My son-in-law recently purchased (very cheaply) a 2005 Jeep Grand Cherokee with the 3.7L V6 engine. He claims it misfires when he drives it. I've helped him replace the plugs and tested the coil packs and it seem to start and idle just fine. I also cleared the OBD II codes as well (it also reports the ox sensor bad).
As it seems to misfire only under load I'm thinking fuel system issue. But not being that skilled in newer mechanics I'm hoping some of you who are can provide some tips on what and how to diagnose this further.
Your responses will be greatly appreciated. Thanks.