Mercruiser 5.7L inboard... temp guage says runs cold

Hi.... I have a 350 5.7L 300 HP tournament ski engine in my 1997 Malibu Response.... every year I check the fresh water pump, and change the thermostat.... because historically it has run just over 180 while skiing... and just below 180 while idling.... there is a 160 thermostat in there.... this year, did same as always, but no engine runs too cold.... when grounding the temp guage, it goes to top... when running engine, it will warm up slowly to 140... then temp guage will drop back to 100... slowly go up again, sometimes to 120.. sometimes to 140.. never higher.. and always dropping off to 100 after a fewe seconds.... so, just changed the thermostat again.... same problem. Anyone got any suggestions.... could it be something with the engine blocks main water pump... opened one of the block drain plugs after running a few minutes... plenty of water... but hot. Now sitting high and dry... afraid to run engine for fear of damage... help!

Re: Mercruiser 5.7L inboard... temp guage says runs cold

buy a cheap guage wallmart auto zone hook it up even just hold it to see or mount by the motor for a sec gage.oem one might be bad a hand held infer red temp gage will tell u 2 but the other is cheap to see whats going on

Re: Mercruiser 5.7L inboard... temp guage says runs cold

did you solve the problem ????

Re: Mercruiser 5.7L inboard... temp guage says runs cold

You're thermostat is probably sticking open a little bit, or partially block a little open. Common on raw water cooled engines.
You should be able to tell if it's actully 140? by feel. At 100 you can keep your hand on the stat housing with no problem. at 160, maybe 4 or 5 seconds