Mercruiser 350 Mag acting up

[CobaltDeadHead]

Having some issues with my 300HP, 350 MAG with Bravo 3 lately.


About three weeks ago I was running just at normal cruise around 3500RPM. Engine threw an alarm and then eventually started sputtering out. No noticeable change in oil pressure or temperature. I basically limped back to the marina going slightly above idle speed without issue. I had the marina take a look at it. They ran the boat for an hour and couldn't repeat the issue. Alarm diagnostics didn't bring anything up.

I ran the boat probably a half dozen times since except for last Friday and this Monday. Four of those days it ran fine. Friday I had a couple of friends on the boat for the first time and they wanted to see what kind of juice she has. So, I ran it WOT for a bit. After basically 60 seconds I'd get an alarm with the temp too high; up around 200. Back off the throttle and the temp drops down to 160-175 and alarm goes away.

Monday I took maybe a 20 cruise at again 3500 rpm. Boat ran fine. Hung out on the sandbar for 20 minutes and it became clear our 21 month old daughter needed a nap. She falls dead asleep when cruising at about 10 mph. So, we did just that for about an hour. Stopped back at the sandbar for a 20 minute swim again and then it was time to hit the road. Engine ran fine during the cruise. Start the boat up again (with drive up due to shallow water) and almost immediately get an alarm and notice the temperature gauge is running hot at about 200. Sandbar is adjacent to the channel for our marina, so it was just a 5 minute no wake ride back. I haven't had the marina take a look at it since, but plan on it soon.

This is a new to me boat. Roughly 300 hours on engine. Impeller was changed this spring.

Thoughts?

[BigGib]

First guess would be impeller or thermostat issue? Sand can clog things up, experienced it first hand on the river in our old four Winn's which also had a 350 mag. I've also had sand clog and burst a water line and smoke the water pump in the process.

[akorcovelos]

Impeller most likely. I’m on my 2nd one this season.

[Big Block Power]

I'm with both of these guys. Could have a restriction somewhere too.

[Whippled263]

Check the impeller