Help with engine overheat - '08 Merc 4.3L I/O UPDATE

[momay4000]

Gang,

I have an '08 Merc I/O 4.3L:

Weird thing has started happening this year - boat only overheats for 10-15 seconds only when I start out in the morning going to a full plane out of the I-Bay channel. It does NOT overheat at idle, nor does it overheat cruising at 5 knots from my dock through the channel to the lake. As soon as I achieve full plane, it overheats and I have to shut down.

As soon as I shut down the boat, restart the engine and push it to 4000 RPM at idle, it instantly cools off and things are fine for the rest of the day, including trolling and cruising back in. I can run the boat after that for hours and it maintains a constant temp between 170-175 degrees whether trolling at 2.5 mph or cruising at 35 knots back in.

My guess is I have sand, rust, debris in the system which gets flushed out each time, or perhaps it's an impeller problem (I have replace it every other year but not last year)

Any thoughts would be helpful,

Thx,

- Chris

[Big Dave]

thermostat sticking?? maybe.

[L&M]

thermostat sticking?? maybe.

That would be the place to start providing your confident the impeller & intake screens are clear. Are you running thru any weeds or floating algae getting off the dock or out of the marina?

[ray koziatek]

dildo er i mean ditto hanks (thermostate)

[momay4000]

Thanks for the replies gentlemen,

I-Bay is loaded with very dirty water, lots of floating debris, but I'm not running through anything too thick on the way out such as weeds, algae, muck ,etc.

If the thermostat is "sticking" - should I pull the boat immediately and get it fixed or simply keep an eye on things since it levels out after only a few seconds or so?

Thanks for all the help guys,

- Chris

[lakebound88]

I had the same issue with my Merc 350 5.7 EFI. Come to find out, little zebra messels made their way into a 3 week old impellor and ruined it. Had to pull the boat to get it redone. My volvo 350 (current) is doint the same, except its not overheating it is just opening later on the stat. I believe it is sticking and I purchased a new stat for her this past week. Listen to Hank though, He knows his stuff and is a great guy. I still owe ya Hank!

[ray koziatek]

t stat 15.00 impeller 24.00 guess i should of said t stat thx ,,lak b

[rshubuck]

Diddo! Change the thermostat! Stat Its the cheapest route for now without pulling the boat. Then run her once to see if it fixed the issue. But my guess is its an impeller problem and you will have to pull your boat but its better then repowering your boat it the near future. Good luck

[NitroMusky]

A wise merc mechanic has said always start at the source: the impeller. Like hank said, Make sure that you indeed haven good water supply coming in as the basis to start from, then worry about everything 'downstream' - especially if as you said, the impeller has not been replaced recently, since it is indeed a 'wear item'.

That said I'd do the 'stats at the same time, given the minimal cost and involvement. That is where my money is as far as the problem goes. Sounds like it's sticking. When you turn off the motor, no coolant flows, and temps continue to rise a bit, which finally get it hot enough to free the suspect stat and get it working again until it sits again at the dock for a while and the cycle repeats.

Good luck. Oh and yes...do not wait. You could be sitting on a potential time bomb, a very expensive one at that!

[momay4000]

Engine update,

I like reading this forum, so I wanted to give an update on what the problem actually was with the overheat, so that it may help others in the future. I pulled the boat almost immediately and here's the verdict:

Thermostat was fine, but we still replaced it with a 160 deg. stat

The overheat was from a bad impeller - it looked like it caused a little melting of the water pump so we replaced everything.

Exhaust was fine (not blocked, no evidence of zebra mussels anywhere). Heatgun readings on the engine read 165 degrees, the guage on the dash reads 177 deg. like it always has.

Also, on the day I pulled the boat, my raw-water pump stopped working as well. This was from a bad pressure sensor ($35 part).

Historically I replace my impeller every other year - it will now be every year.

Thanks to everyone for their help - should be back on the water in a week once we get the parts for the raw water unit.

- Chris

[UpGrady]

Chris,

Sorry to hear about the problem... was out on Sunday for a ride and saw your boat was missing from Mayer's and wondered.

The every year on the impeller as well as dropping your lower unit is the safest bet for you with a single I/O and all of the trolling hours you amass during a season. I push my twins to every other year, but if I was out every weekend and putting hundreds of hours on per engine, I would do everything yearly as well. You also have the worst thing in that area with the weed and moss coverage at Mayer's. It could have been the perfect storm where you sucked something into an intake that helped to smoke your impeller as well. Back when I had my old boat and docked there, I could not make it past the channel without reversing to unbind the weeds on my prop, it was horrible...

Good luck and hope you're back in soon!

[jimski2]

A loose fan belt for a water pump was a problem once.