90 240sx Engine Knocking

[240TKE128]

Recently i noticed that a weird knocking sound was coming from engine i took the valve cover off and noticed that the rocker arm on one of the exhuast valves is loose to the point i can push it up and down with one finger, when all the others are unmovable. could this be what is causing the knocking sound if now where do i start with the repairs.

The engine does currently have just over 210k miles been constantly maintenced, and has been well taking care of.

Any information would be much appreciated.
Modified by 240TKE128 at 7:57 PM 8/14/2009

[AeonTorpor]

If it's that loose at speed I would think it'd be likely to make that sound. The cam shaft lobe is pushing the rocker up and the arm is so loose that the rebound is too slow to follow the cam. So when the cam comes full circle it's whacking the arm as well as the piston possibly hitting the valve.

I'm not sure what might cause that, but if I had to guess, I'd say a severely worn or cracked spring/seat.

My guess would be that the head would need to be rebuilt, possibly w/ new valves, rockers and cam depending on the damage done. At worse, the whole engine because of the piston contact.

I'm no pro at this, so if anyone else has more please chime in.