Temperature gauge fluctuating

Hi all

Has anyone ideas of cause for fluctuating coolant temp gauge please?

Car is 1997 MGF 1.8 VVC

Temp comes up to normal while driving, then drops two graduations on the gauge. It slowly recovers to normal then repeats the performance continuously for the rest of the trip. Town driving, nothing too hectic, moderate ambient air temp. Cooling quite important with Aussie summer not far away.

Would appreciate possible causes please.

Thank you

Might be worth checking the thermostat.

There’s bugger all current flowing in these circuits so any high resistance on contacts or earth returns can cause many a headache. This evening I managed to resolve an electrical problem which had evading detection for a couple of weeks; I’d failed to adequately cleaned the paint off a hole redesignated as a convenient earth. A couple of scrapes with my penknife & alls well.
Look for simple problems first, even with our cars the easy jobs still outnumber the difficult ones.

Thank you for those ideas. I’ll be installing a new thermostat in a couple of weeks (hip replacement), and I’ll check the circuit continuity. I’ll let you know what I find.
Cheers, D

Any quick deviation of the gauge is most probably electrical rather than temperature related.

This appears to be cyclic and fairly predictable in its behaviour, so I reckon it is a failing thermostat, opening ok but not closing quickly enough, rather than rebellious electrons. As such, it needs looking at because if it jams in the closed position there will be trouble.

Firstly the temp gauge is dampened and secondly if the thermostat is intermittently failing it will go up from normal upwards and fall but never fall below the midway point (as long as the engine is running). I would put my money on the problem being electrical.

No. If it opens and does not close fully then the engine temperature will fall. Consider what would happen if the thermostat is removed.
This is almost certainly a lazy thermostat. Electrical problems of this type are characterised by being intermittent. This is cyclic and predictable, like the operation of a thermostat.