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40 HP Mariner Overheating

barefoot_maumee

New member
I am having trouble diagnosing what is going on with my 1997 Mariner 40HP. The problem is when shortly after I throttle up to about 3/4 to full throttle my over heat alarm begins to sound (continuous beep). If I bring my throttle down below 3/4 or lower the alarm will stop shortly after (about 5 seconds later). I can cruise all day below 3/4 throttle with no problems. This problem first started last year. My tattle tail on the motor was and is streaming well but I went ahead and put a new impeller in it anyways. Still the same problem. I replaced the sensor on the head (P/N16775A3) with a new one from mercury, still the same problem. My Fluke multi meter has a temperature probe on it, I ran that into the motors tattle tail while at full throttle and buzzing like crazy. The highest that temp got to was 125 degres. Someone can correct me but I don't think that is an overheat situation. I installed a 1/4'' tube on the gauge port with a small 1/16'' orifice to see if at full throttle the impeller was sucking air. I never had air come out of the tube, it was a steady stream the whole time. I did purchase a used alarm module box (P/N 14857A10) that is the brain for the overheat and oil alarm with still the same problem. It actually gave me another problem. It doesn't do the Beep Beep when the key is turned on, and when the motor is running idle you can hear faint intermittent beeps. I unplugged the overheat sensor and the faint beeps continued, when I unplugged the oil tank they quit. So I reconnected only the overheat sensor with still the same high throttle overheating alarm. I checked the voltage at full throttle and that is right on 14.5 volts. I used an inferred thermometer and measured the head temp while beeping, the highest I ever saw was 140 deg. My wide open rpms are 5400-5500. I believe this is correct for that motor. I have had this pontoon for about 6 years and never had a problem until last year. I am hoping I haven't bored everyone with my detail but I really would like to know what is going on. Do I somehow have two modules that are bad and act exactly the same? Do I somehow have two over heat sensors that are bad and act the same? Does anyone have a way to check the over heat sensor? Or is this thing somhow really getting hot? Could the carburetors be somehow leaned out over the years and at full throttle are burning hot enough to set the sensor off? Sincerely the guy that is buzzing when running from a rain storm.
 
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