1100 rpm). I went to Auto Zone and ad them hook it up to the computer and got a code P0440- could this problem really just be the gas cap?

Sounds like a vacuum leak somewhere in the engine bay...

the code also lists the evap canister. is that a possibility. Please excuse my ignorance, the last car i did any serious work on was a '67 Firebird. Also, is there any way to test the evap cannister before i purchase a new one.

Right on the money about the vacuum leak found a hose that had come off and reconnected it. Car runs great again, but am still getting the gas cap message. I guess a fifteen dollar gas cap isn't much compared to what I would have spent at a shop to have the hose plugged back in. You guys just created this forums biggest new fan!!!

You may not need a gas cap if you rectified the vacum leak. Apparently that vacum leak was associated with the EVAP system. I think they will have to clear that DTC with an OBDII scanner or it may clear itself after a number start cycles.

Karloto wrote:Right on the money about the vacuum leak found a hose that had come off and reconnected it. Car runs great again, but am still getting the gas cap message. I guess a fifteen dollar gas cap isn't much compared to what I would have spent at a shop to have the hose plugged back in. You guys just created this forums biggest new fan!!!

Right about possibly not needing gas cap. Unless you are able to clear the codes, your light may show until the car has the right conditions to run another vacuum test on the fuel/evaporative system. If there's no leak in the lines to the canister or filler system, etc., then the light will go off.

Roughtly the conditions for the car to run the vacuum check for and air leak into the evaporative system as the fuel gauge is between 1/4 and 3/4, the engine temperature has to be above a certain minimum and below fully warmed up. In other words, a lukewarm engine being restarted. If it doesn't sense a leak, the light will turn off.

I had a light on and used vaseline on the o-ring on the old cap, ran a rough terry cloth around the inside of the filler where that o-ring has to seal. Then I drove the car about 4-5 miles in 10-20 degree winter morning temp. Stopped at gas station for coffee for 5 minutes. When I restarted the car, the light went off within a minute.

Someone at the box store can use their code reader to turn off the light. I get the feeling they don't like to do that. I borrowed a code reader from one of the stores as a loaner tool. So I think at that time I was able to reset all codes with it myself.

What was the hose for that you found the leak: was it in the engine vacuum circuit or was it a hose in the evaporative control circuit?

The "Check Gas Gap," ad "Service Vehicle Soon," indicators both disappeared after about ten start cycles. The hose that had come off plugged in near the evap canister but underneath the removable cowling that you have to pull off the oil cap to remove. Thanks again for the pointers, the runs great now.