Boat runs hot lately. Ideas on why? (full inboard Chevy 350)

[Prodigal Son]

My boat has a fresh water cooled 350 that almost always ran right around the 160ish degree mark. Period. Then one day, like an idiot, I didn't open my raw water intake. I blew my radiator cap and lost a ton of coolant, scared myself because I thought my boat was on fire (steam and smoke are not that different if you're not expecting one or the other).

Refilled everything, checked my hoses, found a leak or two, tightened that up, replaced the impeller (which was in fine shape) and took her out thinking I was fine. I'm not. It now runs up to 170, holds for a minute and then if you're idling along the temperature will just continue to rise until you go faster. Running on plane the temp holds right around the old mark of 160ish.

What would make it do that? The impeller is brand new and on plane she runs just fine.

When the engine is hot it smells like coolant is leaking on something and burning, but I can't find a leak. My overflow tank is actually overfilled and doesn't seem to be losing any? Thoughts?

I wouldn't be worried if it was only hot at idle and it took a while to get hot because that would almost make sense (not much airflow at 0-3knts), but she runs from 160 up to 200+ in no time flat when I'm not running at 2000 rpm or higher. I ran the boat for an hour and a half today, docked for an hour and then came back and fired it up and the temp was at 180 before I could get my first line off of the dock...like 30 seconds of running to go from zero to 180 and rising.

[welder]

Sounds like you need to flush your block/waterways to get the build up out.

There is a chemical solution and I cant remember the name of it .

[blueh20pc]

My thought is that you didn't tighten down the impeller enough, or the housing is shot.

The first thing that's wrong with anything I did to fix it, is the last thing my hands touched.

Hey, that's gonna be my new quote!

[peterbo3]

How about the impellor housing? If the pump body or plate is slightly distorted, air will be sucked in & the aerated water may not be enough to provide cooling at lower revs. Once you are rolling more water is being circulated & the temp is under control.
Sudden thought.
The heat exchanger normally goes into the header tank & is sealed with large O-rings or gaskets. The exchanger itself is usually bronze or SS & can handle heat. The O-rings /gaskets are not so tough. If raw water is leaking into your closed system then that would explain the full overflow tank. The smell of the burnt coolant could be coolant leaking onto the hot engine. I would still check the pump for leaks but I am thinking the heat exchanger may be the guilty party.

[Capt. Fred]

Could the overheat episode have caused the thermostat to be damaged so its not allowing water to flow at idle? Do you have a water pressure gauge on it?

[Blue Magic]

It sounds like it might be a head gasket. Check your exhaust when your running and see if you see any coolant or excessive steam coming out. Also run the engine and then shut it down for about and hour, When you start it up see if any coolant is coming out of the exhaust.

[Trigger]

Could also check the spark plugs, coolant has a way of making them "very" clean.
If 1 or 2 plugs is dramatically cleaner than the others, could be a head gasket problem.
All of them the same, normal color, could help eliminate a head gasket problem.

[Prodigal Son]

Would an overheat cause the head gasket to blow? I don't think any passages are clogged, at least not with impeller...my impeller was in perfect shape when I pulled it out. Haven't noticed anything coming out of the exhaust when I fire it up after sitting on anchor not running nor any extra white steam (always a little from vaporized water).

[Blue Magic]

Yes...........
One other thing to check is if you have older hoses going from the water pump to the exchanger, check and see if they are soft. If they are soft the hose might have seperated inside and when you put a load on the engine the inside of the hose collapes.

[glassman]

first off what kind of boat and motor merc or volvo, ect... check the thermostate and do a compression test that will tell you if you blown a head gasket also check the water pump even if it is new after all of that which side is hot port or stbd or both if only 1 well check the thermostat housing and see if it is clogged. If still running hot and no blown head gasket check th heat exchanger for clogs ( fresh water or salt water use ? ) if salt take the heat exchanger to a radiator shop and let them dip it and clean out the tubes in it should cost about $50 or so .