Mercruiser 888 overheats once at first, fine for rest of the day. Sometimes.

[larboc]

1973ish 888 mercruiser (302) Ford that runs great but when you first put it in and throttle up to get on plane, the temp gauge climbs up to 190-200 degrees before the thermostat opens and the temp falls back to 160 and stays there for the rest of the day. Never had any issue other than that till yesterday, we were trolling for about 3 hours at slightly off idle. I had to shut the engine off for a false alarm tangle, I fired right back up and started trolling again. I watched the temp gauge and it slowly climbed up to 220 on the gauge (i'm assuming it was boiling lake water). I blipped the throttle a little more and then the temp dropped back down to 160 for the rest of the day. Never happened before. Ran fine after that except there was a little bit of what my copilot described as smoke/steam coming out of the outdrive that smelled like gas for about 30 seconds after it cooled back down. I dunno what to make of it. My first instinct was thermostat but my t-stat housing is sketchy and I really don't want to change it if it isn't the problem. Read a couple people with similar issues that changed it and it made no difference anyway. Impeller is 3 years old but this has happened since day 1. Raw water cooled. I should add that the boat is only ever operated in lake superior since I've owned it so the water is typically very clear and cold. Was 55 degrees lake water temp when this issue happened yesterday.

 

[spoilsofwar]

A three year old impeller is ready for replacement, and at the very least you need to remove the tstat housing and inspect the thermostat... Since you're going in there, "sketchy" or not, I'd just replace it. Both of these things are preventative maintenance items that are supposed to be replaced before they give you issues.

 

[larboc]

The issue with the t-stat is the housing. The housing had frozen and cracked I think at some point before I got it and it's kind of still together with RTV and jbweld and working, if I touch it I think I'll have to replace the whole housing and it's marine specific. I'll probably replace the impeller next spring, but like I said it did this with a day old impeller too.

 

[Scott Danforth]

The issue with the t-stat is the housing. The housing had frozen and cracked I think at some point before I got it and it's kind of still together with RTV and jbweld and working, if I touch it I think I'll have to replace the whole housing and it's marine specific. I'll probably replace the impeller next spring, but like I said it did this with a day old impeller too.

Suck it up and replace the busticated t-stat housing. A used T-stat housing from Ebay is a hell of a lot cheaper than a new motor.

While at it, replace your impeller. A few $ now will prevent thousands of $ later

Your problems are cracked and busted t-stat housing (per your words), and questionable impeller and t-stat