Overheating

[tunahuntr1]

I have a 1976 Mako I/B and just re-powered with a new 5.7L. mercruiser.My problem is that I added fresh water cooling to the engine and when I put the boat in the water it heats up beyond 180 degrees.I thought I had the raw water pump reversed but that wasn't the problem.I also did rebuild the pump prior to startup.Another thing is the engine like the one I took out is counter-rotating and I am sure the impeller was placed properly.I used to have a different boat with fresh water cooling and I can remember that it had block off plates between the risers and manifolds,what are these for exactly?Is it my problem?I don't have them on my engine.I also have a new thermostat that is 140 degrees.The water is circulating through my engine.Thanks to anyone that can help,I am at whits end.

 

[Chris1956]

Re: Overheating

Tuna, fFrom your description, it sounds like you have several problems. I will assume you replaced an Inboard motor with a sterndrive. If so, consider the following:
The V-8 powered Mercruiser sterndrives with Fresh wate cooling usually had a "through the hull" pickup for raw water to the heat exchanger, and a belt driven raw water pump. The sterndrive impellers were inadequate to supply the raw water needs needs for V-8s.

Normally, the block-off plate between the risers and the manifolds was the boundary between the fresh water cooled block and manifolds, and the raw water cooled risers. If you don't have it , all that nice antifreeze is going overboard.:'(

 

[tunahuntr1]

Re: Overheating

Thankyou for your feedback Chris,I replaced my 5.7 L. with a 5.7L new engine.I replaced the exhaust manifolds and risers and rebuilt the raw water pump from the old engine.I never had an overheating problem with the old engine so I know that the water flow is sufficient.I also have a full tank of antifreeze in the engine that is not going overboard(it is sealed) .My old engine was raw water cooled but no freshwater cooling system,that I added on the re-power.I can run water via a garden hose before the raw water pump and run the boat without overheating but still too warm(about 180 degrees)it should run at about 140.But the problem lies in the fact that the water is being pushed through.When the boat is in the water it doesn't seem as though water going through the raw pump is circulating properly.I was wondering if the exhaust back pressure was the culprit.That is why I mentioned the cut out plates.I do not believe that I need them with the risers I have...Any more Ideas I would surely like them....Tuna

 

[Bt Doctur]

Re: Overheating

raw water cooled engines use a 140 deg. fwc used a 170 .sounds like your looking for rwc temp in a fwc system.most do run 175-180 with the proper size heat exchanger

 

[tunahuntr1]

Re: Overheating

Well hello everyone and a thank-you to Bt Doctur , you were right about the rwc and fwc temps,and figured out my problem with the overheating I now run a nice 175 degrees.It seems as though when I was trying to start my engine that was brand new in crate I replaced the rwc pump housing and impeller with a Sierra brand housing.It had a defect and lunched my impellor.I found all of the pieces in the expel side of the pump.lol.
I went to mercruiser,found the pump housing and new impellor and she runs like a dream.I took the maiden voyage this morning without a hitch.All systems normal.8)