Bluetooth works better.
If you have iPhone 4S or later, no jailbreak is needed. They work with Bluetooth Low Energy right away, no pairing is needed.
I have the LELink adapter plugged in my car since i got it last year, really like it.
This one might work for you.
Engine Link - OBD II vehicle monitoring and diagnosis
$6, works with wifi adaper or bluetooth low energy adapter. No jailbreak needed.