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I have a 96 Miata that keeps overheating. It doesn’t loose any coolant, it just goes into the overflow and sucks back in when it cools. I changed the thermostat, thought maybe it went bad and drained, flushed and refilled with new coolant. Didn’t fix the problem. Last night i read something about taking thermostat out to be able to see flow in order to check to see if water pump was good or bad… pump flow seamed just fine… and even with thermostat left out, allowing full open flow through radiator, still overheated!!! I"M SOOOOO LOST AND CONFUSED!!!

One easy thing to look for is a corroded radiator. From the rear of the radiator, look for missing fins between the tubes. If you see a few spots where they are missing, you need a new radiator. If you see green and white coloring, you should get a new one. If you notice any missing ones from the front side, don’t wait to replace the radiator. You may have to pull the shroud back to see much of the back of the radiator. It may be bolted or fastened with screws at the top.

Have you changed the radiator cap. If it’s not holding pressure, it would cause the symptoms you describe. It’s a lot cheaper than replacing the radiator.

Ed B.

Coolant expanding into your overflow tank is normal and expected.

Do you have any other reason to believe your engine is overheating?

Why do you think it is over heating?

Please define “overheating.” If you’re not losing any coolant there may not be anything seriously wrong. Does the temperature guage read high? Has anyone measured the ACTUAL temperature of the coolant, with a thermometer? I’d try a new radiator cap, and see if that makes any difference, before going any further.

No odvious or excessive damage that i can see…other than just a few eraser sized dents in the fins on the front from rocks over the years. Thanks though.

Many thanks to all, i am verrrry thankfull for any and all suggs. Well… by overheating i mean the temp guage slowly climbs… goes on up into the red…junps alllll the way over against the stop post… and steam ends up boiling out from under the hood. Textbook overheating i think…i just can’t figure out why.

You did well, check it in another two years. The other replies are right about the flow back and forth to the overflow. In the old days, without coolant recovery, we weren’t allowed to fill the radiator to the top. We had to leave an inch or two for expansion. Or was it a half inch? It was at the end of the sixties anyway.
This is a good system; if there is anything floating to the top of the radiator, it ends up floating at the top of the reservoir. It helps keep the inside of the radiator clean. It was unintended but it works that way.

Don’t forget to check that the thermostat is fully opening, and at the correct temperature.

You’ve Got To Stop Running This Miata Until You Have It Figured Out!

Many thanks to all, i am verrrry thankfull for any and all suggs. Well… by overheating i mean the temp guage slowly climbs… goes on up into the red…junps alllll the way over against the stop post… and steam ends up boiling out from under the hood. Textbook overheating i think…i just can’t figure out why.

The steam and high temperature indication is Mother Nature’s way of telling you that you may be causing serious damage.

How’s the temperature, driving? How’s the temperature, idling?
Is the cooling fan working? Tow it to a mechanic if necessary, but stop over-heating it.

You may be one over-heat short of serious damage or one over-heat beyond.

I’d replace the radiator - it’s probably plugged, so while coolant flows around the outside and through the engine, it does not flow across the fins to get cooled.

With the age of the vehicle, the problem could be with a restricted radiator. A radiator can look fine on the outside, but have restrcted cores internally. This reduces the cooling capacity of the radiator where it can no longer remove the heat from the coolant fast enough to prevent the engine from overheating.

One way to check for this is to take an infrared thermal gun and point it at the raditor cores with engine running. The cores that have little or no coolant passing through them will be much colder than those that do.


My Inexpensive Infrared Non-Contact Thermometer Could Check That Before Replacing It.

You’re possibly right, but a new radiator could be the first in one of many expensive guesses.

Lots of ideas, but replacing the radiator cap is certainly number one in my view. Easy, cheap, and it might work. Otherwise I wonder if the heat gauge sensor is faulty and leaking so, once under pressure, it allows water to get on hot parts and also reads wrong. Can you tell where the steam is coming from? Start it up, with the hood open, and let it run. Watch everywhere for steam. Look at the underside of the hood because if the water pump is leaking it will usually get on the fan, get thrown around and make a line of dried crusty stuff across the underside of the hood. And check your radiator hoses - if they are original they are pretty old now. Miatas are not usually found with cooling system problems.

Ok, replaced radiator cap…no diff. There was a slight leak from water pump, so I replaced it too, still no diff. I was beginning to suspect a blockage of some sort, some where, based on facts that it only starts to get overly warm when actually driving, if i pull over and let it idle it cools down pretty fast AND am a bit more concerned about this part… the temp coming from heater just isn’t very warm at all. Is it possable for there to be a blockage somewhere in the engine block itself? If it was a blockage in the heater line… that wouldn’t cause the rest of the system to overheat would it? If it was just a partial blockage in the radiator wouldn’t the WHOLE system being really hot make the heater blow just as hot?

You could have the cooling system pressure tested and check to make sure you don’t have a head gasket leak. However the infrared test or even the hand test (the hand of a veteran mechanic) can usually spot a clogged radiator, much cheater than replacing anything else. If the radiator is clogged the heater may be equally clogged, but let’s start with one thing at a time.

It also could be a bad hose.  A new looking hose and even a hose that feels solid can be collapsed inside. Those are the types of things that an experienced mechanic can usually detect quickly and cheaply.