97 Mariah Z222 Shabah Overheating???
 

First off, I haven't torn into the problem yet...

Out on the lake, boat ran fine all day until going back to docks. This was the only time that day I went WOT when I got an audible alarm---> temperature gauge was around 250F. Was hearing gurgling noise from the thermostat housing. So I fired boat back up and drove at a no wake speed without a reduction in temperature. At the time, there didn't appear to be water in oil and had no issues with cranking or driving.
I am looking for help to pinpoint issue/opinion without having to completely tear into everything. The water impeller was replaced approx. 5-7hours of use ago.

Could it be the thermostat, engine circulating pump, or defective water pump/impeller? Curious as to why the gurgling noise from what appeared to be from thermostat housing/ front of boat?

97 Shabah Z222
5.7L carburetor Mercruiser
Alpha gen2 outdrive
Approx. 555hrs

Thanks for quick responses!

Re: 97 Mariah Z222 Shabah Overheating???
 

Thermostat blockage or failure or cooling system blockage would be my first thoughts if you recently replaced the impeller.

Re: 97 Mariah Z222 Shabah Overheating???
 

I'm really hoping it's just the thermostat...I guess I can pull it and check to see if it's bad...it would be the easiest to replace, without having to break open the stern drive to inspect impeller, although I probably should just to be safe.

Re: 97 Mariah Z222 Shabah Overheating???
 

Is the boat in the water or on a trailer? If in the water you can try pulling the hose from the outdrive to the thermostat housing and run the engine. If you get a geyser from the hose then you know the pump/impeller is ok. When the old impeller was changed were there pieces missing from it? If so then they may be stuck in a cooling passage.


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Re: 97 Mariah Z222 Shabah Overheating???
 

Good thought, I'll check the schematics to know which hose to pull and try. Even though it will only be running for mere seconds, I hate to get the bilge filled with any water...maybe I'll use a bucket to attempt to catch the geyser!

When I did replace the impeller 3 yrs ago and not many hours ago, it was fine and in good shape with no broken pieces.

Re: 97 Mariah Z222 Shabah Overheating???
 

Actually that might be your issue, 3 years and very little use since then may have caused the new impeller to go a bit brittle and break up. Normal change period is every 3-5 yrs on an impeller.

Re: 97 Mariah Z222 Shabah Overheating???
 

Just for my education... I thought the 5.7 had water pumps on the engine? Is there also a water intake impeller in the stern drive? Last year I had 2 separate episodes of overheating during the boating season but they immediately went away (within 2-3 minutes) and I could not figure out what happened.

Re: 97 Mariah Z222 Shabah Overheating???
 

The gurgling sound from the thermostat was water boiling at 250 degrees

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Re: 97 Mariah Z222 Shabah Overheating???
 

I bet bad impeller as mikey t suggests. You can.see.the impeller start sucking water on the muffs. The leakage around thebmuffsbwill very briefly stop as it sucks water and fills the lines.to the water pump. Then shortly after that you should see it flowing from the outdrive exhaust ports and the property when revved.

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Re: 97 Mariah Z222 Shabah Overheating???
 

One thing I did notice is that the engine temp gauge when overheating barely went over 250 degrees F. Is anyone familiar with the max. range that the engine temp gauge goes too? If the gauge is capable of going higher...this leads me to believe that maybe the thermostat was partially stuck. If the gauge only goes to 250, then it could possibly be either the thermostat or the impeller.