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Old 21-11-2010, 01:05   #1
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Hot Running Engines

Hi All
I'm running 2 255HP Mercruisers (5.7Lt) on my 31' cruiser.
My problem is how hot these engines run.
When we run at 1200RPM the two gauges sit on about 1/4. Push them up to 2000 RPM and the gauges start to climb to 1/2 . Anything over 2000 RPM and we're up to 3/4 and that's when I start backing off and they take their time to cool down to about 1/2.
I've had the starboard headers recently replaced with no real change compared to the port engine.
Both engines are in good condition and use no oil.
Is this temp range normal?
When I compare them to a car engine they are running a lot hotter.
Many thanks.
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Old 21-11-2010, 01:13   #2
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Quote:
Originally Posted by Flybridge 31 View Post
Hi All
I'm running 2 255HP Mercruisers (5.7Lt) on my 31' cruiser.
My problem is how hot these engines run.
When we run at 1200RPM the two gauges sit on about 1/4. Push them up to 2000 RPM and the gauges start to climb to 1/2 . Anything over 2000 RPM and we're up to 3/4 and that's when I start backing off and they take their time to cool down to about 1/2.
I've had the starboard headers recently replaced with no real change compared to the port engine.
Both engines are in good condition and use no oil.
Is this temp range normal?
When I compare them to a car engine they are running a lot hotter.
Many thanks.
Check your thruhulls! They could be sucking air.
This way or that? On planing hulls the screen should be facing forward. On Displacement hulls facing aft.

.
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Old 21-11-2010, 03:48   #3
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You ideally need a guage that gives you a reading in degrees centigrade or farenheit. Most diesel engines run around 80 degrees centigrade (180 farenheit) I'd worry about running an engine too cool as well, problems like glazing the liners are far more likely then.


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Old 21-11-2010, 05:03   #4
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Thanks for your reply but these are petrol engines. ie 5.7lt each
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Old 21-11-2010, 05:33   #5
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Gas (petrol) engines should run at a similar temp.

Have you checked the gauges or checked the engine temp with a separate meter or temp sensor?
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Old 21-11-2010, 07:32   #6
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have you opened up the hatches and actually felt the temp of the motors, as you bring the rpms up? do this first!!! feel all around where you can touch. check both motors at same time . i would add regular numerically graduated gauges even if temporary. the gauges you have are what are known as idiot gauges, because they don't tell you anything they just look pretty. do not use the senders in the motors, because senders and gauges are usually matched, and you don't know codition of yours. if regular don't mix mfgs. if you get normal reading with the numerical gauges i would swap out to them or at the least graduate your gauges to the numeric. are you fresh or raw water cooled? this dictates what you need to check. more variables with closed system. as far as heat build up you could have weak impellers, a restriction, air in you system if closed, even in an open system you could have air, thermostats sticking(strange for both at same time) are you inboard or inboard-outboard. your strainers could be partially plugged list goes on!!!!!
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Old 21-11-2010, 08:40   #7
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The Mercruiser 5.7L Engine uses a 160 deg. F (71 C) thermostat, which will be its nominal operating temperature.

Mercruiser 5.7L engine Specifications - Performance Product Technologies
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Old 21-11-2010, 12:11   #8
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Hi Mike

It's a closed system and they are shaft inboards. The raw water pick up pump has been replaced on the starboard engine.

The cooling systems have been checked. I must admit I haven't been down there having a feel.

The gauges are the original Mercruiser dash.
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Old 21-11-2010, 12:30   #9
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... I must admit I haven't been down there having a feel...
Keep it that way.
The safe touch temperature for unpainted metal for 10 seconds is about 132 F, making an "up to temperature" (160F) engine block, “too hot to safely touch”.
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Old 21-11-2010, 15:30   #10
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ever seen these laser, point at, thermometers? wonder if you can rent or borrow one to see if you have a hot spot/restriction?
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Old 21-11-2010, 16:35   #11
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water pumps are these inboards?
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Old 21-11-2010, 16:53   #12
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A couple Questions.
How long have you had the boat?
How long has this be happening?
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Old 21-11-2010, 18:20   #13
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A Question that Has Bugging me forever!

Quote:
Originally Posted by delmarrey View Post
Check your thruhulls! They could be sucking air.
This way or that? On planing hulls the screen should be facing forward. On Displacement hulls facing aft.

.
Thanks for clearing that up. I've seen them turned both ways on some displacement vessels.

kind regards,
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Old 21-11-2010, 18:21   #14
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Yes these are inboards shaft drives.

I've have the boat for a few years now,but we have always used it in enclosed waters so therefore speed wasn't important.

Now we are doing more outside trips and I want to be able to hit it if I need to without worrying about the temp.
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Old 21-11-2010, 18:28   #15
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If that's all you've been doing for the past years it maybe normal to have them get warmer under load. They may have been running cold previously.

As suggested above get yourself a aftermarket temp gauge and verify what your temp really is just for the sake of security.

You can remove the aftermarket gauge after you know what temp you're really running.
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