Mercruiser 260 Overheating Mystery

"Hi, im hoping someone can hel

"Hi, im hoping someone can help me with this one. I have a 1987 Mercruiser 260 in a 19' Four Winns Boat, Alpha One outdrive. Recently the boat has begun to run hot, but not appearing on the gauge. I noticed it when I smelled burning rubber, then when I shut off the engine, the engine dieseled for a minute before it sputtered and finally shut down. I opened the engine cover and it smelled hot and felt hot. Checked the gauge and it was fine but increasing as it sat there. It seems (im not 100% sure) that I can drive at slow speed without a problem, but it overheats when at higher speeds. I replaced the thermostat and the lower unit water pump, but still the same problem. I also removed the hose from the t-stat housing to the bottom of the exhaust manifold on the port side and ran the engine and plenty of water came out. I removed the brass plug on the exhaust riser on the Port side and ran the engine and water came out a few inches from that hole. After I replaced the water pump, I noticed there was considerable more water coming out of the Port side exhaust. Both manifolds feel just about the same temp to the touch. I am thinking about ramoving the exhaust risers to check for obstructions but wanted to check here first.... anyone have any thoughts?"

"[i]smelled hot and felt hot[/

"smelled hot and felt hot
A $15 infrared thermometer is a little more scientific. A running engine should feel hot. Heat has no smell.

Maybe your drive coupler is burning up, not cooling related at all.

Clogged risers would make the engine overheat.

But first step is to verify your gage and see if the engine really is getting hot or not."

"Hy Stat is right about the in

"Hy Stat is right about the infrared temp gun; it will locate hot spots quickly. I assume the engine is cooled w/sea or lake water and not a heat exchanger. Is the circulating pump belt tight? Have the exhaust components ever been replaced or acid cleaned? Alot of rust, scale and mineral deposits can accumulate on the walls of these parts and restrict waterflow and heat transfer."

May have a broken shaft on the

May have a broken shaft on the engine circ pump.

"Thanks for the replies! The e

"Thanks for the replies! The engine feeling hot was more that it was more heat coming off the engine than normal, the smell was the smell of burning rubber (probabaly the exhaust tubes).

I removed the Starboard side riser, there was some scaled rust inside but plenty of room for water passing through. I shined a light through the passages and it really didn't look to bad. I installed a new gasket and put back together. I removed the t-stat. I also removed the hoses from the t-stat housing and checked for blockages, found nothing. There is 2 ball valves inside the housing for the hoses that go directly to the risers... Those appeared to be good. I moved them both inside and out freely and they popped back in fine. I put everything back together and ran the engine and now there is more water coming from the Starbord side exhaust than the Port side as before!?! It appears to be exactly opposite. Port side now is down to a spray, Starboard side has water running out.

Engine is fresh water only and cooled with lake water.

I will get an IR Thermometer.

BT Doc: Circ pump.... Interesting thought.... will check that!"

"The inspection of the risers

"The inspection of the risers is not the LARGE opening.
It is the small rectangle holes on the out side.

Te amount of water that comes out of the left or right at the transom does not matter it is just a simple path of least resistance.

Here is a rough pic of what to look for.


"

"Thanks! I appreciate the draw

"Thanks! I appreciate the drawing! When I removed the riser, all 4 openings were clear. There was some coating of rust, but not even close to blocking any of the passages.

Good to know that it doesnt matter if more water is coming from one side or the other. I thought for sure that was a crucial symptom."

"there was some scaled rus

"there was some scaled rust inside but plenty of room for water passing through"

When you had them off you should have cleaned them and the elbows. That would prevent a future problem and eliminate a guessing game.

Nice drawing! Is this the same

Nice drawing! Is this the same for a FWC system?

"For a "closed cooling&#34

"For a "closed cooling" system that has a HEAT EXCHANGER, the bottom view portion, the four small holes are sealed off to keep the anitfreeze/water mixture in the engine and manifold and heat exchanger circulating as in a auto with a radiator for comparison."

"I should add, the four holes

"I should add, the four holes are sealed off with a stainless steel block off plate."