Incorrect water temp dial reading

Any external measurement will always be lower. It isn't a valid test.

 

My coolant temp gauge is suspect, I have had issues with it indicating too warm, ~220F, on occasion. There are a few of us here with similar issues. At this point, I suspect the gauge is the problem.

I've done infrared temp gun readings as well, but I use the thermostat housing, or the aluminum coolant pipes on the right side from the water pump, or the coolant pipe on the left side, compared with gauge reading and the operating state of the radiator fans. I have found that the coolant pipes when the engine is fully warmed up (after driving) will be at around 90-92C, maybe 94C when the fans comes on - but the gauge can be reading 195 or 220 or in between, without consistent correlation.

Recognizing Brian's point that an external reading is not directly comparable to the sensor reading in the coolant flow stream, it's important to establish consistent reading points and compare external temp readings for different operating conditions - gauge indicating normal, gauge indicating high, radiator fans operations, etc.

On occasion, I've seen where the coolant temp gauge is indicating high (220), then will suddenly drop to 195 - in the space of 1 second, like a clock ticking, so not indicative of a stuck thermostat opening where the drop would be less rapid.

After having searched here and read many threads about the coolant temp, I have reached the tentative conclusion that the fault lies with the gauge itself. Several members have written of similar issues that were resolved by removing the gauge panel and cleaning the electrical connections on the back of the coolant temp gauge, and also cleaning the wiring harness ground connection points under the dash. I have put off doing that myself until the gauge LED lights arrive so I can convert the instrument panel lighting to LEDs (any day now!), I will be checking the coolant temp gauge at that point.

I haven't read of the coolant temp sensor giving problems of false readings or inconsistent, randomly changing readings, but that is still a possibility. I'd be interested to know of problems with that component.

Each case will be different, though, and I wouldn't want to lead you to an assumption that it's just the gauge and that you don't have an overheating issue. You want to be sure that your thermostat is functioning correctly, and that the rest of your cooling system is in good shape - radiator, hoses, water pump - before focusing on the gauge.

 

Check that you have a really good 12v+ at the gauge. That is the cause of many of the problems you describe. On some cars when the lights or rad fan goes on the gauge will suddenly move. It is due to a voltage drop on the circuit. The instruments are very voltage supply sensitive and is what that kit that TacElf makes for the oil pressure is designed to solve.

 

Agreed, that's my plan along with cleaning the gauge contacts.

I know there is a voltage drop issue to the dash gauges, right now when I turn on the running lights my fuel level, oil pressure, and coolant temperature needles all go up by a few needle widths! (The oil temperature gauge in the console doesn't move)

 

I notice my temp gauge fluctuating over the last 25 years. It is indeed a voltage issue. My car has EFI and uses a separate temp senser from the one in the dash. The EFI system logs all parameters including water temperature. It is rock steady while the factory gauge tends to move around. I am always concerned when the dash gauge goes up. It is comforting that I don't see a corresponding change on the EFI temp reading.
However, I think you will see your dash gauge change within a range of readings. If it gets above that range, definitely take precautions and check it out.