Need help! I’m stuck and boat overheating can’t figure out why

[Iwantacarver]

Boat still overheats at the dock it sucks

[Phrancus]

Some thoughts:

Any rpm, only difference is a moving boat

Overheating is either nothing to exchange heat into (throughput) or excessive heat production. Idle and higher RPM are causing problems so I'd discard the latter. So no or not enough throughput. Isseus at any RPM so let's rule out regulating equipment like pump (high RPM working, low not working would imply slipping or wear) and thermostat (cooling is started at operating temp when moving, so it opens)

What is the relation between moving/not moving and continuously exchanging heat into coolant into water?

differences moving / not moving boat:
intake: stern drives have little holes at the front. When moving there is pressure but thats a little bit compared (I think) to what the pump does to the throughput. Nevertheless: check if the pump can pull water in. En route are the little holes and the bellows (I take it you have sterndrives, I'm not familiar with other setups or gas engines)
a thought: can the bellows be sucked so that they 'implode' if they are too weak and/or the intake is difficult? And when on the move, the pressure is enough to supply the pump?

throughput: can't think up a difference. when there's something reducing flow, it does so at speed too. I can imagine high and low RPM to react differently to a lime buildup/ squished hose and such) but no relation to speed.

exit: discharge into the exhaust. If this is not good enough, the water can't exit. Argue against this is that it does so at all RPM when not moving. If only at low RPM, perhaps the power of the exhaust and water flow pushes something open that is not open at dock. Like te other bellow or a flap at the exhaust in the sterndrive.

Other difference I can think of would be the angle of the boat and thus the engine and the water intake. But cavitation would then be a thought when on the move and not when not moving.

One last thought: I have an oil cooler (I have diesels, not your type of engines, don't know if you have those too) that is cooled by it's own heat exchanger. Theoretically, if that exchanger is not working properly, the cooling fluid system would work fine but the oil could still overheat and cause trouble. Don't know what that would trigger in terms of an overheating alarm.
Can't come up with a difference with the oil in moving/not moving though, only RPM and that was not a clear difference.

One weird far out of the box idea: any weed growth in the dock? I know it sounds stupid but if there something like mud/growth that can block the intake when you are docked... it's not there when you're out and about.


Perhaps some thought of a detail is triggered by the thoughts above.

[Viper]

All good suggestions but I'd start by wanting to know what brand of engines are we talking about unless I missed something. Would also help to know the model of the boat and what the production year is. The more info about the application, the better.

[Iwantacarver]

2005 carver 36
Crusader 5.7 350 gas engines

[tomschauer]

Replace the raw water impellers

[Iwantacarver]

Impellers are brand new

[Viper]

When the impellers were replaced, where the old ones intact? If pieces were missing, did you backflush the system to get them out? You should probably do that even if they were intact in case there are some pieces still lodged in there from a previous overheat/owner. This is where the exact model umber of the engine would help; pretty sure the engine oil cooler is before the raw water pump in that vintage, so while there may not be any impeller pieces in there, there might be marine growth. After the pump, depending on the model, the first item in line would either be a fuel cooler module or the transmission oil cooler, then on to the heat exchanger, all are suspect for blockages.

When the impellers were replaced, did the inside of the pump housing and cam get inspected for wear? New impellers won't do much if there is excessive wear in the pump. Which brings me to the next set of questions;
- how long have you owned the boat?
- where in Canada is she?
- is this a fresh water or salt water application?
- if fresh water, has she ever been in salt water?
- do you do any shallow water anchoring?
- are there a lot of weeds in the marina?

Also, what model number 36 are we talking about here, an aft cabin or sedan bridge? We need to be more specific, it can make a huge difference and have a bearing on how we diagnose and make recommendations.

[bud37]

QR_BBPOST Boat still overheats at the dock it sucks

Both engines exactly the same ?

[Iwantacarver]

Both engines the same - and impellers were never ruined changed for maintenance regular

[bud37]

QR_BBPOST Both engines the same - and impellers were never ruined changed for maintenance regular

And is this when the overheating started.....after the impeller changes on both engines ?