I have a 91 jeep cherokee 4.0 stock everything and it just started misfiring and back firing. It started doing it when I had put some good ol' arco gas in it because it was a little cheaper then anywhere else. It started to back fire and miss after it was warm and after letting it sit for an hour or two. I pulled my plugs and they were replaced not so long ago. So was the plugs and distributor cap. Its not throwing any codes at all and the gas mileage has gone way down hill. It has now started doing while im driving and before it would do it at the initial start up when warm. Any suggestions. Your help will be greatly appreciated

 

put in some octane booster and see if that helps with the misfiring... were you having this issue prior to putting in the gas? You may have gotten a bad batch.