Miata won't warm up - and it's not the thermostat?

Scratching my head pretty good here.

My '97 Miata daily has not had great heat all winter. I had chalked it up to a partial clogged heater core, but last week I back flushed it with no grossness found inside.

Looked around and found all of the hoses to be running rather cool (120F-ish). So I threw in a new thermostat. No change.

Of course this morning it was 12* when I left the house so I let it warm up for 10 minutes. When I got in it was barely off full cold, warmed up to near-normal by when I get on the highway... and then the needle DROPPED as I got on the highway. Didn't start to recover until I got back off.

To me, this SCREAMS open thermostat. However, I just replaced it.  Both same temp, and I have the old one handy to test.

Walk me through this GRM. The low will be 3* in a few days. I need heat, as does my engine.

Is there something that could allow coolant to leak past the thermostat?  Does the temp start to come back up if you turn the heat off?  

Trapped air bubble in the head?

In reply to rslifkin :

That's the thing, I don't think so. It's really straightforward.

I'm going to put cardboard in front of the radiator for now to have some heat on my drive home. But I can't imagine I have 2 bad thermostats in a row (although I will test the old one).

In reply to chaparral :

How would that cause... too much cooling?

Blaise said:

In reply to chaparral :

How would that cause... too much cooling?

No, but it could cause funky gauge readings and poor coolant flow to the heater core, making it seem like it's running cold even though it's really not.  

I'm checking temps with an IR gauge, not the dash temp. Engine is actually running cold.

Do you have an oil cooler? Something like 70% of the motor cooling is done by the oil, so maybe an aftermarket oil cooler is over-cooling?

And I wouldn't put it past 2 faulty thermostats in a row. You should be able to literally feel the thermostat open as the engine idles and warms up (hold onto the hose, or watch the temp with IR gun).

Finally, do you have anything blocking the coolant passage from behind the thermostat back to the water pump? (this is where coolant circulates if the thermostat is closed, maybe internal to the head) if that is blocked, then water may be pushing its way past the thermostat. You could have crud built up, or I know some 'racing' parts were sometimes built to block off those passages for people who ran no thermostat and wanted to force all the water to go through the radiator.

No oil cooler. Bone stock daily driver, bought it from original owner who put 170k miles on it. Was on touring tires. 100% sure there's no performance-cooling mods.

No modifications to the thermostat neck. The thermostat itself sealed right up against the housing as it should.

I'd start by testing both thermostats.

If the engine's running cold, the new thermostat might be bad, or the thermostat housing might be shot and leaking coolant around it!

Did you test the thermostat you removed to see if it was faulty?

Sounds like low coolant to me.

I went through this with my Miata last winter. Did the cardboard over half the radiator thing and got heat which just indicates the thermostat isn't functioning. Once the weather warmed I changed the thermostat (the old one wasn't very old) and now it has proper heat. My money is on a second bad thermostat in your case.

Boil water on stove, drop in thermostat, watch it open, clean out pot before significant other finds out.

oldopelguy said:

Boil water on stove, drop in thermostat, watch it open, clean out pot before significant other finds out.

I'll use her copper pot to make her extra angry :)
 

I'll test the old one tonight. Having 2 bad thermostats in a row would be really crappy. For the record the old thermostat was NOT stuck open.

Is there any failure mode in which the cooling system can overpressurize and cause the thermostat to open?

They rarely stick open, often they stick closed.

If your radiator cap somehow was welded shut, it could cause an overpressurized situation, but that would cause a great many issues.

What are you using for coolant?

My Ford Ranger does this on cold days.   I can watch the temp needle go to the normal operating position then the thermostat opens and the needle drops to a 1/4 warmed up.   Over and over.    

I used to worry about it but gave up.  

The big diesel truck guys with huge radiators sometimes have louvers over their radiators to close so the engine will warm up on cold days.  

Blaise said:

oldopelguy said:

Boil water on stove, drop in thermostat, watch it open, clean out pot before significant other finds out.

I'll use her copper pot to make her extra angry :)
 

I'll test the old one tonight. Having 2 bad thermostats in a row would be really crappy. For the record the old thermostat was NOT stuck open.

Is there any failure mode in which the cooling system can overpressurize and cause the thermostat to open?

Yes, if the internal passage is blocked preventing coolant flow anywhere but through the thermostat, it would cause the thermostat to be pushed open.

Blaise said:

oldopelguy said:

Boil water on stove, drop in thermostat, watch it open, clean out pot before significant other finds out.

I'll use her copper pot to make her extra angry :)
 

I'll test the old one tonight. Having 2 bad thermostats in a row would be really crappy. For the record the old thermostat was NOT stuck open.

The failure mode I've seen more often is that the thermostat opens early and then doesn't open fully when needed. I don't think I've had a thermostat that was stuck fully open for quite a while.

If you got your thermostats at the local parts place, you might want to try splurging on an OEM thermostat or at least a different brand of thermostat.

BTW, I assume you're not using one of those 'lower opening temperature for "improved" cooling' thermostats.

No, stock temp.

Lukewarm heat from the vents could be lots of things, I personally wouldn't 100% trust the IR thermometer. I diagnosed a similar issue a while ago (replaced thermostat like you did) and it turned out that the gauge temperature sensor was out of spec. 

You may want to check my thread, it's literally the same problem as yours:

https://grassrootsmotorsports.com/forum/grm/bewildering-cooling-issue-too-cool/134461/page1/

If you don't trust the sensors, use an OBD-II reader to see what the ECU thinks the temp is. The ECU and the gauge use different sensors on an NA Miata. But it's far more likely that a minimum-cost thermostat is bad out of the box.

From experience, go with OEM.

iceracer said:

From experience, go with OEM.

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