2005 Chevrolet Cobalt Engine doesn't idle

My engine blew a while back and I replaced it with a used engine from an 04 Pontiac Grand Am stating it only had 40k miles on it. After swapping all my Cobalt parts over to accommodate my needs including my old sensors that worked perfectly, I started the car. It fired up first try and immediately died. The car does the same to this day. It only runs if I keep my foot on the throttle and VERY rich. All other driving situations it runs normal. On cold days it struggles to stay running upon start up with my foot on the gas pedal until the coolant level reaches around 90 degrees. Until then it has a bad misfire and backfires through the intake. On a hot day after driving it for a while it runs perfectly normal at idle every now and then. After I fill the tank in any weather it idles high with a misfire. Any help will be appreciated, thank you.