Variation in Engine Temperature at Idle between Twin Engines

Repair or modification of Boston Whaler boats, their engines, trailers, and gear
Tates2
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Variation in Engine Temperature at Idle between Twin Engines

Postby Tates2 » Wed Aug 05, 2020 3:46 pm

I have twin 2002 Mercury OptiMax200-HP engines on a Revenge 25. The engine have 580-hours. I have checked these two engines and found a large discrepancy [in the time needed for engine warm up] between them.

The port Optimax 200 engine idles at 560-RPM, and its temperature goes to 100-degrees-F from 70-degrees-F in two minutes; this engine has always run smoothly.

The starboard OptiMax engine idles at 580-RPM, and its temperature goes to 93-degrees-F from 70-degrees-F. Even after 15 minutes the starboard engine temperature never reached 100-degrees-F. This engine normally runs rough at idle speed, ,and it has been the most problematic of the two engines.

When the two engines are running in 60-degree-F saltwater, both have similar temperatures in the 120-degree-F to 125-degree-F range, varying only about three degrees at most

[What is causing the temperature difference at idle speed]?

[Is the cause of the temperature difference at idle speed due to] a thermostat?

jimh
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Re: Variation in Engine Temperature at Idle between Twin Engines

Postby jimh » Wed Aug 05, 2020 10:28 pm

The engine temperature of most outboard engines is regulated in two ways.

At low engine speeds (less than 1,000-RPM or so) the engine temperature is controlled by the action of a thermostatic water valve. The valve opens at a certain temperature, usually in the range of 130 to 140-degrees, allowing more cold seawater to flow into the cooling system. This action controls the engine temperature from idle speed up to an engine speed where a second system enters into operation. The engine temperature at idle for a modern outboard engine should be rather warm. A warm engine runs more smoothly than a cold engine. A warm engine means a temperature of more than 150-degrees and perhaps as high as 185-degrees. The exact range of temperature at idle speeds will depend on the temperature the thermostats open. The thermostat should regulate the temperature without regard to the seawater temperature, if the system is working properly.

At higher engine speeds (usually beginning around 1,500-ROM or so), a second cooling system regulation system comes into action. This second system opens a large valve to allow a very high volume and flow rate of cold sea water to come into the cooling system. A typical mechanism for the operation of this valve will be the water pressure in the cooling system. Cooling system water pressure is proportional to engine speed. These valves are often called a pressure relief valve (PRV) because they open at a certain threshold of water pressure in the cooling system. When the PRV opens the engine is generally rapidly cooled by the much higher volume of colder water now flowing the cooling system. The exact temperature the engine will stabilize at will depend on the temperature of the seawater. If the seawater is cold, say 60-degrees, the engine temperature will tend to be lower. If the seawater is warm, 80-degrees, then the engine temperature will tend to be higher. Also the engine speed will affect the temperature. Running at full-throttle will tend to raise the operating temperature.

The combination of these two regulator systems for engine temperature tend to produce a situation where the engine runs warmer at idle speeds, sometime as warm as 180 to 185-degrees, then cools down to a lower temperature at higher engine speeds, somewhere around 120 to 140-degrees.

As I read your narrative of the two engines, there is only a variation of 7-degrees between the two engines after a few minutes. But I infer that the starboard engine never gets above 93-degrees-F, while the port engine reaches 100-degrees-F or higher. This variation could be due to several causes.

The starboard engine that runs a bit colder may have a water pump producing more water pressure. This gives the engine more cooling at idle speeds. However, the action of the thermostat should compensate for this. If the starboard engine never warms to more than 93-degrees at idle speeds, the cause if probably the thermostat in that engine is not able to shut to a closed state completely.

The cause of a thermostat not being able to shut completely could be due to debris that has lodged in the thermostat and blocks the valve from closing, or from a rupture in a seal in the thermostat. Or, there may not be a problem with closing completely, and the thermostat is just opening too soon, that is, opening when the cooling water is still cold.

I don't have first-hand knowledge of the cooling system of the OptiMax 200, but usually the thermostats in the cooling system are not particularly hard to replace. I recommend you inspect the thermostat in the starboard engine to see if there is debris in the valve or a rupture in a seal. A new thermostat is not generally very expensive, so you could just replace the thermostat as a test to see if the behavior of the engine temperature is affected.

Also note that thermostats are available with different temperature thresholds. Seawater at 60-degrees-F is rather cold, so perhaps your engines could benefit from changing to thermostats with a higher temperature threshold. This would allow the engines to warm up to a better operating temperature range.

