Temp suddenly drops down to 100 degrees and then Overheats

[donearly]

Hello and thanks for reading my post. I've spent years reading but have never posted before. Hopefully in the future I can help others as this site has helped me in the past. However, I really need some help here.

I cannot seem to find anyone with this issue. I'm in Clearwater, Florida (Salt) and run my boat all year long. It's in a slip (not on a lift) but I flush it out after ever use. Gear is 5.0 Mercruiser (Non-EFI) with Alpha One Gen 2 Drive, Raw water cooled. A few months ago it started to overheat under load at high RPM. I used a IR Thermometer and found the manifolds and elbows were getting warm. I replaced the manifolds, risers, elbows, water pump housing and impeller and thermostat (160 degree).

Now here's the problem: When under load and higher RPM's (2,500 +) out of nowhere the temp gauge will suddenly drop down to 100 degrees and then climb back to 170 degress (Normal temp reading for this sender). If I don't back down on the throttle to 2,000 RPM's or less the temp will climb until the alarm sounds. If it sounds I back off and idle for a few minutes and it cools down again. Sometime I can run for an hour at high RPM and Load without the problem showing up. But it's happening more and more lately.

Any ideas? I sure would appreciate any input you might have. Thanks in advance!

 

[alldodge]

Howdy

Go to a hardware store and pick up some 1 1/4 ID clear hose. Then put it inline between the intake hose from the drive to the thermostat housing.

What year is your motor, and do you have a serial number?

 

[Darrenmb]

Sounds a bit like its starving for coolant.. the coolant has to be flowing past the sensor for it to read its temp, if air is flowing instead of water, it wont see it as hot but cold, i dont know a whole lot about boats, still learning myself, but have seen this a few times in cars! Hopefully someone with a bit more boating knowledge will show up and point you in the right direction.. but until then, inwould at least check your impeller and hoses at a minimum..

Good luck

 

[boatman37]

Yep. In my experience that indicates it is reading air temp when it drops that fast.

 

[Scott Danforth]

Welcome aboard

could be a sticking thermostat, however I would also look at the O-ring between the drive and gimble