Hot but not
 

Just got a 1.8 NA that can idle all day and rev it up with no issues but take it for a drive and under a mile of driving it creeps up and pins the gauge. Go back to idle and sit and it comes down in a minute. Strange thing is it shows NO other signs of running hot except the gauge. I've even jumped out and put a thermal gun on hoses and thermostat housing and its all under 200* Ever seen this? Could sender or gauge be bad? I know the two wire sensor is for fans and ECU other single wire is for gauge and gauge alone?

Fans do work fine, they come on like they should.
System had been flushed.
I have replaced the Thermostat and Radiator (not because of this issue but just as maintenance).
Timing is at 10*

ideas?

It sounds like you may have answered your own question. I hear you say your car is running normally and the only thing saying you're hot is the gauge. I'd be inclined to question the gauge then. Or more likely the sender. Have you tested that on its own?

Agreed but why would driving have any bearing on it?
Also I've owned plenty of these cars and never seen this issue before.
Thx

Really good question and I don't know the answer. But one of my rules for life is to keep eliminating stuff until you find the problem. The sending unit seems easy to check too.

I know this isn't the same, but I'll share that I once had an oil pressure sending unit on a Jeep that acted all wonky when the engine got hot. It went non-linear and then started bouncing around. That was 25 years ago so it took me a while to search for the problem. My mechanic knew exactly what it was. :-)

Sounds like poor bleeding of the system after the flush.

+1

1st time I'd buy that but I went back in and replaced the new thermostat with another new thermostat and jacked up front, it idles and reved for 2 hours with a funnel with water in it. I think its bled this time.
I never had issue with having air in a stock miata system before. Some of the reroutes I have done has issues but not stock setups.

Having to add coolant every so often? Just because you have bled the system doesn't mean you aren't leaking a little and re-creating the air pockets. Theoretically you can have a slightly blown head gasket (maybe?) that is bleeding air into the coolant?

Just thinking out loud.

It has crossed my mind.
I picked up a new sensor for gauge tonight and I'll cross fingers and try that tomorrow.

I was having a problem with cooling on mine as well btw... Found that one of the radiator clamps on the heating hoses failed.

I was having a problem with cooling on mine as well btw... Found that one of the radiator clamps on the heating hoses failed. It was dripping ever so slowly but because it's so close to the turbo it never showed on the ground.

Mine did that once when I had the factory undertray off. It would do just fine sitting still but I drove it 5 miles down the interstate and it was trying to overheat.

Ok, under tray is installed.
Replaced gauge sending unit.
Replaced dash cluster.

Same thing, idles for ever no issue. Drive it a mile or so at 35 mph 4th gear and it creeps up and pegs gauge. Than moves around that side of the gauge than if you stop and return to idle it in a minute it returns to below the middle on gauge. Shows no other signs of being hot.
I'm lost.

History on the car is it has sat for 3 years before I got it. Don't know if that had anything to do it this. 1995 with 70K on clock.
I guess nest is pull water pump?

Little long but you can see the wild swings of the gauge.


This was seconds later with temp gun.

Ok... What happens if you sit still and keep the motor revving at 2,250 rpm? Does the temp go up? Harder to do would be to get up to highway speed and then put the car in neutral and coast at idle speeds for as long as you can and see if the temperature goes down.

This car isn't on a mega squirt is it? It would be interesting to see what the ECU is seeing the temperature as.

Good idea.
This car is untouched stock with 70k

True.

Well been sitting at 21psi for 15 min..... So.... That's not itihttps://cimg3.ibsrv.net/gimg/www.mia...41aaa53d04.jpg

True.

Well been sitting at 21psi for 15 min..... So.... That's not itihttps://cimg3.ibsrv.net/gimg/www.mia...41aaa53d04.jpg

Well I I can't explain it with the cooling system holding i figured head gasket ok. Guess not. Bubbles and bubbles.. Any other explanation before I pull the head?


No thermostat in motor.