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Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 15607
Experience:  Degree in Marine Technology. Gas and diesel marine mechanic.
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Have a 1981 260 Mercruiser with an overheating issue, has a

Customer Question

Have a 1981 260 Mercruiser with an overheating issue, has a new water pump, exhaust manifolds, new risers, new thermostat housing and and thermostat. Still overheats.Engine runs excellent!
Submitted: 2 years ago.
Category: Boat
Expert:  Jason replied 2 years ago.
Hello my name is ***** ***** am I speaking with?
Is it overheating at idle, or at higher rpms? Do the exhaust risers stay cool even though thee engine is hot, or are the riser hot as well?
Is this an inboard setup with transmission, or a sterndrive? When did it last cool properly?
Customer: replied 2 years ago.
Overheats at higher rpms, risers stay relatively cool, and it's a sterndrive. Not sure when the engine last cooled properly. I'm scratching my head on this one. The only other thought was a blockage in the exhaust tube but have inspected and found none.
Expert:  Jason replied 2 years ago.
What kind of temps are you getting? And more importantly, are you reading them with just the boats gauge, or have you verified it's running hot by using an IR temp gun on the engine?
Customer: replied 2 years ago.
200 plus with an IR temp gun
Customer: replied 2 years ago.
I need someone's fresh perspective on this one!
Expert:  Jason replied 2 years ago.
On engines of this age the most common reason for overheats is the water pocket cover grommet in the sterndrive. When you changed the impeller out obviously you noticed the rubber tube that drops down out of the upper half of the sterndrive. On top of that tube is a rubber grommet that seals the tube to the sterndrive on the top end. Rubber grommets don't last for ever, and they dry rot and fall apart like any other piece of rubber. When that happens (and it does happen, all the time). Not only does water leak out of that area, and the engine not get a full supply of cooling water. But it also sucks air in. The "test" to see if you have a bad grommet would be to take the main line that runs from the inner transom plate to the thermostat housing and splice in some clear tubing so that you can see water flow. Go and run the engine, and look for air bubble in that tubing. If they are there, the grommet is no good. Click this link below to pull up a picture.
#3 is that grommet.
Does that all make sense?
Customer: replied 2 years ago.
Makes sense but the grommet is new.
Expert:  Jason replied 2 years ago.
Okay when you asked your question there is a little box that you are supposed to type in everything you have done in order to try and figure this out yourself. Since I have no clue what you've done... why don't you fill me in now.
Customer: replied 2 years ago.
Have installed a new water pump in drive and replaced upper water tube bushing. Replaced all the exhaust manifolds, risers, thermostat housing and thermostat along with several hoses that needed to be replaced. This are the log style manifolds and have the separation plate installed below the riser where it attaches to the manifold. Have done numerous flow checks to see if the water was flowing into block and manifolds and everything appears to be okay. Have inspected the exhaust tube for blockage and found none. Engine runs excellent. Got a suggestion?
Expert:  Jason replied 2 years ago.
Do you have an IR temp gun?
Customer: replied 2 years ago.
Yes I do.
Expert:  Jason replied 2 years ago.
Do this form me. Take the boat out and duplicate the problem. Then measure the temp of the thermostat housing. The sides of each exhaust manifold. And then the tops/sides of each exhaust riser. Let me know what you get for each temp.
Expert:  Jason replied 2 years ago.
Good morning. Just checking in. Have you had a chance to run the boat yet? -Jason

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