Mercruiser 3.7 /470 1984

TKrause

Seaman Apprentice
Joined
Apr 19, 2012
Messages
46
I have a 1984 Century 3000 Cuddy with the always interesting 470 powering it. This year upon de-winterizing it and starting it i noticed it was filling the overflow bottle and it's not looking like coolant, it's very clear and watery, at idle only this seems to occur, at 1500 rpm's it no longer fills it but it doesn't draw it into the riser either.
Tonight i pulled the pressurized cap off the riser and proceeded to fill it as i didn't see any coolant. Within seconds it was draining out the gimbal ring ports at the bottom where heated lake water would come out i assume. I fired the engine up and proceeded to add coolant, within 20 seconds or so it was pushing out the top of the cap (the engine had only been running for 2 min tops, so not even near being warmed up), my question would be is it possible i'm seeing two issues? Both a bad H E and a bad head gasket? Note: there is no coolant in the oil, I have not changed it this season yet and it is still golden brown from last season.
 

stonyloam

Vice Admiral
Joined
Mar 13, 2009
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5,827
It could be that you have a bad exhaust elbow and the seawater exiting the exchanger is mixing with the coolant (rust hole or bad gasket) A compression check will tell you about the health of the head gasket.
 

TKrause

Seaman Apprentice
Joined
Apr 19, 2012
Messages
46
Thanks Stony, am i right in guessing that the way to check the elbow is to remove it and inspect?
 

stonyloam

Vice Admiral
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Mar 13, 2009
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Yeah, take a look. Inspect the gasket surfaces too. While you have it apart you can check your exhaust shutter too.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
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Jul 23, 2011
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50,228
Time to do a pressure test of the system. You either have a bad riser or manifold, or your heat exchanger has an issue
 

TKrause

Seaman Apprentice
Joined
Apr 19, 2012
Messages
46
Thanks again Stonyloam, pulled the riser and found the open over one and a half ports that should have been blocked. Cleaned and reassembled and bingo, all appears to be working as designed. No bubbles or abnormal movement in the coolant. And no overflowing expansion tank.
 

stonyloam

Vice Admiral
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Mar 13, 2009
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5,827
The blind squirrel strikes again LOL! Gotta keep those old 470's hummin....
 
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