Thermostat Question

D

DayCruiser

Ensign
Joined
Sep 24, 2004
Messages
953
Took the boat for the first time yesterday. 3.0 Mercruiser with 1989 engine, 4 cylinder.
Last year a new impeller and thermostat(160 degree) was put in.
In forward gear the temp would reach 160 and then drop to 158 and sometimes go over 160. Idle under bridges it would go a little over 170. A long idle period through the Marina it approached 180 but never went over 180.
Could the thermostat be not be opening far enough or getting stuck closed? I say not opening far enough because there is not a bigger drop in temps when it does open.
Or is the impeller shot again? I only cranked it in the water but know this is a wear and tear item. I would like to get more than one season out of it.
Should I just change the theromostat to narrow it down?
 
f_inscreenname

f_inscreenname

Commander
Joined
Aug 23, 2001
Messages
2,591
Re: Thermostat Question

It sounds like you are in proper operating temperature. If it will make you sleep better at night. Change the impeller again (never hurts) and put a 140* stat in it for the summer.8)
 
T

Tom97Sundancer

Cadet
Joined
Aug 19, 2003
Messages
23
Re: Thermostat Question

Speaking of impellor replacement, can you tell me how often I should replace mine? Should it be done before every season??

Thanks
 
Haut Medoc

Haut Medoc

Supreme Mariner
Joined
Jun 29, 2004
Messages
10,645
Re: Thermostat Question

Should I just change the theromostat to narrow it down?

If you have a raw water cooled system, you should be running a 140* thermostat.......;)
I'll bet your impeller is fine.....;)
 
bruceb58

bruceb58

Supreme Mariner
Joined
Mar 5, 2006
Messages
30,481
Re: Thermostat Question

If you run in salt water, you could also be getting reduced water flow because of rust build up in the risers.
 
C

Charlie D.

Cadet
Joined
Feb 20, 2007
Messages
18
Re: Thermostat Question

Daycruiser I once had a boat about the same age with a similar problem. Turned out to be the water circulating pump on the engine itself. I took my pump off and it looked fine, changed it anyway and the problem went away. Just a thought, or something else to check.
 
Boatist

Boatist

Rear Admiral
Joined
Apr 22, 2002
Messages
4,552
Re: Thermostat Question

You can take the thermostat out and see if that is the problem but more likely the drive impeller or circulation pump.

Thermostat should be wider open at 180 degrees than 160 so I would not suspect the thermostat.

Plugged exhaust manifold will cause over heat at high RPM and loads because can not get enough water flow thru the manifold to cool at high RPMS.

Pull the hose from the outdrive to the engine on the water not on muff and see if you have a good flow at idle speeds. Impeller work as a displacement pump at low rpms and as a centrifugal pump at high RPMS.

Do not leave it off more than a minute or so as this is also cooling water for the manifolds.
 
ziggy

ziggy

Admiral
Joined
Jun 30, 2004
Messages
7,473
Re: Thermostat Question

i think he's fine. a thermostat starts to open at the degree value stamped on the t-stat. it is full open at + 20*. that puts him right where he is for proper operateing temp. ya haven't said yet. but if yer rwc, the 140 stat would be correct. if a closed system, fwc, the 160 would be proper........if yer concerend that the t-stat isn't workin proper. take it out and put it in a pan of water on the stove and cook it with a thermometer in it. my old icky lookin t-stat did proper under this test. but i did replace it because it was icky lookin. only difference i noted in the new one was that it opened just a shade further than the icky looking one when brought up to 160*...i have rwc...
 
D

DayCruiser

Ensign
Joined
Sep 24, 2004
Messages
953
Re: Thermostat Question

Ya its rwc. I think, I will go with the 140 thermostat first and then if that doesn't keep it a little cooler. Go down the list of other advice listed here.
It is just running warmer than when the impellar was put in last year/all year and the water hasn't even warmed up yet. That is what worries me, when the lake temps get much warmer
Thanks again for all the helpd:)
 
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