High Coolant Temp, Normal?
 

My 1990 Miata is showing very high coolant temperatures of 230* F+ up to 260* F while driving through stop and go traffic. This is displayed through an aftermarket water temperature gauge that is tapped into the thermostat housing with the thermoswitch (upper radiator hose). The fans strangely do not come on. I bought a new water thermoswitch from Autozone but that also failed to turn the fans on when the coolant temperature reading from the aftermarket was 207* F. The fans are confirmed to be working yet through the TFA-GND test. The temperature gauge in the dash sits between 11:30-11:45. Is this normal? Could the gauge be faulty? Thanks in advance.

Your fan relay is not functioning correctly. Either the relay is bad, or the terminals are corroded.

I don't know where the coolant fan relay is on the 1.6s. Do a search.

The driver's side fan is the "coolant fan", it will come on when your thermoswitch says your engine is hot.

The passenger's side fan is the "a/c condensor fan" it will come on when your A/C is on.

With the A/C off, in slow/stopped traffic, you should start to be cautious if your water temps go above 215. If your water temps are rising above 230 in the same situation, you need to be worried. If your water temps are showing above 250 at any time, panic should be setting in - as in "I need to either turn the car off right now, or else find a way to drive continuously at a speed of at least 30mph until my ---- cools down".

Alternatives to shutting the car down are:

1. best alternative - turn the heater on. You want it blowing as hot as possible, as fast as possible - so full hot, and full fan speed - might wanna open your windows, drop the top, and point the vents away from you.

2. plausible alternative - if you've seen mythbusters, then you know "plausible" means that it could theoretically work, but I can't prove that it ever has. Turn the A/C on!! This is a plausible solution because the A/C powers the second fan on the radiator, and so turning the A/C on is going to begin to move air through the radiator. It's also possible that this may not work, because turning the A/C on is going to create an additional heat source in front of the radiator, that fan is going to have to overcome the heat in both the condensor and the radiator - no way is it going to bring it down to normal operating temps, but it could bring it down from "panic" to "worried". You'll want to make sure that the A/C fan works before trying this though - if the A/C fan doesn't work, you're just going to make things worse. Also, if you decide to shut the car down because of heat, turn the key back to the "on" position, and turn the A/C "on". If the car's off, it won't be pumping the heat into the front of the radiator, but the A/C fan might come on and draw a ton of heat out of the water that's sitting in the radiator. Give it about a minute and start the car back up - your temps will still show high initially, but as that cooler water starts circulating through your engine, you'll notice a pretty nice drop in temps over about a 20-30 second time frame.

Sounds like your sensor is on the way out. Mine did that for a while before I replaced it. It would have weird temp swings of almost 30* depending on if I was using my heater or not. Put a new one in and all was well.

Your fans are switched by the thermoswitch in the thermostat housing. The fan not coming on means that it is satisfied with the temperature. You've also renewed that thermoswitch.

Your coolant gauge is powered by a thermister on the water outlet on the back of the head. 11:30 to 11:45 position indicates good temperature.

So, you have three independent temperature sensors that seem to agree and point to everything being normal. And, you have an aftermarket sensor that you've jury rigged into your thermostat housing that is scaring you to death.

First thing I would do is verify that aftermarket sensor. A pot of boiling water should do the trick.