2016 Mercruiser 4.5l 250 Engine Temperature

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Phil Chieco

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Can someone tell me what the operating temperature of a 2016 MerCruiser 4.5 l 250 horsepower engine should run at. This is the new version of the engine. I've noticed that it never gets above 125°. On all my other mercruisers they ran between 170 and 180 degrees. Any assistance would be appreciated as I'm not sure if I need a new thermostat or sending unit or this is normal. Thank you
 
I can not tell you exactly. RWC will have a different temp thermostat than FWC.

Get an IR thermometer and aim it at the temp sender. It will be 10F to 20F lower than what the temp sender is sensing.

Check the connection to the sender and any harness connectors for corrosion and tightness.
 
Maybe I wasn't clear enough in my question. My temperature gauge is working and goes up to about $125°. I'm just used to seeing running temps in the 170 to 180 range on my 3.0 and 4.3 mercruisers.
 
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But a temp sender basically a probe that changes resistance as the temp changes. So some added resistance from a bad connection will skew the temp reading.

Not sure of the resistance values but you could pull the sender, measure resistance, then immerse in hot water, wait a minute or two and measure again and it should be a different value. Not sure what they should be but probably in a service manual.

Depending on the sender it may use ground as part of the circuit so if the screwed in connection is corroded or someone used Teflon pipe thread tape or pipe dope that could be an issue also.
 
ddurand said:
But a temp sender basically a probe that changes resistance as the temp changes. So some added resistance from a bad connection will skew the temp reading.

Not sure of the resistance values but you could pull the sender, measure resistance, then immerse in hot water, wait a minute or two and measure again and it should be a different value. Not sure what they should be but probably in a service manual.

Depending on the sender it may use ground as part of the circuit so if the screwed in connection is corroded or someone used Teflon pipe thread tape or pipe dope that could be an issue also.
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Thanks for your advice. I started up the boat this spring and I let it warm up to temperature on the gauge it was 120 and I did put a temp gun to it and it registered just about 120 to 123. The bottom of the risers were about 130 and the top of the risers were about 135. So I'm thinking it is the thermostat and the engine's been running much cooler than it should. Any additional thoughts on this topic. Thanks
 
Thermostat is a maintenance item. Replace if questionable. Keep in mind there may be a thermostat for closed cooling and a lower temp thermostat for raw water cooling. Get the proper one.
 
as ddurand said, 135 might not be too cold, you need to give more info.

Does the boat have antifreeze & a heat exchanger - with seawater only cooling the exhaust ( aka FWC or closed cooling) , or does it pump seawater through the entire motor ( RWC). If it’s RWC, and you run in saltwater, I believe the merc thermostat should be fully opened by 140 degrees, so would not be surprising to see those temps. looking at the merc parts, i think 8M0089715 is the thermostat for a RWC version of your motor - not sure though if you have a 4.1l/250, or a 4.5l/274. But this is a 140 degree thermostat. Holy cow these new motors are complex, with 499 million parts!

The FWC thermostat runs 20+degrees hotter. the theory being that it’s not good to have 150+ degree saltwater running through the motor.

if you are running an RWC boat in freshwater, I suppose you should use the hotter thermostat?
 
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A relative has one of these (2020 Mercruiser 4.5 200 hp) and it has a 140 thermostat and runs pretty cool like 135-140.
 
alk said:
as ddurand said, 135 might not be too cold, you need to give more info.

Does the boat have antifreeze & a heat exchanger - with seawater only cooling the exhaust ( aka FWC or closed cooling) , or does it pump seawater through the entire motor ( RWC). If it’s RWC, and you run in saltwater, I believe the merc thermostat should be fully opened by 140 degrees, so would not be surprising to see those temps. looking at the merc parts, i think 8M0089715 is the thermostat for a RWC version of your motor - not sure though if you have a 4.1l/250, or a 4.5l/274. But this is a 140 degree thermostat. Holy cow these new motors are complex, with 499 million parts!

