What do I check next on overheating issue?

[codyj24]

2003 Mercruiser 6.2L w/Bravo3
Engine overheats while idling and when applying 70% or more throttle.
Going 10-30 mph - the temps stay somewhat comfortable.
Boat is 24' Starcraft used only in freshwater lake.

I replaced raw water impeller and thermostat (both looked fine) - but overheat issue remains.

Any ideas?

 

[poconojoe]

I'm far from an expert, but here's some things I can think of....

Is it truly overheating? How are you judging that? Not that I'm doubting you, but we need to look at all aspects. The gauges on our boats can sometimes be inaccurate.
Overheating is definitely a big deal and not to be taken lightly, so it's good that you're on top of this.
An overheating I/O engine is way more detrimental than an overheating car engine.

Is the correct (temperature) thermostat installed and correct gasket?

I wonder if you have some sort of blockage somewhere. Sometimes broken parts of an old impeller can Lodge in somewhere. Same for parts of a broken exhaust shutter (flapper). Exhaust shutters can melt apart if you had an extreme overheat situation. The metal parts can fall down into the wye pipe and cause a blockage. I found one of those metal plates all the way down inside my drive when I replaced the impeller.

There's also a water pump on the front of the engine, just like a car. Maybe that is not functioning fully or perhaps the belt is slipping or the pulley is slipping?

Sometimes air can get into the system and cause poor cooling. Maybe a loose hose clamp? I've read where you can add in a temporary piece of clear hose somewhere so you can see if air bubbles are in the flow of water.

Just some ideas from a guy that's no expert, so let's see what the experts say. I'm sure they will come up with something.

 

[Lou C]

If you're overheating when idling & at higher speeds but not at moderate revs, that points to low raw water flow, if you have a new impeller, did you change the housing and wear plate as well? The housing ususally doesn't need to be replaced unless the plastic was distorted from a bad overheat but the wear plate should be replaced if it was even the least bit scored. These open or raw water cooled systems depend on a high volume of cool water in and warm/hot water out to keep the engine cool. I might put the boat in the water and disconnect the raw water output hose from the impeller and see how fast it fills up a 5 gallon bucket. If the flow is weak you may have what's known as Bravoitis where a fitting in the transom mount corrodes and necks down the raw water feed hose that goes through the transom. This requires pulling the drive to even diagnose. I'll leave it to the Merc guys to comment further.
The other thing it can be is your exhaust elbows, when they start to corrode and get clogged with rust flakes that can cause hot running as well. Not as much of a problem in fresh water than salt, but still can happen and on a 2003 boat you're talking a 19 year old exhaust system. Have the elbows ever been off to check?

 

[poconojoe]

Great explanation by @Lou C .

Just another thought... Correct me if I'm wrong, but I think it is possible to install the impeller with the vanes facing the wrong way.
However, I believe you had this problem before changing the impeller, so it's likely not that.

 

[tank1949]

2003 Mercruiser 6.2L w/Bravo3
Engine overheats while idling and when applying 70% or more throttle.
Going 10-30 mph - the temps stay somewhat comfortable.
Boat is 24' Starcraft used only in freshwater lake.

I replaced raw water impeller and thermostat (both looked fine) - but overheat issue remains.

Any ideas?

Go buy a cheap inferred thermometer. Determine if the thermometer reading is close to your boat's temperature readings. That old of boat, I'd pull risers and rod them out.

 

[Lou C]

This varies by engine & cooling system design but I have found that the area right under the thermostat housing reads the closest to the temp gauge, usually about 15 or so degrees less than the gauge. My engine has a 160 stat and it runs from a little under 160 to at most 175 after coming off plane, but it will start to cool down at idle pretty quickly. I leave an IR temp gun on the boat and keep track of temps when things are working well. Then I have a basis to compare to when something changes.
For example the exhaust elbows are usually at 90-100*F at idle, then after coming off plane they will rise to 125-135*F, but soon cool off after idling.

 

[kenny nunez]

If every thing suggested checks out then you probably have a case of “Bravoitis”. This is a phrase that the Donzi and other sport boat owners call it. From the age of your drive you may be getting exhaust gasses if your exhaust exits through the drive. Also there may be a foreign object in the in coming water line.
Mercruiser recommends with Bravo drives to have 2 water supplies. Add a through hull high speed strainer with a valve plus a “Y” connection from the drive connecting the incoming water to the through hull fitting to the raw water pump.

 

[Scott Danforth]

Overheating is a lack of cooling water flow.

Check for bravoitis, check the PS cooler for debris

 

[codyj24]

I'll get to work pulling elbows/risers apart for inspection.
I will also get an infrared thermometer to check when running to verify temps.
Boat did have some steam coming out of captain's side exhaust ports and engine started to stumble when temp showed high when idling.
I did not notice any visible issues on impeller or thermostat when I changed both of them - I didn't replace the wear plate.

The Bravo 3 drive was off and rebuilt 2 years ago and then taken off a month later due to lube filling up in the bilge area due to bad o-ring on drive. Nothing was visibly bad - but who knows.
thx!

 

[flashback]

Replacing the elbows solved it for me, the original elbows were trashed. I also replaced the exhaust hoses as the hose from the Y pipe to the down elbow was toasted. post a pic of yours when you get them off.

 

[Scott Danforth]

The Bravo 3 drive was off and rebuilt 2 years ago and then taken off a month later due to lube filling up in the bilge area due to bad o-ring on drive. Nothing was visibly bad - but who knows.
thx!

Unless you look for bravoitus, it may not be initially obvious.

 

[QBhoy]

Hi. Is it alarming as an overheat ? Or just showing high on gauge ? What’s it running at ?
If it’s genuine, I’d check the power steering cooler for leak or drawing air.
If it’s overheated previously, the exhaust flapper may have melted and blocked up somewhere, but perhaps unlikely if you have the side exit option too…unless it’s affected up stream of this.

 

[codyj24]

Pulled off exhaust manifolds and risers and cleaned them up - they weren't clogged or too bad looking. Flappers and hoses all looked fine. Replaced belt driven water pump, cleaned thermostat housing again and put a new 160 deg t-stat and gasket. Still getting hot quickly when trying to troll or go at slower speeds. Once started you can see temp rise and fall after t-stat opens and then after a little bit it will start rising again past 210 deg. Putting infrared on t-stat housing will show 200 deg while risers are a lot cooler.

 

[Scott06]

this guy shows how to check for bravoitis

I would think u can do it with out janking the engine if you are young and skinny (aka not me)

Here is a thread on testing water flow from raw water pump - it think 7.5 qts in15 sec

Is this normal water flow? Bravo 2 outdrive

Hello I was wondering if anyone can verify if the water flow i have coming from the transom to my raw water pump is sufficient? 1998 Maxum 2700 SCR 5.7 Mercruiser 2-barrel Bravo outdrive. I started a thread earlier on air in the cooling system. If anyone else has any ideas or tests I'm happy...

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