inconsistent temperature gauge readings

I have a pair of 1988 Merc 135 outboards on 23' CC. Last summer, a couple trips after water pump impeller changes, I noticed that the starboard engine was showing higher temps than normal at mid range rpms(1000 to 2500rpm)- the port remained normal.<br /><br />I shut the engine down and let the other one push us home. The next day I fired them both up and found that at idle the starboard ran cool. At the mid range it showed hot but if I got up on plane at higher revs it cooled back down and showed normal.<br /><br />When the starboard engine shows high temps, I occasionally stop to check the pee hole water and find it 'touchable' - probably 110 or so - certainly not the high end temps being indicated and only very slightly warmer than its twin.<br /><br />As far as I can tell the temp sending units are small cylinder shaped items slightly smaller than a dime in diameter and about 1/2" tall. they appear to lightly touch the valve cover. The stamped detail on these items reads: 438106 33/03 12 volt. I can find no other senders. <br /><br />I'm reasonably confident that the overheating is not real but the false reading are driving me crazy. Can anyone help?

Re: inconsistent temperature gauge readings

On your engines you have temp sensor and an overheat switch. The temp sensor is "usually" tan wire and the overheat switch is tan/blu wire. If the temp gauge is reading "H", the warning alarm should go off. So either your temp sensor is faulty, overheat switch is, or warning alarm is.<br /><br />If the temp sensor is working properly you should measure a resistance between it and the block and the resistance should change as the temperature of the block does.<br /><br />If the overheat switch is working properly you should see an open circuit between it and the block and if you ground it to the block that should set the alarm off, if the key is turned to the "ON" position.<br /><br />Do you have a water pressure gauge? What are its readings? Have you ever serviced the tstats or poppet valve assemblies?

Re: inconsistent temperature gauge readings

Thanks Andy. As soon as the rack storage thaws out I check ouy your suggestions.<br /><br />As far as i can tell there are no t stats installed on either of the outboards. No, the poppits have not been serviced but when I took the boat to a Merc service location thinking that I had an overheat condition, i was told that they were working.<br /><br />Bob

Re: inconsistent temperature gauge readings

If you are running with tstats you will want to install them.