HOT Miata

[ChristianL]

By now, you all know how hot my car is, right?

Well, today it really was.

The cooling system has always run really nicely on the car, and is pretty much up to the task of spirited driving. Dual-core metal radiator, new water pump ~10k miles ago, new coolant/water about ~200 miles ago, etc. Everything looks and works well. The car's been driven hard with no problems.

Then today it went for emissions, and while Beth was waiting in line with it, the temp gauge hit "H", and the contents of the radiator ended up in the overflow tank.

According to the guy at VEIP, the upper hose was hot, the radiator was cool, and whatever was in the overflow tank was boiling. His diagnosis was a bad thermostat.

They filled the radiator with straight water, put the radiator cap back on, and Beth drove it back to work with no problems.

So since I have to troubleshoot this when she gets home, does anyone have any ideas what I should look for?

[ChristianL]

Also, let me make sure I get this straight.

Coolant/water goes in the radiator. Flows through the lower hose to the engine. Gets pulled through engine by water pump, which puts it through the thermostat to the upper hose back to the radiator. Right?

Also, when thermostats fail, they fail open, don't they?

[mistanfo]

I believe that a thermostat is designed to fail open, but I have had 2 fail closed, 1 fail open. However, since it is a cheap fix, I would recommend trying to replace it first. If you want to test it, remove it, clean it well, and put it in a pot of water, and then boil the water. The thermostat should open long before the water boils, though you would need a thermometer to make sure that it is opening at a "normal" temp.

[djarum69]

No Christian, cold state on a thermostat is closed, so if it starts up cold, and is therefore shut and refuse to open you are stuck with no radiator. They can design it to try to fail open, but it usually fails closed.

If the radiator is cold but you still have a timing belt (water pump is running) then you either have a total restriction in the top or bottom hose, or you have a bad thermostat. It's an easy fix, but remember the RTV.



Edited By djarum69 on 7/12 09:52 at 2005

[ChristianL]

Thanks for the info, guys. I guess I should know better than to assume a $8 part would be that fail-safe. I'll probably pull off the hoses and change the thermostat anyway.

BTW, I checked Samantha's records, and it looks like PBC did the timing belt/water pump about 6k miles ago. I've heard good things about their work, but when Mike and I drained the cooling system a couple months ago, we found it was just filled with the crustiest water you've ever seen outside of a sewage plant.

When I changed the radiator a couple weeks before I sold it to Samantha, I put a 50/50 mix in, and I don't think she ever touched it, so I'm wondering who worked on the car at PBC. I almost called Pete to let him know.

[GoKart]

They probably reused the coolant. I'm sure more coolant would be on the service ticket if it was included.

As Ben mentioned, use plenty of high temp silicone and DO NOT over torque the thermostat bolts.

[ChristianL]

Took a few minutes tonight, and figured out the problem.

I don't know about the later cars, but the 1.6 has a temp sensor perched right on top of the thermostat housing. With a slight jiggle, mine came off with a broken wire.

As you can probably deduce, this controls whether the fan goes on, and since the wire broke, no signal was sent, fan didn't go on, car overheated quickly.

So does anyone know how this is hooked up? I didn't have the time (or daylight) to pull the thermostat housing, but when I grounded the sensor wires to the housing, the fan went on. Is there something inside the housing this wire attaches to? I'm hoping this will just be an easy soldering fix.

[bill_keksz]

No sensor on the 1.8.
Does it unscrew?
Did the wire break right at the sensor? If so, replacing the sensor ( & tstat housing, if inseperable) is probably the best solution.

[ChristianL]

There's a wiring harness that runs in front of the cam pulley cover that clips into a female connector on top of the tstat housing. This in turn appears to be bonded to the housing somehow. It's the wires that run from the female connector down into the housing that broke.

I've just never popped the tstat housing off, so I just didn't know if the factory grounded the wires to the housing itself or if there was something else hiding in there.

[Junkyard Dog]

Ground the wire with the key on and the fan will turn on.If it doesn't you've got other fan/wiring problems.Pep Boys has a rocker switch that fits in the blank hole next to the dash dimmer control rather nicely.I've been using this to turn on my fan for over a year now since the sensor from mazda was around $80.Just run a wire to your broken end from the switch and ground the other side.Simple!

[REDFUN]

A long shot here. I seem to remember having the same problem with my 323. The thing like half crumbled from old age but still worked. Here's the long shot; this may be a standard Mazda part and can be had at a junk yard.

[SEMX5]

i know i'm a tad late with this reply (not on the boards much) but just as a heads up-the only time the radiator fluid had been handled in hers was at PBC.

it ran hot at a local miata drive the next weekend, but after letting it cool off it didn't run hot again for us (the wire may have started to break at this event?)

Speaking of PBC quality... the coolant in yours doesn't surprise me alot. The one time we had both miatas worked on there, we had a few issues that concerned me . . .

the valve cover gasket was not properly seated on the '91 and leaked oil ontop of the spark plugs
one of the spark plugs in the '91 was so loose it wobbled
one of the hoses on the '01 was damaged/attached incorrectly, and dumped alot of the coolant on the drive back to Richmond
the belts on the '01 were badly cracking and they put them back on the car without saying anything
guy working on mine used my engine bay as a "toolbox" and scratched the paint in a couple spots

they paid a local dealership to replace the hose dumping coolant, so i'll still go through them for parts... but i probably wont use them for service again.

[ChristianL]

The thing like half crumbled from old age but still worked.

That's exactly what seems to have happened to me, too. If I can't fix it, a friend has a spare one. I'm finding that Mazda's prices on 1.6 parts are starting to rival those from Maranello....

David, interesting comments on PBC. Like I say, I was surprised that I put in a nice green (and yummy-tasting, you didn't drink any, did you? It's not as good as brake fluid. ) 50/50 mix in July, then to see it less than a year later as cruddy muddy water. Yellow maybe, but not rusty brown.

[SEMX5]

(and yummy-tasting, you didn't drink any, did you? It's not as good as brake fluid. )

i've heard coolant has a sweet taste, not at all harsh or bitter like i found brake fluid to be