10M 454 closed system overheating

[Trojan10m]

I have a 1982 10m, with 454 350hp closed cooling that is overheating and melting exhaust hoses. I am at a loss as I have done just about everything except replace manifolds and circulating pump. The impeller was new and checked good flow. There is no coolant/ water leak outside the motor and no chocolate milk inside the motor. I have checked all basics and in between many times... raw water pump fins, tranny cooler, pickup tube. Here is what I have done so far....

Last year I overheated, found out no antifreeze in tank. Filled it up with new antifreeze and took trips to downtown for dinner 15 mins each way. After trips checked the level in heat exchanger and it was a little low. I swapped heat exchanger side to side to see if problem followed it did not. I opened the end caps of the exchangers and backflushed and check tubes for build up. Nothing was there and looked good. Put it back together and still losing antifreeze. Pressure tested heat exchanger on motor with radiator pressure tester, it held good pressure. But sill losing fluid. Pulled off the heat exchanger off as I had a yearly vacation and pressed for time and plumbed it as an open system without splitting the risers as I wanted to go back to the original system. It ran a little hot at 200 removed thermostat runs 180. Was still running hot so took the block down to short block thinking it could be a head gasket. Really had my doubts along with our friends but running out of ideas... head gaskets were good, but found flatten cam shaft during process so replaced. Out of time and gambled made the trip fine 1.5 hours each way, ran 180 all day long. Finished the rest of the year like that with no problems but still wanted the closed cooling back on the boat.

Fast forward to 4 weeks ago same set up still on the Boat and take a weekend trip 2 hours each way. Boat ran good no problems on the way there. On the way back it ran great no problem but more steam from that side, temp was good. Checked after trip and exhaust hoses were melted from riser to 90 fitting and to the T fitting other the other bank. But good after the T fitting. Melted so bad is ran out the ends of the hoses. After removal it burnt through the inside layers.

I ran motor with the exhaust hoses off to check flow it has good water coming out. I took the heat exchanger to radiator shop it test good, did multiple test no problems. Remount everything back to stock with heat exchanger and no thermostat and new exhaust hoses and filled with water. Headed downtown for fireworks at cruising speed 2800. 5 minutes into the trip someone smells rubber and temp is climbing fast to 150. Opened the hatch and water peeing out the overflow. Turned off and idled there and back.

I am going to do the circulating pump in the next day. My thought is maybe the fins corroded away from the inside. It’s 35 years old I know it’s a slim shot but it’s $40 and cheaper than exhaust and motor.

Please let me know if you have had this problem ideas thoughts anything. Thank you. David

[prowlersfish]

If your burning exhaust hoses your issue is low raw water flow . As far as a water pump for $ 40 is a Automotive Pump ? Will it pump in the direction the engine turns ?

[RWS]

suggest you invest $50 in a decent I R thermomitor

run both engines and compare the relative temps at different locations.

this way you can compare temps and isolate the problem

if you have side exhausts, might check for obstruction in the chamber - possibly some marine growth?

RWS

[mikeandanne]

Those hoses need to be installed at just the right angle on the riser to allow the water to swirl inside the hose to cool it.Sometimes people step on or sit on the hoses while working down there and push the hose down, just something to check if you have good flow from the riser as you say.

[P-Dogg]

suggest you invest $50 in a decent I R thermomitor



if you have side exhausts, might check for obstruction in the chamber - possibly some marine growth?

A must-have item! Great for checking everything from packing temps after running, and more importantly, seeing which can of beer is the coldest!

Zebra mussels???

[Trojan10m]

Well didn’t do the water pump yet. Pulled hoses off after trans cooler and before heat exchanger and checked flow between the motors at idle. Gave the motors some rpm and the flow is solid out of hose Of good motor and no change on the bad motor. So I think it might be a bad hose from the thru hull to pump and it being collapsed under load. It is hard to believe a reinforced hose with metal fails, butI I will find out. Thank you.

[Trojan10m]

Well replaced hoses to sea water pump, to tranny cooler and back to heat exchanger. Disconnected hose from exchanger feed hose and ran motor the flow didn’t change, checked sea water pump. Just in case it change during the last little bit, imppellar was good. than ran motor with the hose discounted from sea water pump from tranny cooler and flow was great,when revved up a solid 1 1/4 flow 12inches or more. Thus I think the only thing it can be is the tranny cooler.

With that said...
Who has the most ecomincal replacement. And thank you alll...

[prowlersfish]

Have you looked at the cooler to see if its plugged ? They can be cleaned . Beacon Marine should carry them . Also the Cam in the water pump could be worn . Your really need to look at the risers Common issue .

[tonydelgreco]

I had the same issue except I did not melt hoses. Changed only one side of port engine, manifold and riser after I blew a hole in the exhaust manifold during a backfire. Motor temp looked good on the gauge but was steaming. Checked with Infrared gun and new manifold holding at 150 but old manifold at 220. Moral of the story is the manifolds and risers play a major part in cooling the closed system and should be changed every 5 years.

Tony

[Trojan10m]

Update and thank you. I got a hook tool and picked and backflushed 7 full impeller blades and enough little pieces to make two more. Overheating problem fixed...
I also used the good motor and pump to back flush the other motor and pulled four impeller blades out of that motor. So now I can confident both motors cool great. I ran 2 hours there and back for my vacation on no problems. Thank you again.