Re: Pressure relief valve
Without the valve the motor may run too cold at idle. The obvious effect is that it may not idle as strong as it could. A lot depends on where you are (water temp). At planing speeds it's possible, although not likely, the block could not be filling up with water completely.
With todays fuel I'd put one in there if it were mine. If you do a lot of ideling the plugs may last longer. Make sure you have the correct Champion plug. When they took the valve out they could have changed plugs for a different heat range.
