2000 ox66 250hp

[<cobia24>]

the engine runs great except at 3800 rpm's the engine will jump to 4000rpm's and then back to 3800. had the o2 sensor cleaned but it did not help. other then that, it has a rough idle in gear at low rpm's but it sounds like they all do that. any idea's?

[<jb123>]

was the sensor just cleaned or was it tested ?? has the motor sat for any amount of time ?

[<cobia24>]

the moter hasn't sat at all . It started at the end of last season so when I had the water pump and thermastats done I had them clean the sensor don;t know if it was tested. any other idea's could it still be the sensor?