A lot of Merc's are that way. Run the motor and then remove the cover. Put your hand on the block and see if its hot. If its running hot you will only be able to hold your hand on the back of the block for a second or so. Most likely your motor is fine.
it has never been changed at it only started at the end of last year. the outboard is all original factory parts except the power pack the prop and the seals behind the prop.
I just replaced the impeller on my 1990 Merc this year!The old one still LOOKED good,but $25 is cheap insurance against a $1000 repair bill.Change it and have fun running the boat without the worry of over heating.
change it before it blows up and you have impeller particals all throughout the powerhead. Most times thery cannot all be removed without ripping the engine apart.