What is normal operating temperature?

[tonyiiiafl]

My 1994 390 CPMY has twin 7.4 Merc Bluewater gas carbureted engines. Port has 1100 hours, Starboard has 40. Each has 160 degree thermostats. Under normal load, running at 3000 RPM's, 15 kts, both operate at 170 degrees. if I open I up to 3750, (NOT WOT)I will hit 180 -185. Both are pretty much the same temps, so they are even. What do you all operate at? Does this sound normal?

[bud37]

Two questions ...are you in salt water?......closed cooled ( heat exchanger ) or raw water?

[Midnightsun]

Closed cooling around 180. Raw water cooled around 160. At least this is what I have seen on all my boats over the last 30 years. That being said, it should be constant and not increase or decrease with rpm or loading.

[km1125]

I wonder if both your raw water pumps are originals, meaning both have around 1100 hours. Not sure on the Merc pumps, but I just checked the cam on my Crusaders (with 1200 hours) and the new cams were 25% thicker than the old ones I replaced due to wear.

[tonyiiiafl]

Closed cooling. Raw water pumps and impellers are new last July.

[bud37]

So , considering you have heat exchangers , if you have good flow thru the exhausts then with the temps a little high, my guess would be the exchangers may be getting crudded up....tipping point for salt water issues is around 160 I believe, perhaps someone who has done the exchanger flush/ clean-out will chime in here.
Running engines at that temp you mentioned is certainly not alarming in any way, way better efficiency , just the salt crusting problem.

[Viper]

Closed cooling 160 to 180 and raw water cooled is typically set up with 140 stats in case it's heading for salt water. Lots of fresh water boats that are raw water cooled will still have the 140 stats and should be replaced with hotter ones such as 160 for better engine performance.

[tomschauer]

my 1998 Merc bluewater freshwater cooled mpi 7.4 engines run about 170-175. my starboard has begun to hit 180 when running at 3200 or above, it drops back quickly to 170 when I back off the rpm's. I believe its time to change the impellers. I believe the 1996 and up mpi vortec roller motors run a bit hotter (on the gauge) due to the newer reverse cooling flow. The water hits the heads first to reduce the spark knock (detonation) and enable higher compression on lower octane fuel. The actual block temps are the close to the older numbers of 170.

[mjk1040]

Geez! Viper wish I saw this thread a month ago. I just replaced the thermostats in our Crusaders with the OEM 140"s! The Crusader manual doesn't mention application difference for fresh water verses salt water usage? Just states 143 degree thermostats. Oh well both are now equal and reactive the same.

[km1125]

Source of the post Geez! Viper wish I saw this thread a month ago. I just replaced the thermostats in our Crusaders with the OEM 140"s! The Crusader manual doesn't mention application difference for fresh water verses salt water usage? Just states 143 degree thermostats. Oh well both are now equal and reactive the same.

Yea. Same engines. I just replaced my port thermostat because it was stuck open and would not come up to temp unless at cruise. I put a 143 but mostly because I didn't want to dig in and change the other one. I really should think about that though. I think the thermostat was only $14, but the gaskets were just as much.