I have a 91 merc 135 that overheats after about 2 minutes at cruise speed. Water spray out the tel-tel seems good(as good as the other one, I have a twin setup). Water pump is new, poppet valve looks ok but I don't know how to check it exactly. I took the poppet out and took it apart and looked at it and didn't see any holes or nothing in the rubber, I did make the gaskets out of generic gasket material that is probably a little thicker than original. What can cause the motor to do this at half throttle? I also noticed dirt dobber nest in the back exaust port( the one below the cowling)when I bought it a few weeks ago so we sprayed water up through those ports on both motors to knock the nests out. <br /><br />Can anyone help?
.<br />Anyhow, that could be the entire problem. If these motors have had any salt water usage, look very closely around the mating surfaces of the head and the block (head gasket area), and the exaust port plates. If you see any white, powdery material, it is salt-creep. Salt-Creep is aluminum oxide (an aluminum rust)...A corrosion that is started by salt water left in the cooling circuit form improper flushing with fresh water after use.<br />You may have to pull the heads if you see this, because inside, there will be more---much more,sometimes to the extent restricting, if not, completly blocking water flow.<br />But before doing this, test it with your hand. Run it until you get the alarm, then shut her down. with your hand, touch different areas of the block and heads. Of course, it will be warm, but should not burn you. Just in case, be carefull ,using light, quick checks. If any area is hotter than you can stand to hold your hand or fingers to for 3 seconds, it's too hot to operate without grenading.<br />And the hot spots will be close to the areas of restriction.<br />For safe measure, don't inspect the thermostats...Replace them. they're only like 9 buks, and usually only last as long as the impeller does (Should be replaced at the time impeller is changed). Also, they have a tendsancy to corrode shut, if left to sit for a while.<br />If oyu feel no hot spots anywhere, chances are good that the sending unit is shorting-out.