Re: Mercruiser Overheating...
Darren<br />Let me start by saying I do not know your engine but in general I can give you a couple of ideas.<br />Ofcourse it can be a bad thermostat, If you remove and put in a pan on stove should open at about 150 degrees. Also most engines have a hose coming from the outdrive and hook up to engine near the thermostat. If you take this hose off and start up this is water coming from outdrive, run it over the side or into a bucket and start up. If you have good flow your outdrive inpeller is ok. My best guess is that thermostat and impeller will be ok and your problem will be in manifold. On saltwater engines the manifold last about about 6 years if you always fush with fresh water after each trip. If you do not flush then fails even sooner. What happeneds is Manifold starts to rust inside then the rust starts to flake off and flow out to riser but it gets caught in the exit of the manifold or entrance to the riser. This blockage slows water from flowing thru the engine and at high speed the engine overheats. When you slow down have enough flow to cool the engine. You can pull the riser and clean out the rust and scale from the water passages and it will be ok for a while. If this is problem may want to replace the manifolds or riser. Reason is that as rust and corrosion get worse the manifold will rust thru and let water flow into exhause passage. Water will then run back to exhaust valve and rust it. If valve is open also flows into cylinder and rust it. Water can fill cylinder then when try to crank engine will freeze and maybe bend rod or break piston as can not compress water. On some engines when water leaves the engine it goes thru a short hose to the exhaust manifold, if you pull this hose and run a longer hose over the side engine will cool down fast. You can not run too long as have no cooling to exhaust manifold but long enough to see if this is your problem. This water will be hot coming out. Sounds like you have two engines one thing you can do is feel temp of exhaust manifold, it will be a lot hotter if it is plugged up. I hope this makes some since.
