Engine Overheating

[spoolsout]

1995 Merc 454 Gas engines. Port engine runs hotter than the Stbd engine when at cruise, about 175/180 vs 160/170. Real problem occurs when I throttle down to idle or shut it off and restart it. Temp jumps to 200+ and alarm goes off. Throttling up in neutral does nothing, however if we get up on plane it starts to drop back down. It is not consistant in that sometimes it doesn't overheat at idle which is what has me confused. Checked the strainer already and will be replacing the thermostat this weekend. Any other ideas?

[Bullish]

Sounds like the thermostat might be hanging up on you at different times.

I assume this boat is fresh water cooled.

Do you have good exhaust water coming out the pipe? If you have good water coming out your exhaust, you can rule out a lot of potential problems like clogged strainer, clogged intake grill on bottom of boat, bad impeller, bad water pump, etc.

Thinking further.....Have you ever cleaned the inside of the intake grill on the bottom of the boat. I keep my boat in NJ too and every Spring I clean a lot of worm-growth out of the grill with a very thin flat-head screw driver. Perhaps water is forced through there on plane but can't get through at idle.

[Nutz]

My guess is a bad impeller. If you are running it stays cooler as water is forced thru by the motion of the boat. At idle or while revving it overtemps. If it isn't the impeller I would think there is something that is keeping the water from getting to the exchanger. If you know your impellers are good I would look at the cores of your heat exchangers. I know on a cummins you clean them by pushing a 3/16 in rod thru the core.

[hammerhead]

How about checking the risers, when was the last time they were replaced? under high rpms the water might have enough pressure to keep the engines cooler, low rpm lower water pressure, maybe not enough to keep cool?I have 1997 volvo gas engines 650 hrs, my port side always runs hotter than my starboard side, replaced risers, thermostat, and still the same.Let us know how you make out. Harry

[spoolsout]

Thanks for all of the input. Engines are closed system FWC. Only the risers and heat exchanger have raw water in them. Checked the grills at he beginning of the sason, about 4 weeks ago and they were clear. We will be replacing te thermostat this weekend so hopefully that will fix it. Don't think it's the risers since the engines were rebuilt last year and only have about 100 hours on them. Water pump is an option but we will start with the thermostat first. Thought about the heat exchanger since we have already replaced the stbd exchanger but antifreeze appears clean. I'll post as we work through the process of elimination.

[spoolsout]

Replaced the thermostat and although it was somewhat frozen did not cure the problem. Pretty sure at this pont that it is a raw water pump impeller. Being replaced this week so we'll see how that works out.