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.

Wait, what? No thermostat in motor? I thought you said it was stock. You should run a thermostat.

I wonder if the water pump impeller still has all of its blades at full size.

I also wonder about an air pocket.

It had a thermostat (tried two) I pulled it out to see if water pump was moving water. It is, and air.

Still don't believe this so I did a compression test
1 170
2 170
3 175
4 180

1 - 4 left to right. https://cimg7.ibsrv.net/gimg/www.mia...2a684bdbe0.jpg

Plugs look good. Head gaskets are often misdiagnosed using the funnel. Let it warm up, you'll slowly see the fluid level rise as the coolant expands. Then the thermostat will open and the fluid level will fall quickly. That's when you need to stop the engine, pull the funnel, and cap the radiator. Any longer and you're just not allowing your coolant system to operate under pressure, which it needs to, and keep it from cooling properly. Once you do this, let it cool and then watch the pressure gauge. If it immeiately spikes to high pressure, you have a bad head gasket. It should slowly rise until your radiator cap pressure (usually 1.1 bar), then bleed down/up/down etc.

Did you pull your coolant out of a pond?

That's all I use.

So did you determine if it even is overheating yet? Unless I missed it you are getting ready to pull the head but no mention if you verified it.

Last drive I let it sit at the pined mark on gauge and I could hear it boiling when I got hood open. The plugs for an few hours of use have signs of heat on the electrode.
Far as I can tell its all it could be. Or cracked block or head.

I'd say HG, or a restriction that was missed, or WP. Just because you see flow, doesn't mean the pump is up-to-par. And yea that's some shitty coolant.

HG now changed, I'll put rest back together tomorrow. No real determining factor on the cause of the issue other than exhaust valves look a bit hot. Water pump looked ok, timing was right on, and HG looked ok. I'll put it new water pump in and cross fingers.

Oh if the head gasket was actually bad, and it was actually over heating, you'll need a new head and valve cover.

So I change the head gasket nothing remarkable didn't look bad change the water pump while I'm in there change timing belt start car up idle for 15 minutes brought it to 2,000 RPM started overheating again boiling out of the radiator cap interesting enough one of the heater hoses never got hot I had flush the heater core before had no issues. I've just bypassing the heater core we going to try that would a blocked heater core cause a car to overheat?https://cimg3.ibsrv.net/gimg/www.mia...18b29aba35.jpg
https://cimg8.ibsrv.net/gimg/www.mia...bcbc3dc358.jpg

Bypassed it seems to run fine???

Could a blocked heater core cause over heating?

Well answer is YES!
Put many miles on it tonight, no issues. I have owned many Miatas and worked on many and never seen this issue before. Guess what I ll be checking 1st next time.
The new owner picked it up today and is thrilled!
Thanks all!
Now back to the slowest MSM build ever.