Open cooling system temp question

Ristin

Seaman
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Aug 4, 2017
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58
1984 Seaswirl Sierra OMC 3.8L

Owners manual indicates engine temp should be between 150 & 170 degrees. Does that mean at all times like in a car where once the engine is warm the themostat keeps the water temp at whatever setting the thermostat is? Reason I ask is today I noticed my water temp gauge indicating around 120 for mid range cruising and during WOT it climbed to 140. In July on our very first trip out I remember seeing 180 at WOT and I think around 160 at lower rpm.
Wondering if my thermostat is stuck open or if gauge/sender is going bad.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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you probably have a broken or stuck thermostat or a bad gauge sender.

use an infared thermometer to diagnose, replace as required.
 

Bondo

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Ayuh,.... If ya haven't replaced the impeller in the raw water pump,....

I'd start there,....
 

Lou C

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The lower than normal temps could be as simple as a piece of rust holding the thermostat open a bit. Happens a lot in raw water cooled engines. Might need a bit of cooling system maintenance as noted above.
 

Ristin

Seaman
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Aug 4, 2017
Messages
58
Thanks fellow boaters!
The thermostat was stuck open about an 1/8" Nothing holding it open that I could see but it needed to be changed. The o-ring was rock hard and I wonder if it wasn't the original thermostat.
Now the amazing part of the story. I live in a rural area and we have a small marine shop just outside of town. From the road the shop looks like a place you wouldn't want to stop and most of the boats parked there are old & tired. I would have never gone their except a friend of mine who has 10 boats told me the shop was really good. I took my old thermostat & o-ring with me and asked if they had a new one. Doggoned if they didn't have the kit in stock and it was cheaper than buying on line & paying shipping.
I installed the new parts and fired her up on the muffs. Temp gauge climbed to 180 and stayed there and once warmed up the engine idled smoother and quieter. AWESOME!!!!!
Used my infrared thermometer and couldn't find a temp over 154 anywhere in the cooling system. Think the stock temp gauge reads high.
 

Lou C

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11,798
Normal OMC thermostats are a 160* unit. The normal temps I have seen are 160-175. I also check temps with an IR temp gun. I have found that the area on the engine that seems to follow the dash gauge most accurately is the part of the intake manifold that is right under the thermo housing. When my dash gauge says 160*, this area reads 145-155 or so. If you stock gauge is in fact reading high check all the connections for the temp gauge (sending unit on the engine, terminals on the temp gauge) for corrosion and excessive resistance.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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Now pull the drive apart and replace the raw water pump
 

Ristin

Seaman
Joined
Aug 4, 2017
Messages
58
Normal OMC thermostats are a 160* unit. The normal temps I have seen are 160-175. I also check temps with an IR temp gun. I have found that the area on the engine that seems to follow the dash gauge most accurately is the part of the intake manifold that is right under the thermo housing. When my dash gauge says 160*, this area reads 145-155 or so. If you stock gauge is in fact reading high check all the connections for the temp gauge (sending unit on the engine, terminals on the temp gauge) for corrosion and excessive resistance.

Although the connection didn?t look damaged or corroded, I cleaned the terminal on the sender before deciding to pull the thermostat housing. The gauge would indicate lower than actual temp if there was excessive resistance.
I?m ok with the 34 year old gauge being a bit off. 🙂 kinda like the owner
 

Ristin

Seaman
Joined
Aug 4, 2017
Messages
58
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After changing the thermostat I gave the boat a bath and went to lake Pend Oreille. Family was away so this was my first solo trip.
Yesterday cruising though the no wake zone of Buttonhook Bay the temp gauge stayed at 100, today, 180 indicated which should be 160ish. Boat runs so much better now.
 

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