Temperature Gauge

[NOLA Proline 22]

All,
I have a 2000 Mercury 200 Saltwater series and both when i am cruising & idling the temp gauge will jump from just above 140deg. to just under 220deg., stay there for a few minutes and then drop back to 140.  It isn't a gradual climb or drop but a quick spike.  Any thoughts?  Thanks

[231CALIFORNIA]

short or open or high resistance intermittently in wiring to guage? sensor bad?

if not electrical. sticky thermsotat? not working smoothly, or plastic bag or piece of impeller in cooling circuit?
impeller old?
that stuff likely would move guage smoother though.

[kregars]

Had a semi-similar experience with an old Chrysler/Mercruiser that was due to a failing Thermostat.  It would close even though the water temp should have been high enough to sustain constant temps.  Took a tech about a week to diagnose till he pulled and boiled the thermostat and watched it fail to closed for no reason.   Again, different motor/setup but annoying as I'd replaced the temp sensor, sending unit, etc (strange thing I had the SAME thing happen to a 1989 Dodge Ramcharger with the same type though closed loop 5.2 liter engine).  Luckily after having the problem with the boat, I knew what the issue was likely to be and ultimately was the problem.

[wjm7947]

Check your ground wire!!

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