The FWC thermostat runs 20+degrees hotter. the theory being that it’s not good to have 150+ degree saltwater running through the motor.

if you are running an RWC boat in freshwater, I suppose you should use the hotter thermostat?
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Hello. Engine is RWC in fresh water. No antifreeze. The engine is the new mercruiser 4.5 250HP IO 2016 year. As I mentioned, when I put a temp gun to the thermostat housing and it was pretty much 120 -123 degrees. I believe this is the correct thermostat 8M0089715 at 140 degree. I guess it's a pretty inexpensive part to see if this works. Do I need to replace gaskets as well?
 
if you are running in the 130-degree range, my guess is that you have the 140 degree thermostat, and are fine as is. water is pretty cold this time of year, and at idle speed would not be surprised if you stayed cool.

But - another question - was this motor always yours - or did you buy the boat used? A lot of people, for one reason or another, pull the thermostat out completely. Usually, to hide an overheating problem.

Yes, you generally need to replace the gaskets. Please don't order the part number I gave you, but instead look up with your engine serial number.

 
alk said:
if you are running in the 130-degree range, my guess is that you have the 140 degree thermostat, and are fine as is. water is pretty cold this time of year, and at idle speed would not be surprised if you stayed cool.

But - another question - was this motor always yours - or did you buy the boat used? A lot of people, for one reason or another, pull the thermostat out completely. Usually, to hide an overheating problem.

Yes, you generally need to replace the gaskets. Please don't order the part number I gave you, but instead look up with your engine serial number.

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Thanks for responding. I purchased the boat with 70hrs on it and I know the prior owner did not swap out the thermostat. The screws have no markings on them. See pic. The temp gauge has always been at around 120 and when I ran the boat and brought it up to temperature, I put a temp gun to the thermostat housing it registered just about 120 to 123. The bottom of the risers were about 130 and the top of the risers were about 135. So do you think I'm good with my current thermostat? Wasn't sure if I would get better performance out of the boat if it ran a little warmer.
 

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Not sure if you would get better performance, but at a warmer temperature your engine would run more efficiently. I've never done it, but people do take their thermostats out, and throw them in a pot of water on the stove to witness them opening. I always figured if you are already taking them out, might as well just replace?
 
In fresh water I’m not sure if this happens as much but I can tell you from years of salt water experience with raw water cooled engines:
Cast iron engines rust internally & shed rust flakes
These rust flakes can lodge in the seat of the thermostat & keep it from closing
Then the engine never really warms up esp at low speeds
On my OMC 4.3 V6 this happens every few years. I used to just replace the ‘stats but then found another shade tree mechanic way of dealing with this: get a small hammer & tap all around the stat housing after running the boat up on plane to get it open. The tapping vibrated the rust flake loose…. It’s worked for me a few times!
My brother has a 2020 version of this engine & it runs at about 140. My old OMC came with the 160 stat & that’s where it runs. I tried a 140 but the engine took too long to warm up & I had condensation in the exhaust manifolds.
BTW, I have not found the oft repeated “water temps over 140 degrees will cause a salt water engine to clog with deposits” to be true. When I replaced the cyl heads 6 years ago after 15 years in the salt I found nothing of the kind just an even layer of surface rust. Maybe that happens in the warm salty Gulf Coast but definitely not here in Long Island.
Ideally if I repower this boat I will add a closed cooling system.
 
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alk said:
Not sure if you would get better performance, but at a warmer temperature your engine would run more efficiently. I've never done it, but people do take their thermostats out, and throw them in a pot of water on the stove to witness them opening. I always figured if you are already taking them out, might as well just replace?
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When I install a new one I like to test them; I use an old fashioned radiator thermometer & a pot of hot water…
 
BTW I have winterized that engine a few times in my brothers boat & here’s my take on it:
A very nice upgrade on the old GM 4.3 V6. If I didn’t know better I’d swear Mercury bought the old tooling from GM. Almost everything is nearly the same as the old GM 4.3, cast iron construction 90* V6 with balance shaft. The big difference is the fully integrated fuel & ignition system which lets it run like a modern auto engine. The downside is the very high parts cost & there is no way around that because it’s not GM marine any more you are stuck paying Mercury parts prices which are 2-3x as much as GM marine engine parts.
 
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