Rough idle on 1/4 tank of gas

[Shiwnath]

Recently i've noticed that when I'm at 1/4 tank of gas, my car will start idling rough and my check engine light would come on. After I would fill up, it would still drive a bit rough. But after i drove around 50 - 100 km after filling up, the car would operate normally and the check engine light would come off. I scanned the code and its a P0300 random misfire code. Also i've been getting a crappy gas mileage of 17 mpg of mixed highway and city driving(mostly city). Could anyone possible shed some light on why this would be happening? I'm starting to lean towards a bad fuel pump or water in the tank. I tried running injector cleaner with antifreeze, but there was no improvement.

[sleepyalero]

hmm, either you need to find which cylinder is misfiring since it threw a random misfire code, or sounds like it needs a new fuel pump... or both?

let some other people also get in on this, see if im right or not. :p but those sound like pretty reasonable causes..

[colonel6632]

good chance water in the tank if it's been warm and cold for you outside.

mine did that too, get gas at a new place, and keep it full and keep adding gas line antifreeze and see if it gets better

[Shiwnath]

Recently the weather has been changing between warm and cold so that could be the cause. I guess I'll just keep filling up at the half for a while.

[Shiwnath]

Well I finally figured out what the problem was. It had nothing to do with how much gas i had in the tank(this was just a coincidence). About a year ago I changed my plugs to Bosch platnum plus plugs because OE plugs were more expensive than my broke ass was willing to pay. I've read about people having problems with these plugs on the 3400, but I ignorantly still decided to buy them and put them in. Today I went to the dealer and bought the right plugs and put them in. Now at idle its as smooth as ever and the check engine light went off after 10 minutes. I'll never make a mistake of putting garbage Bosch plugs in my Alero again. AC Delco FTW.

[sleepyalero]

glad to see you fixed it.