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Old 11-06-2020, 20:00   #1
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Lehman90hp sp90 diesel overheat

My Ford Lehman SP90 overheated. Overheating causes its own problems. But how do I determine what caused the overheating in the first place?

I replaced both hoses from expansion tank to the heat exchanger because one was leaking. The exchanger and exp. tank were serviced in Aug and I have not used the boat since then.

I replaced the leaking cap on the fresh water pump. My pump has two exits and the cap covers the one my engine does not use.

I replaced the expansion tank lid.

I started it and it heated to 220 before I shut it down. Usual operating temp is 180-190.

There is moisture under the fresh water pump where the shaft enters the block. I assume this means the bearings or seal are shot. I ordered a new fresh water pump but delivery is stalled due to Covid.

Is it the pump? Could it be the thermostat? Or what else?
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Old 11-06-2020, 22:56   #2
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Re: Lehman90hp sp90 diesel overheat

Welcome aboard CF, Kipawlak.

First I would be way concerned about the sea water side of the cooling system.
Is the sea cock turned on?
Is the sea water filter clean?
Is the raw water pump impeller OK?
Was the heat exchanger cleaned when it was serviced?
Have you got a good flow of water though the exhaust outlet?
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Old 12-06-2020, 00:31   #3
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Re: Lehman90hp sp90 diesel overheat

If you're new to marine engines, the salt water side needs to be flushed with a chemical that removes marine growth. Most people do it yearly, but depends on where you're at and sea water temps.
The heat exchanger being partially plugged with marine growth or old impeller pieces is a common cause of overheat. A stuck thermostat is probably second.
Moisture on a pump is common depending on the temperature difference between the water the pump is moving and the engine temp. Some humidity conditions will cause moisture to form on waterlines, pumps, etc. Usually when pump seals fail, water drips from a weep hole.
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Old 12-06-2020, 09:41   #4
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Re: Lehman90hp sp90 diesel overheat

Thanks for the reply.
Salt water impeller changed and heat exchanger serviced late last season. I cleaned strainer last month and water is pushing out the exhaust.

Kj
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Old 12-06-2020, 09:43   #5
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Re: Lehman90hp sp90 diesel overheat

Ok I will check re growth. I know there was salt in fresh side due to heat exchanger leak. So what chemicals? Shd I have a pro do that?
-kj
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Old 12-06-2020, 10:28   #6
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Re: Lehman90hp sp90 diesel overheat

Common salt water flushes are Rydlyme and Barnacle Buster.
Attached Files
File Type: pdf Inboard_descaling_instructions.pdf (1.40 MB, 7 views)
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Old 12-06-2020, 11:32   #7
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Re: Lehman90hp sp90 diesel overheat

I had the same problem with my Ford Lehman. I cleaned the heat exchanger, replaced the impeller, cleaned the raw water filter....no joy. I then replaced all intake hoses. ..... MUCH JOY. ... As the seawater intake hose ages it loses its rigidity. The soft hose collapsed enough to restrict water flow, but not enough to see with my eyes.
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Old 12-06-2020, 11:44   #8
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Re: Lehman90hp sp90 diesel overheat

you did not say under what circumstances the oheat occurred. ie load or noload. if noload, then i'd go for the thermostat next. could be stuck closed.

seems you've done most of the stuff for the seawater side. if oheat is only under load, then more seawater side checking and cleaning may be in order...
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