Proper Engine Operating Temperature?

I have 7.4 Mercruisers gas engines in my boat. One engine runs in the high 140's and the other one in the mid 150's.

I think they should be running around 165-170 degrees.

What is the factory recommendation?

Dave

Have a look here.

rochepoint said:

Have a look here.

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Great resource, thanks!

Great link, thanks. They say 160 so they are running a little cold.

Want are the exact model numbers, bet there is a complete manual available online?

rochepoint said:

Want are the exact model numbers, bet there is a complete manual available online?

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How could I find that out?

This site has the manual covering your engines, quite a large PDF.

rochepoint said:

This site has the manual covering your engines, quite a large PDF.

Click to expand...

Thank you.

When I get on my desktop, I'll download it.

Before you get too upset about temperature differences be aware that dashboard gauges are not entirely reliable. Borrow or buy an infrared thermometer and check the engines directly.

The real question is how is the engine cooled?

If it has a heat exchanger and operates on fresh water about 180F is the norm.

If its sea water direct cooling the temp will be closer to 140 F so the salt is not kicked out to plug the engine.

boatpoker said:

Before you get too upset about temperature differences be aware that dashboard gauges are not entirely reliable. Borrow or buy an infrared thermometer and check the engines directly.

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I'm using the numbers from the B&G engine management system, not the dash gauges.

FF said:

The real question is how is the engine cooled?

If it has a heat exchanger and operates on fresh water about 180F is the norm.

If its sea water direct cooling the temp will be closer to 140 F so the salt is not kicked out to plug the engine.

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It's raw water cooled.

I had a Criscraft that was raw water cooled and it called for a 140 F thermostat for the reasons FF dexcribed. When running the gage would show about 145 or so.

jleonard said:

I had a Criscraft that was raw water cooled and it called for a 140 F thermostat for the reasons FF dexcribed. When running the gage would show about 145 or so.

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It just seems that running at this temp creates inefficient combustion chamber temps that results in carbon build up and much lowered fuel economy.

I believe FF and beachbum29 is spot on here.

Sea water cooled must run stone cold (130) or less to keep from salting.

Closed loop cooling can be run cool at 170 or even 160 but the hotter the better. I'd say run 190 if you can. Especially if you're running at a 40% load or less. That probably includes well over 50% of TF members. One should do everything one can to get the operating temp up. And if your cooling system is in good shape and w about 40/60 (AF/water) we should be able to run well over 200 as w the AF and pressure I think the boiling point is about 240 degrees. I run 190 in my Mitsu.

manyboats said:

I run 190 in my Mitsu.

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It's been well documented that my Cummins runs at 192 no matter what. I've also been told by some "experts" that the newer engines run hotter to achieve a more complete fuel burn. I'd like to know what RickB's take is on this.

beachbum29 said:

It just seems that running at this temp creates inefficient combustion chamber temps that results in carbon build up and much lowered fuel economy.

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Many boats like yours are sold to fresh water lake dwellers where raw water cooling is ok at 160 to 180F. Years ago when I worked at a LI boatyard the annual chore on Atomic 4s was to clean out the cooling passages, never possible to get them 100% clean with cratered engines quite common.

For salt water cruising heat exchangers became a much desired item or run them at 140 as FF says where you correctly note combustion temps are possibly too low.

Why not convert to fresh water cooling? Easy enough to do with all sorts of interesting kits to choose from. It will add to the resale value of your boat too.

SeaHorse II said:

I've also been told by some "experts" that the newer engines run hotter to achieve a more complete fuel burn.

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A few years ago there were projects to develop ceramic pistons and liners so that engines could be run without cylinder lube or coolant in order to increase efficiency. I am not sure where it went but all that seems to remain are a few aftermarket ceramic coatings.

Running hotter has its limits as far as benefits go before it leads to emissions issues and complicated fixes to reduce them. It also leads to issues with thermal cycling of engines that don't operate for long periods or at constant loads for long periods.

This gets back to the angels dancing on a pinhead issue of what are you willing to pay for a one percent increase in efficiency to drive a recreational boat around for a few hours a week? Steering a straight line might provide better returns.

Back on topic ... once you heat seawater above about (about because the makeup of seawater varies considerably with location) 150F or 65C you will start to plate the liners with insoluable calcium carbonate. That forms an insulating layer that just makes things worse and requires acid washing to remove it along with a bit of your cooling passages.

Keeping a raw seawater cooled engine in the 140 - 145F range will provide good service and better if lower but then you run into lube oil issues due to moisture.

Very interesting posts. Thank you. Guess I'll leave well enough alone.

I agree w Tom. FWC.

It just seems that running at this temp creates inefficient combustion chamber temps that results in carbon build up and much lowered fuel economy.

Not that !! much poorer efficiency and you save the weight and maint of an extra water pump, and heat exchanger.

About $1000 worth of parts for a car engine , worth it for most from new , seldom worth the hassle on a used boat.

If the engines are Sterling , Rolls Royce , Lycoming ,Allison or some other brand of classic or exotic it might be different.

Just priced the kits. At $1K per kit plus incidentals & labor, it's not worth it to me.

I figure it would run me $3500-$4000 in the end.

rochepoint said:

This site has the manual covering your engines, quite a large PDF.

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just downloaded it. thanks again.