260 Mercruiser - Still Overheating

[samm835]

I have a 1986 260 (350) Mercruiser with closed cooling system. I have been having an issue lately with the motor overheating, I took the boat out, replaced the foot impeller(old one was in perfect shape). I have recently taken the circulating pump out and replaced along with all the hoses that are on the motor. I am NOT just throwing parts at it, but the motor is over 30 years old so I am changing the old out as I go. I took the heat exchanger end caps off, and look inside and everything looked clean and nothing was blocking any holes junk filled. The risers are cool to the touch, but manifolds are hot, and the thermostat casing gets hot, so what should be the next place to look for blockage or worn out parts? Thanks for the help

 

[kenk63]

I replace the thermostat

 

[harleyman1975]

Thermostat, circulating pump (on front of motor), belt tension, with engine running is there bubbles in coolant tank? (that would be a sign of a blown head gasket) do a pressure test to see if cooling system is holding pressure, do a compression test to make sure you don't have a head gasket problem...Just some things to try.

 

[samm835]

Thermastat, circulating pump both new, an when heat exchanger cap off there are no bubbles.

 

[kenk63]

T stat new, doesnt mean it works, pull it out. boil a pan of water throw it in and see if it open`s

 

[samm835]

Agreed, but I took out as well for a short run......and it still crept to overheat. The t-stat isnt the issue, I dont believe. (Be nice if it was)

 

[blamtro]

I had the same issue on the exact same motor minus mine is raw water cooled. Mine happened after hitting a small sand bar in a no wake zone. Thought I sucked up enough sand to ruin the impeller but it was in perfect shape. To fix my problem I had to pull the water pump, all block drains, and thermostat and blast everywhere with the garden hose. In the end I got less then an 1/8 cup of sand out of everywhere but it fixed my overheat.

 

[Bondo]

Agreed, but I took out as well for a short run......and it still crept to overheat.

Ayuh,..... So "Test" each leg of the raw water circuit,.....

The water line comin' in from the drive, to the H/E, pull it off, point it straight Up, 'n at idle the water should run 4 or 5" above the end of the hose before it falls away,....
Pull the lines goin' to the risers,.... Flowin',..??

Also oughta check it over with a shoot, 'n read thermometer, Hot to the hand don't mean squat,....
'bout anything over 100? or so is painful,...

 

[rick3452]

I recently went through High Speed Overheat. It was time anyway so I replaced Everything, Exhaust manifolds, elbows, Impeller, Thermostat, checked for blockages etc.
What solved it was was to ADD a Through Hull Seawater pick up tapped into the line to the Seawater pump. Instant success... No mater how hard I run it it stays on 160. Mine is a 2000 5.0 Bravo 3 closed cooling.