misfires under load for few miles then runs fine all day

Question

In park or nuetral idles good even when reved. Once in gear it misfires/hesitates at an even throttle till hits 3rd or 45mph. it then smoothes itself out.No power going up hills either at this time. Once driven for few miles there are no problems what so ever ( drive to work- 8 hours sitting- starts and drives like new) but when left overnight sitting for apprx. 12 hrs the problem returns. I've changed/replaced: maf sensor, fuel pump and filter, crank position sensor(relearned), plugs. NO CODES.

Answer 1

A basic trouble shooting method that some shop uses to duplicate a misfire condition is they perform a Power Brake , this is where the brake is aplied and the acclerator pedal pressed ,when the misfire here occurs it igntion.2 possible problem the catalytic convertor is plugged. or at the igntion coil there a short to ground there where you can use a salt water spray and spray at the ignition coil are and check for secondary igntion leak.. You can use a exhaust back pressure test to check for a plugged catalytic if you do not have one you can go the long route, by removing the front 02 sensor on both sides but you will have to secure it in a safe location and plug the connection or you will set a mifire code, test drive and note the performance if the perfromance returns the suspect is a catalytic convertor.