Diagnosing misfire in jeep V6 engine?

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.

On Sat, 25 Aug 2012 20:27:53 -0400, BobMCT >
wrote:

>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.

If it only misfires under load I'd be thinking ignition system issue.
But it could be either. If your ox sensor is bad it could be running
too lean perhaps causing misfire under load.