Re: 140 hp mercuriser 1985
About the only thing I would suggest is to get the OEM manual for your engine and study the cooling system along with the sections in the manual on the system.
Here is a link to a 3.0L engine for a newer model than yours, but the cooling systems are very similar.
http://www.boatfix.com/merc/Servmanl/13/13covr2.pdf
Scroll down to the manual outline, under section 6, click on sections A, B and even C and read them. I think you will understand it after that.
