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Old 05-23-2007, 11:53 PM
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Default mercruiser operating temperature

i have a 1979 120 hp mercruiser inboard 4 cyl. what is the average openating temperature and is it normal for the engine to feel like it is getting hotter after i shut it down and let it sit for a while? thanx!!
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Old 05-24-2007, 01:11 AM
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Default Re: mercruiser operating temperature

yes, when you shut it down the cooling water is no longer circulating so it will get a little hotter. Mine runs about 180 since it has a 170 thermostat in it. if yours runs hotter you should replace the thermostat with a 160 and if you don't know when the impeller was changed last you should change that also.

At that age you are also due for new manifolds and risers. If they start to leak then you will ruin the motor.
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Old 05-24-2007, 01:27 AM
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Default Re: mercruiser operating temperature

Raw water cooled engines should operate between 140 and 160 degrees. Fresh water engines should run at 170-210 degrees.
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Old 05-24-2007, 06:49 AM
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Default Re: mercruiser operating temperature

It is normal for the for the Eng to heat up when it shut down and sits for a while ,
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Old 05-24-2007, 07:05 AM
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Default Re: mercruiser operating temperature

Mine is raw water cooled and runs around 160.
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Old 05-24-2007, 07:46 AM
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Default Re: mercruiser operating temperature

The hotter it runs the better, up to a point. However, raw water cooled engines shouldn't run over 160, as salt will begin to crystallize in the engine over that temp.
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Old 05-24-2007, 07:57 AM
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Default Re: mercruiser operating temperature

Thanks didn't relize that about salt-- now I know why my engines had 160 --It pays to read THT
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Old 05-24-2007, 04:13 PM
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Default RE: mercruiser operating temperature

My 4.3 RWC runs at 175. Has since I took it out on the sea trial. It only has 30 hours on the motor. Is this too hot? I have always wondered about the temp.
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Old 05-24-2007, 04:43 PM
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Default Re: mercruiser operating temperature

Does it have the original thermostat? Most consider the beggining of scale formation in salt water systems to start around 170 degrees. Generally, it's a good idea to run the motors at 160 or less. If Mercruiser put a 175 in there knowing the motor was going to be raw water cooled, it may be ok. Mercruisers generally run 160. Volvos generally run 143.
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Old 05-24-2007, 06:02 PM
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Default Re: mercruiser operating temperature

many mercrusers run 140 thermostats
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Old 05-24-2007, 06:47 PM
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Default Re: mercruiser operating temperature

Before shuting it down,let it idle for a few mins!That goes a long way cooling it down.
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Old 05-24-2007, 08:25 PM
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Default Re: mercruiser operating temperature

Of course you're ASSuming the gauge is accurate, FB.... I was in the hardware store the other day, had a whole rack of thermometers. Do ya think they all read the same temperature? Well, some were close. I've gotten to the point, what the gauge reads isn't as important as watching for CHANGES in the reading. If it normally reads 173, the goes to 185, start looking.
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Old 05-24-2007, 10:55 PM
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Default Re: mercruiser operating temperature

I agree with you Bill, My guage says 180 or so but has since new so actual is probably closer to 170.
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Old 05-25-2007, 09:39 AM
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Default Re: mercruiser operating temperature

Check the temp with an electronic/laser temp gauge - about $30 at Harbor Freight and most automotive parts houses.
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Old 05-25-2007, 12:11 PM
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Default Re: mercruiser operating temperature

Great now, my boat runs 175 degrees all the time. I had a overheating problem before and replaced the Impeller and thermostat and I already did a new water pump. My problem was a rusted thermostat. f I get a whole of a laser temp gauge do I just point it at the motor or any specific place? What else would help my motor run cooler?
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Old 06-03-2007, 08:43 PM
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Default RE: mercruiser operating temperature

I'm curious too ... new (to me) boat, it overheated shortly after I got it so I changed the thermostat. The impellor was going to be next.

Currently it appears to be running around 185 degrees when underway and at slower speeds it gets up around 200.

Sound normal? What temperature is considered overheating?

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Old 06-03-2007, 10:33 PM
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Default Re: mercruiser operating temperature

My fwc Merc 5.7 is locked at 158. Once it warms up, it basically never varies. Got about 160 hours on it.
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Old 06-03-2007, 11:22 PM
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Default RE: mercruiser operating temperature

Quote:
casey344 - 6/3/2007 5:43 PM

I'm curious too ... new (to me) boat, it overheated shortly after I got it so I changed the thermostat. The impellor was going to be next.

Currently it appears to be running around 185 degrees when underway and at slower speeds it gets up around 200.

Sound normal? What temperature is considered overheating?
I would consider anything over 170 overheating on a raw water system and anything over 230 overheating on a fresh water sytem at 16 psi.
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Old 06-03-2007, 11:27 PM
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Default Re: mercruiser operating temperature

The range on my temp gauge is all "safe" then? I wish there was an unsafe area listed, like a red-line.

These gauges are foggy and I'm going to replace them, I guess I'll look for a higher range.
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Old 06-04-2007, 02:25 AM
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Default Re: mercruiser operating temperature

I should say that running over 170 degrees in a raw water cooled motor may cause problems in the long term, but its not dangerously overheated until you're into the 200's. I'd be pretty nervous by 205 degrees. Basically, whether youre fresh water or raw water cooled, automotive or marine, it's not dangerously overheating until you boil over. Raw water systems aren't pressurized and boil at 212 degrees. Fresh water systems are pressurized and boil at upwards of 260 degrees.
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