The poor running characteristics of the port engine at idle are probably due to the engine being too cold for best operation. You can probably cure those running problems with a new thermostat in the cooling system.

biggiefl
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Re: Variation in Engine Temperature at Idle between Twin Engines

Postby biggiefl » Thu Aug 06, 2020 11:29 am

To simplify that, replace your thermostats and report back.
On my 24th Whaler. Currently in the stable: 86 18' Outrage, 81 13' Sport(original owner), 87 11' Sport, 69 Squall(for sale cheap).

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Don McIntyre - MI
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Re: Variation in Engine Temperature at Idle between Twin Engines

Postby Don McIntyre - MI » Thu Aug 06, 2020 3:54 pm

Or, switch the thermostats and report back.

Tates2
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Re: Variation in Engine Temperature at Idle between Twin Engines

Postby Tates2 » Thu Aug 06, 2020 4:09 pm

Yes , I will replace or swap out the thermostat. That seems like the logical thing to do.

Jimh--thank you for the comprehensive response--very helpful.

i will report back.

Thanks
Tates 2

jimh
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Re: Variation in Engine Temperature at Idle between Twin Engines

Postby jimh » Thu Aug 06, 2020 5:52 pm

Tates2 wrote:Yes , I will replace or swap out the thermostat. That seems like the logical thing to do.


The water pump impeller could also be affecting the engine temperature at idle.

Anecdote: my engine was running very warm after extended operation at idle speeds. I noticed this one day when we had to run at very slow speed for about an hour in thick fog. The engine temperature kept climbing, and reached about 195-degrees. At that point I put the engine into neutral and increased engine speed for about 30-seconds. That opened the pressure relief valve, and the engine immediately cooled down to about 140-degrees. Later I had the water pump impeller replaced along with the wear plate and seals--what is called a water pump re-build kit. In addition, a new procedure for seating the impeller was used. The result of the service to the water pump impeller was the engine idle temperature now remains in the range of 170- to 180-degrees-F. I have not kept the engine at idle for an hour to see if the engine temperature eventually rises.

biggiefl
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Re: Variation in Engine Temperature at Idle between Twin Engines

Postby biggiefl » Fri Aug 07, 2020 10:23 am

His engine is running cold, not hot.
On my 24th Whaler. Currently in the stable: 86 18' Outrage, 81 13' Sport(original owner), 87 11' Sport, 69 Squall(for sale cheap).

Tates2
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Re: Variation in Engine Temperature at Idle between Twin Engines

Postby Tates2 » Fri Aug 07, 2020 12:21 pm

The new thermostat lists for $108, Mercury suggested if I remove it check it by placing it in hot water to see if it closes. Not urgent since it runs in normal temperature range 120-125 while in use.

biggiefl
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Re: Variation in Engine Temperature at Idle between Twin Engines

Postby biggiefl » Fri Aug 07, 2020 2:23 pm

Go aftermarket, they should be only a few dollars unless Opti's run something weird. I think a stainless one is around $12. It sounds to me like your one thermos is not closing correctly and why it takes longer to heat up. No need to put in water as it is obviously opening as it should. I personally would take out the one on the cold engine and see if a piece of sand or corrosion is keeping it open a tad.
On my 24th Whaler. Currently in the stable: 86 18' Outrage, 81 13' Sport(original owner), 87 11' Sport, 69 Squall(for sale cheap).

Tates2
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Re: Variation in Engine Temperature at Idle between Twin Engines

Postby Tates2 » Sun Aug 09, 2020 2:50 pm

A little more research reveals it may very well be the poppet valve per the shop manual OG960500 and above, Pg 4B-7 , Poppet valve controls water flow at high RPM, If poppet valve is stuck open at low RPM, the engine will not reach the proper operating temperature (run cold), and will run rough at idle. Sounds like the symptoms. When I look into it I get back to the post.

Thanks

dtmackey
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Re: Variation in Engine Temperature at Idle between Twin Engines

Postby dtmackey » Tue Aug 11, 2020 10:37 pm

In haphazardly read the thread and didn't see where there were any tests done on the gauge or sending units to verify they are registering correctly before throwing parts at the motors. Since the senders and guages are not calibrated, I've see difference in temps on motors. When ever I have a temp question or concern on an outboard or diesel inboard, I always verify with an IR temp gun and find that is a great tool.

D-

biggiefl
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Re: Variation in Engine Temperature at Idle between Twin Engines

Postby biggiefl » Wed Aug 12, 2020 11:24 am

It could be a poppet valve but it could also be a simple thermostat which I would try first.
On my 24th Whaler. Currently in the stable: 86 18' Outrage, 81 13' Sport(original owner), 87 11' Sport, 69 Squall(for sale cheap).