I have an issue with the temp on my 1993 350 260 hp mercruiser engine.
-Bravo drive
-New Belt driven raw water pump
-The second New thermostat
-removed risers and manifolds and all are clean and water runs through freely
-the risers are stainless and look really clean. Teh manifolds are 2001 but there was no blockage or buildup at all....
-At idle the Temp fluctuates from 140 to 165. Engine heats up to 165 the thermostat opens and cools to 140 and the cycle repeats itself.

I've owned the boat for 8 years and she has consistantly run around 150 to 155. I've never had that much fluctuation in my temp. Is there an issue?

Could there be a restriction in my water flow?

Thanks for any info.....

Only question I have to offer, when you changed the thermostat, is the new one the same temp range as the old one?

I assume this is fresh water cooled and not just raw water.

How does it run under load?

Does the raw water first pass thru a oil cooler before going to your heat exchanger? May be plugged up.

Belt isn't slipping?

Did the engine get very overheated at some point and you have head gaskets issues? Eroded impeller on your fresh water side?

There was no blockage of your manifolds in 2001? That was a loooong time ago. Your manifolds are on the raw water side? That is a mistake right there. They should be fresh water cooled with a solid gasket going to the riser.



150-155' seems a bit low operating to me?

I'm not an I/O guy but I assume the Bravo drive sucks raw water up thru the skeg. Is there a plastic piece going from drive, to hose, to into the boat? that may be cracked or loose s#^&#^&#^&#^&#^&g in air with the raw water causing fluctuations.

Only question I have to offer, when you changed the thermostat, is the new one the same temp range as the old one?



The thermostat is the same range as the old ones. I've changed them myself the whole time I've owned the boat. It is the one the book calls for. It's a 140' thermostat.

I assume this is fresh water cooled and not just raw water.

No. It is only raw water.


How does it run under load? Last year I had an issue where the temp ran up to 180 and fluctuated back down to 140 before beginning the process all over again. There was at the time also a vibration from the motor. It turned out to be the raw water pump. That was replaced in the off season

Does the raw water first pass thru a oil cooler before going to your heat exchanger? May be plugged up. Without the thermostat in the water flow and exhaust is strong and steady. Is the oil cooler a big deal????



Belt isn't slipping? No, the belts are nice and tight...

Did the engine get very overheated at some point and you have head gaskets issues? Eroded impeller on your fresh water side? No fresh water side....as i said got to 180 last year. No water in the oil....it's nice and clean and at the correct level.

There was no blockage of your manifolds in 2001? That was a loooong time ago. Your manifolds are on the raw water side? That is a mistake right there. They should be fresh water cooled with a solid gasket going to the riser.

The manifolds were replaced in 2001. When we took them off they were clean and we hooked a hose to them and water flowed through freely. Could they be the issue?



150-155' seems a bit low operating to me? It's always operated like that since I got it...

I'm not an I/O guy but I assume the Bravo drive sucks raw water up thru the skeg. Yes

Is there a plastic piece going from drive, to hose, to into the boat? that may be cracked or loose s#^&#^&#^&#^&#^&g in air with the raw water causing fluctuations. I don't know. I'll have to pull the boat to see that...


Thank you for your help....


..

165 is hardly overheating.
if it stays under that, in all conditions, I wouldn't worry about it.

165 is hardly overheating.
if it stays under that, in all conditions, I wouldn't worry about it.


I agree that the 165 isn't over heating. I guess my concern is the fluctuation and why it is so drastic. When I had the muffs on it and the thermostat was out the exhaust water flowed unrestricted. Now I expect a bit of restriction whe the thermostat is in and closed but I was actually getting a flush back through the drive when the thermostat closed back up (cooled to 140'). That never happened in the eight years I've owned the boat.

So, could the thermostat housing be clogged so far in that I can't see that when the thermostat closes it restricts the water flow or could the engine water pump (not the raw water pump) be bad?

If your are not getting over 200 you are fine!! 165 is great!!

I guess I'm going to need a new thermostat housing. There is a port in the housing that allows water to flow over the thermostat, down to the engine water circulating pump and then out to the risers and manifolds. The engine water pump doesn't pump any water until the thermostat opens to allow the heated water out of the block. So one port in for the main feed from the raw water pump, then a port to take direct flow through to the engine pump, but between those two ports there is a chamber that allows water to rush up to the ports that feed the manifold and risers which is on top of the thermostat. Well, I took some plumbing wire brushes to try and get some of the build up out of the ports and for the most part I was able to. I couldn't get anything to go up through that center port nor could I even see it by looking inside the housing. Blocking off all other ports with my hands and a hose I was able to back flush that invisible port.......what popped out was two muscles....totally blocking the water flow to the manifolds and risers when the thermostat was closed.

Anyway, cleaned it a little more and flushed it some more and water flowed through pretty good. Put it back on and now my temperature swing is between 150' and 160' at idle and 150' to 165' at 3,000rpm. It was better than that last year so I'm either looking to get this housing acid dipped or buy a new one if they are not too expensive.

Thank you for all of your input and advice. If your temp is spiking and dropping to the extremes of your thermostat range with your I/O this may be an area you want to look into.