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Old 24-06-2018, 11:41   #1
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Brand new Mercruiser overheating

My sonís 33 ft Maxum has 350 block engines with Bravo outdrives. The starboard engine was just replaced (brand new not rebuilt). New exhaust manifolds & risers and new raw pump. Itís a FWC engine. It can run for several hours and then the temp climbs and high temp alarm goes off. Other times it gets hot much more quickly. Weíre stumped and the mechanic stands behind his work but hasnít been able to figure it out. Any thoughts? Weíre getting frustrated. Thanks for any thoughts.
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Old 24-06-2018, 11:50   #2
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Brand new Mercruiser overheating

First verify with an IR thermometer itís really getting hot.
If it is, since itís intermittent and not related I assume with being run hard, itís likely a sticking thermostat. It could be an air bubble in the cooling system too.
Does it have bleed screws on the fresh water side? For the last 20years or so GM Small block engines have ďreverse coolingĒ meaning the water flows backward from what it used to, this is a more logical cooling loop, but can trap large volumes of air, so there is usually one or more bleed screws in the cooling loop. Usually one in the thermostat housing.
Pure swag, but easy to do and costs nothing and canít cause any harm. Start with easy and cheap, eliminate the easy and cheap stuff, then graduate to hard and expensive.

If this engine is connected to a water heater, that also can cause a large air lock.
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Old 24-06-2018, 12:03   #3
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Re: Brand new Mercruiser overheating

Thanks for such a quick reply. After the first episode of overheating the mechanic came onboard with an IR thermometer. Running hard the engine was 169 degrees. The gauge ran 10 degrees hotter changed both sender and alarm sender. The older engine ran 10 degrees cooler. The mechanic said thatís not unusual. We donít have a hot water heater and ran for 4+ hours before overheating the first time. Could you run that long with trapped air w/o overheating?
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Old 24-06-2018, 12:52   #4
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Re: Brand new Mercruiser overheating

169 isnít hot I donít think.
Your saying itís alarming at 169?
If not, how hot is it when it alarms? Iíd try a thermostat, easy and cheap. I really donít think that is it, but easy and cheap to eliminate and you may get lucky.
The air bubble thing can be tricky, it moves around and causes trouble, I assume it could move into the water pump and make it stop pumping water.
Again, see if it has a bleeder screw and if it does open the screw and with fresh water cooling system full it should flow out coolant, but it has to be completely full as the bleeder screw ought to be real close to the top of the system.
Iím sure googling how to bleed a Chevy cooling system will show you how, assuming the Marine engine is reverse flow, and has bleeder screws.
My old 454ís didnít, but likely they were too old.
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Old 24-06-2018, 12:59   #5
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Re: Brand new Mercruiser overheating

Sorry I wasnít clear. When the mechanic was onboard we ran the boat. It didnít overheat but the new engine ran 10 degrees hotter than the older original engine. Read with IR gun. Didnít know if that was a concern. I donít think itís air b/c the senders are right next to the bleeders and antifreeze ran out when replacing them. But Iíll certainly try bleeding again
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Old 24-06-2018, 13:29   #6
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Brand new Mercruiser overheating

Thing about the bleeders is this air bubble can move around.
Read this, it may help
https://ls1tech.com/forums/generatio...ng-system.html
Also many people will drill a small hole in the flange part of the thermostat, this will slowly let trapped air out when the thermostat is closed and allow a tiny bit of water to circulate when the T stat is closed.
I think its a good idea myself, and youíll notice a lot of thermostats come this way with a loose brass rivet in the hole.
I think drilling a small hole is a good idea and hurts nothing that I can see
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Old 24-06-2018, 18:16   #7
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Re: Brand new Mercruiser overheating

Where is the raw water pickup? Thru-hull or from the outdrive? That it runs for 4 hours without overheating suggests something other than trapped air, but doesn't rule it out.


If raw water is supplied to the raw water pump by a pump in the foot of the outdrive, that impeller could be bad and a good engine-mounted raw water pump could conceivably pull enough suction to keep the engine cool most of the time.


Just a shot in the dark
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Old 24-06-2018, 18:23   #8
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Re: Brand new Mercruiser overheating

I appreciate the reply. The seawater pump is fully rebuilt and belt driven off the engine. Bravo 3 outdrives do not have a water pump in the outdrive
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Old 24-06-2018, 19:22   #9
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Brand new Mercruiser overheating

He has a point though, a raw water impeller isnít hard or expensive to change, and I have seen one that was new, but the rubber part slipped on the metal part and didnít pump much water.
Check all the simple stuff first, make sure no cooling hose could be collapsing for example. I have never personally seen it, but there are many stories of hoses that separate and the inside collapses.
Check the gasket and face of the raw water pump, perhaps itís sucking air?
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Old 24-06-2018, 19:25   #10
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Re: Brand new Mercruiser overheating

Hummm thatís a good thought. Years ago a collapsed transmission hose cost me a transmission.
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Old 24-06-2018, 21:35   #11
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Re: Brand new Mercruiser overheating

170 isn't hot. Probably has a 165-70 thermostat.
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Old 25-06-2018, 04:29   #12
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Re: Brand new Mercruiser overheating

During normal operation the new engine runs 170 the older engine runs 160. Curious to know if a 10 degree difference between engines is odd. When the engine overheats the temp gauge pegs.
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Old 25-06-2018, 05:18   #13
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Re: Brand new Mercruiser overheating

Pull the thermostat and run without it. Check to make sure the new engine came with a marine and not automotive thermostat. New auto engines are designed to run real hot to meet emission standards
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Old 25-06-2018, 11:48   #14
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Re: Brand new Mercruiser overheating

The two Yanmars in our Cat are a little more than 10 degrees apart and it never worried me, figured each reading was 'normal' for that particular engine. If and when one of the engines varies from its 'normal', I'll start the process of finding out why.

I'm actually hoping it is a difference in the senders and gauges, not truly actual temp differences. When we get back on the boat in a couple of weeks I will be taking my IR thermometer with me and trying to verify once and for all.

I realize my answer doesn't help the overheat alarm situation but threw it out there hoping you were not fixated on the 10 degrees. I would be amazed if the two engines were actually identical heat wise; I would think a few degrees either way would be more the norm.
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Old 27-06-2018, 01:18   #15
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Re: Brand new Mercruiser overheating

+1, 169įF is not hot ..
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