Coolant Loss ???

[Dr D]

I have a 4cyl 165hp Mercruiser in a '87 Sea Ray Seville Cuddy. Been running great....no overheating.
2 days ago i took a ride with 5 others onboard. All fluid levels were fine Boat ran "slightly" hotter than usual 175-177', but no higher. Engine DID NOT overheat.
Haven't used the boat since. Checked the coolant level today, and saw that no coolant was visible. Ran the engine a short while, then added about a quart which topped it off.

Question is; what would have caused the coolant level to drop so dramatically ?
Does the coolant evaporate or what ? The engine was pushing a lot of weight with 6 people onboard, but ran well at an avg of about 3500>3800rpms ??

I know overheating or running hot is not uncommon with the engine I've got, but .....it's got a brand new impeller too...
Any ideas??

Thanks

 

[CliffA]

if you happen to have a 3.7L mercruiser engine you may want to look at the web site 'breezeworks' which is dedicated to these engines. I had this engine in a 1986 Chris Craft. it had a big 4 barrell carb and had 190 hp. Ran great and did not give me any trouble but i did have a slow loss of coolant and had to top off the level every week or two. there were no signs of coolant leaking and no signs of coolant being burned in the combustion chamber so i thought there must be a small leak in the closed cooling system that was allowing coolant to escape through the exhaust cooling water. when i bought my current boat the Chris Craft sat unused for around two years. when i pulled it out of storage i found the coolant tank empty and the #3 cylinder full of coolant. since i did not have any use for the boat any longer i gave it to my brother-in-law who is an old 'shade tree mechanic' like me. he tore the engine down and found a very small leak in the head gasket. when the boat sat unused for a long time the coolant leaked into cylinder #3 over time. these engines were known for head gasket problems and for issues with the water cooled alternator. coolant leak can also be caused by a cracked exhaust manifold. there is a lot of good info on the breeze works site about known issues and the needed repairs.

 

[Dr D]

Cliff,
Thanks for the information, and the reference to "Breezeworks".
The possibility of a head gasket leak seems completely viable. I've been topping off the tank as needed.....loss of coolant does not yet seem too critical. I'm hoping there's no issue with a cracked exhaust manifold.
Thanks again for the input ......