possible overheating issue

[Tmakaro]

I have a 4.3 mercruiser TKS. I just put in a new thermostat because the old one was sticking a bit. The temperature is about 175-180(just a little past 175) when warmed up and cruising. When I go from cruising to idle I notice that the temperature rises a little bit and I get a few beeps from the alarm and then it drops back down to the normal temperature of just past 175. This never used to happen.

My old thermostat would stick a bit while warming up and get to about 180 then before it opened up. The alarm would go but then the temp would drop to 175 or just a hair under.

Prior to putting in the new thermostat I did have the coolant in my engine block drain while cruising because a plastic drain plug decided to break. However, the alarm caught that and I shut the boat down right away so I don't think I have done any damage.

I checked the impeller and it looked fine so I don't know what else would cause this beeping. The oil pressure reads fine and the gear oil jar is full.

What else may be causing this? My risers starting to go? The engine is 10 years old and has 450 hours on it.

 

[alldodge]

Do you have open or closed cooling?

What is your serial number ?

Are you in fresh or salt water?

Assuming Alpha drive

 

[Tmakaro]

closed system that I added about 8 years ago at 51 hours on engine.
1a618334
salt water
yes alpha drive - impeller is in excellent shape

 

[alldodge]

closed system that I added about 8 years ago at 51 hours on engine.

Looking up the serial number it list a belt driven water pump for Alpha and Bravo drives for closed cooling. The Alpha drive is most likely unable to move enough cooling water into the heat exchanger. I would suggest installing a 160* to help it run a tad cooler, but even that may not be 100%

https://www.mercruiserparts.com/bam/subassembly/31494/13798/210

Looking at the closed system, it could be with Alpha drive they used thru hull

https://www.mercruiserparts.com/bam/subassembly/31494/13798/190

 

[kenny nunez]

2Xs with AD. With the only incoming water supplied by the drive pump you are on the ragged edge. The best thing you could do is with a belt driven raw water pump pulling from a through hull fitting. Tie the drive pump to the incoming through hull fitting with a “Y” connection. Mercruiser recommends this in some of their service manuals. This way you can run the engine on the drive pump with a flush out.

 

[Tmakaro]

I would suggest installing a 160* to help it run a tad cooler, but even that may not be 100%

If you mean a 160 degree thermostat, that is what I have always had in there.

 

[alldodge]

If you mean a 160 degree thermostat, that is what I have always had in there.

In that case the Alpha drive is barely able to keep the motor within safe limits. You need to Y the connection with a thru hull as mentioned, or go straight thru hull with belt driven pump

 

[Tmakaro]

In that case the Alpha drive is barely able to keep the motor within safe limits. You need to Y the connection with a thru hull as mentioned, or go straight thru hull with belt driven pump

Hmm, it’s always runs 170 to 175 with the needle on the guage straight up. I thought that was normal. Are you saying a 160 thermostat is supposed to keep the engine at 160? It does keep the temp at 175ish at WOT.

 

[alldodge]

Yes, a 160 stat should keep motor at 160 unless the water your boating it is real warm. I run a 140* in my 509 and it stays right there until I open it up, and then 165* is about as high as it will go

 

[Tmakaro]

I’ve had the boat for 7 years and it’s always run at just below 175 on the gauge and held there pretty steady. Last year it started warming up to about 180 and then setting off the alarm. A few seconds later the thermostat would open and it would cool back down to just under 175. This year my engine overheated because the blue plastic drain plug on the block broke off. I caught it pretty quickly because of the alarm. So after fixing the drain plug I replaced the thermostat which I thought was sticking. Now I am seeing the temp guage go from just under 170 to 180, the thermostat would open up and quickly cool down to about 170. Then it would climb back up to 180, sound the alarm for a few seconds, the thermostat would open up and it would cool down quickly again to about 170. Repeat. Doesn’t seem to matter if I am going fast or slow.

 

[Tmakaro]

In that case the Alpha drive is barely able to keep the motor within safe limits. You need to Y the connection with a thru hull as mentioned, or go straight thru hull with belt driven pump

It’s more than enough because when the thermostat opens, it cools right back down very quickly.