Overheating

[Waltz]

OK, hopefully someone can shed some light on my problem. I have a 2002 Carver Mariner 35ft, with twin 5.7 Mercs. I noticed that my port engine temperature guage started to show above the middle (at around 190-200) degress. Overheating alarm does not go off. Normal op temp i believe is 150-160. The following has been replaced, but still shows on the high end: new temp sensor, thermostate, water pump, risers and manifolds, impeller and hoses. I also checked the guage, which works fine. Using IR temp guage it shows the engine at bewteen 125 and 145. Is there something I'm missing?

Someone told me that using an after market temp sensor (not Merc product) could result in a false reading, is that true?

[gonzo]

It looks like the guage is the problem. The easiest way to check it, is to use a jumper wire from the sensor of the stb engine to the port engine guage.

[Waltz]

Thanks, but I tried that. I went under the dash and connected the port engine to the starboard engine guages and it read the same.

I'm going to try buying a Merc sensor and see if that works. Maybe there was something wrong with the after market sensor.

[Boston]

is it a marine exhaust or a closed system
either way you could have a blockage in it somewhere

doesn't sound like the gauge if you switched them and that one engine still reads high
best of luck
B

might check your fuel air mixture
if that engine is running lean in can make it overheat

[pistnbroke]

if you connected the sender on the good engine to the gauge reading high and it still reads high then its the gauge unit itself thats faulty....

if the fault follows the sender ie always on the gauge with the non merc sender then its the sender. and yes they do vary ..resistance cold and hot differs with make /part number .
One way to be sure whats going on is to remove the sender extend its wire and earth and put it in an electric kettle ....when the water boils you will know where 212 F is and if you have a thermometer you can track the reading as it warms up..

[kenJ]

You may want to swap sensors between motors, see if the problem follows the sensor or stays in the port engine.

[Frosty]

Connect the guage on the good engine to the sender on the hot engine then!!

Or maybe it is hot.

[kenJ]

One other thought, make sure there is no trapped air in your cooling system. Often times the thermostat housing is the highest point. Air will collect there causing a bubble. The air in the bubble doesn't transfer heat to the coolant so the metal gets hot causing a higher than normal temp to be indicated. This might be why the IR gun indicates normal temps when pointed at the engine, but the temp reads high.

[Boston]

way to go Ken
given the angle of most marine applications that could certainly be a possibility

[Waltz]

pistnbroke, you are correct. I at first used an after market temp sensor. Installed Merc sensor and all is well. Seems Merc has a monopoly, can only se their products and the engine will work fine. Thanks all who provided advice...that's why we're here.

[HughGWrecktion]

There is nothing stopping the after-market sensor from adhering to the specifications of the the Mercury sensor. You can't blame Mercury because the after-market company screwed up in creating a sensor with the correct specifications.

[PAR]

All companies make defective and slight error parts. It's the cost of mass production. With twins it's important to keep apples to apples, so if one sender takes a dump, replace both. Also make sure they are located at the same place on the engine.

[HughGWrecktion]

I don't necessarily agree with that "replace everything in sets" for everything. It makes sense for say tires because they will wear at the same rate (well front may wear faster or slower than the rear), but for a non-wearing component like a temperature sensor I would perhaps buy two and keep the spare on-board since the one you don't replace could outlast the new one.

[pistnbroke]

unfortunatly temp sensors of the type used in vehicles and boats do deteriorate with cyclic use as they contain a spring which due to the heat cool cycle deteriorates and eventually you have a high resistance connection.

[PAR]

Agreed Piston, the bimetallic spring does have a life span and they also have slightly varying resistance which can mean one sender will read slightly different then another if they are different brands, age or type. If you want equal information, you need a level playing field. I treat my wives the same way, replace them all, start over again.