I have a 06 cobalt with a new engine and the this problem is driving me batty. It runs great then it random misfires every so often at mid rpms about 2500 or 3000. Sometimes I can go for a hour before it will miss and sometimes 30 seconds, it stumbles then goes back. I can re create it by reving the motor in neutral and it will miss eventually. Now heres the confusing part, all I get is a Po 300. And I can make it not miss by unhooking the maf sensor then I cant make it misfire..... I replaced the maf sensor with a used one from the junk yard and it does the same thing. Also when it does miss it goes into open loop then when it goes back into closed loop the misfire is gone, Ive watched the scanner and checked sensors and replaced lots of parts but I am getting no where with it, is there anyone who may have any idea what may be causing this????
I have a 2.2, new injectors and pump. went through looking for air leak and there were none, new seals on intake. new 02 sensors new plugs, coils. Swapped out maf and map. great compression, po300 was on cylinders 2 and 3