Typical Engine Temps 350 Magnum

D

DHutch

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Joined
Jun 24, 2007
Messages
14
Looking for feedback from Mercruiser owners regarding typical engine temperatures I should be seeing.

Last fall I purchased a used direct drive ski boat. It has the carb'd Merc 350 Magnum Tournament Ski engine, 1995.

Temperature gauge was not working last year. Test drove the boat last fall before purchasing and again once at our Lake in 2022 – boat / engine ran good.

Replaced the temp gauge, used a genuine Mercury OEM new replacement.

I also inspected the impeller in the raw sea water pump. Blades looked good & tight while rotating the shaft.

With fake a lake on the driveway idling for 15 minutes, the temp slowing climbed to 160F and stayed there. Lots of water pumping out of exhaust.

Finally launched boat a couple weekends ago.

Idling at the dock for approx 10+ minutes, again the temp slowly climbed to 160F and stayed there.

Out on open water running anywhere 2000 - 3000 RPM, the temp climbed quickly to approx 175F, stayed at this temp consistently, the needle didn't move. A couple brief 4000 RPM WOT for 20 seconds, also stayed at this temp.

When I reduce speed down and go off plane, engine temp drops momentarily to approximately 120F and then while trolling off plane climbs back to 160F in approx 30 seconds. And then back to approx 175F back on plane 2000 - 3000 RPM.

Oil pressure is consistently 40-50 psi. Engine sounds good, not missing, no hesitation, no warning buzzers, no obvious signs or feedback of stress on the engine. I also rechecked all hoses – retightened and no leaks. I will be replacing thermostat.

Sorry, a lot of info - but is temperature around 175F normal? Dropping down momentarily to 120F when off plane is this typical?
 
Lou C

Lou C

Supreme Mariner
Joined
Nov 10, 2002
Messages
11,841
My boat (not Merc but same temp thermostat) will idle at 160* and it stays at that temp at low speeds. At higher speeds on plane it may run a little below 160; however after coming off plane it will temporarily rise to 175, and then after idling or low speed use will drop back down to 160. Dropping down to 120 is lower than what we typically see and suggests a thermostat not closing all the way due to a bit of sand or a rust flake keeping it from closing all the way. I have found this to be a common problem with raw water cooled engines especially in salt water (more internal corrosion) and shallow water use (sand).
 
Scott06

Scott06

Vice Admiral
Joined
Apr 20, 2014
Messages
5,668
DHutch said:
Looking for feedback from Mercruiser owners regarding typical engine temperatures I should be seeing.

Last fall I purchased a used direct drive ski boat. It has the carb'd Merc 350 Magnum Tournament Ski engine, 1995.

Temperature gauge was not working last year. Test drove the boat last fall before purchasing and again once at our Lake in 2022 – boat / engine ran good.

Replaced the temp gauge, used a genuine Mercury OEM new replacement.

I also inspected the impeller in the raw sea water pump. Blades looked good & tight while rotating the shaft.

With fake a lake on the driveway idling for 15 minutes, the temp slowing climbed to 160F and stayed there. Lots of water pumping out of exhaust.

Finally launched boat a couple weekends ago.

Idling at the dock for approx 10+ minutes, again the temp slowly climbed to 160F and stayed there.

Out on open water running anywhere 2000 - 3000 RPM, the temp climbed quickly to approx 175F, stayed at this temp consistently, the needle didn't move. A couple brief 4000 RPM WOT for 20 seconds, also stayed at this temp.

When I reduce speed down and go off plane, engine temp drops momentarily to approximately 120F and then while trolling off plane climbs back to 160F in approx 30 seconds. And then back to approx 175F back on plane 2000 - 3000 RPM.

Oil pressure is consistently 40-50 psi. Engine sounds good, not missing, no hesitation, no warning buzzers, no obvious signs or feedback of stress on the engine. I also rechecked all hoses – retightened and no leaks. I will be replacing thermostat.

Sorry, a lot of info - but is temperature around 175F normal? Dropping down momentarily to 120F when off plane is this typical?
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These temps are ok for sure and I notice more fluctuation when the water temps are cold in the spring.

My boat has a 160 thermostat that your I believe has and runs right about 170. When idling on the muffs it takes a long time to come up to temp, so you seeing 160 then piping up under load seems normal to me.

If the 40 degree drop bothers you maybe try the thermostat but this all looks pretty good. Just get used to where it indicates typically so you can see anything unusual.

Is this a fresh water boat or salt/brackish? if salt would inspect the manifolds and elbows for corrosion.
 
D

DHutch

Cadet
Joined
Jun 24, 2007
Messages
14
Scott06 said:
These temps are ok for sure and I notice more fluctuation when the water temps are cold in the spring.

My boat has a 160 thermostat that your I believe has and runs right about 170. When idling on the muffs it takes a long time to come up to temp, so you seeing 160 then piping up under load seems normal to me.

If the 40 degree drop bothers you maybe try the thermostat but this all looks pretty good. Just get used to where it indicates typically so you can see anything unusual.

Is this a fresh water boat or salt/brackish? if salt would inspect the manifolds and elbows for corrosion.
Click to expand...
Thanks for feedback - strictly a fresh water boat.
 
D

DHutch

Cadet
Joined
Jun 24, 2007
Messages
14
DHutch said:
Thanks for the feedback guys.
Click to expand...
Update on my Engine Temps

I purchased a Sierra 160F thermostat c/w gaskets. Wanted to replace the thermostat and see if this resulted in different / cooler temps shown on my gauge.
When I removed the T-stat housing from the intake manifold, I was not able to pull out the brass sleeve holding in the T-stat. Looks like I would have to damage this to remove and I did not have a replacement sleeve.
Any tips for removing the sleeve without damaging it?

Also purchased an IR Temp gun and measured temperatures at various locations on the engine - measured temps while idling at start-up, idling after warm up and running at approx 2000 RPM for 10 mins.
Temperature right at the sensor on the T-stat Housing never got over 130F.
Temperatures on the risers never got over 90F - these were warm to touch, certainly not hot.
Temperatures on the exhaust elbows approx 125F - hot to touch but not scalding hot.
Temperatures on various locations on intake manifold approx 185F.

I'm assuming these temperatures are ok?
Feedback appreciated. Thank You
 
Scott06

Scott06

Vice Admiral
Joined
Apr 20, 2014
Messages
5,668
DHutch said:
Update on my Engine Temps

I purchased a Sierra 160F thermostat c/w gaskets. Wanted to replace the thermostat and see if this resulted in different / cooler temps shown on my gauge.
When I removed the T-stat housing from the intake manifold, I was not able to pull out the brass sleeve holding in the T-stat. Looks like I would have to damage this to remove and I did not have a replacement sleeve.
Any tips for removing the sleeve without damaging it?

Also purchased an IR Temp gun and measured temperatures at various locations on the engine - measured temps while idling at start-up, idling after warm up and running at approx 2000 RPM for 10 mins.
Temperature right at the sensor on the T-stat Housing never got over 130F.
Temperatures on the risers never got over 90F - these were warm to touch, certainly not hot.
Temperatures on the exhaust elbows approx 125F - hot to touch but not scalding hot.
Temperatures on various locations on intake manifold approx 185F.

I'm assuming these temperatures are ok?
Feedback appreciated. Thank You
Click to expand...
I have never been able to remove a thermostat sleeve with out destroying it

The temps you show are all ok no worries there

The variance you see based off location is due to where incoming water is . Seems to be running well
 
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