Mercruiser 200HP 305 CID V-8 Normal Operating Temperature

[thomascrown]

I have the service manual for this engine. NO where in the service manual or anywhere on this forum, am I able to find normal operating temperatures for this engine, or something similar.

Here is the question. What are considered normal operating temperatures for a 140 thermostat and a 160 thermostat. What temp is considered over heating on these engines?

More back story, if interested.
The manual for my boat calls for a 140 thermostat, however, I bought the boat from the great lakes region and use the boat only in freshwater. (raw water cooled) Before I bought the boat it looks like a thermostat was replaced. My temps normally run between 160 and 180. I use the boat to troll, and normally run around 170 temp. When cruising at WOT it drops to 160, or slightly below. At the dock it sits around 170 never higher than 180.

I just replaced the impeller since while I was out trolling I went into the 200 range at trolling speeds, but at Higher RPM it would come back into the 160-170 range.

After doing the impeller replacement, I am concerned about the 180 temp, I thought I would see a more limited range of temp variance.

I appreciate your input and thoughts.

Thank you

 

[Scott Danforth]

Welcome aboard

you should be running about 140-150F with a 140 thermostat. the 140 is what is recommended for a carburetor and raw water cooling

160hp is what is recommended for carburetor and HX cooling or EFI and raw water cooling

at 200F, you do not have enough water flow. you have a blockage somewhere or your water pocket cover needs replacing

 

[thomascrown]

Welcome aboard

you should be running about 140-150F with a 140 thermostat. the 140 is what is recommended for a carburetor and raw water cooling

160hp is what is recommended for carburetor and HX cooling or EFI and raw water cooling

at 200F, you do not have enough water flow. you have a blockage somewhere or your water pocket cover needs replacing

Thanks for the Reply. Your water pocket cover comment stuck out. Why would you think that is bad? When I dropped the lower, the pipe was in place, and I replaced the whole housing, gaskets and impeller. Things went back in very smoothly. It changed my idle and trolling temps back into the 160-170 range, prior it would creep up to 200.

Also, If there was a 160 thermostat in am I within the operating temps. I have a feeling the previous owners were as confused as I am , and did put the 160 in, as many forums claim the 140 is for salt water conditions, and the 160 is for freshwater.

 

[Rick Stephens]

Water type is not so important - open cooling is the issue. Since with sea water cooled motors there is no pressure built up, the water in the engine will start to create bubbles on the walls of the water passages at anything over 160ƒ. Heat transfer is significantly impeded by those bubbles and hot spots occur. Then the water starts to boil at those locations and the motor gets even hotter, especially locally to where the bubbles started - in the heads around the exhaust valves is first and burnt valves is the result.

Hence sea water cooled motors generally require a 140ƒ thermostat. Which means the thermostat opens at 140ƒ. Motor temps will run above thermostat opening temps.

 

[Bt Doctur]

Always verify your temp with a laser temp to confirm accuracy of your gauge