'90 Miata Overheating when idle

My girlfriend’s 1990 Mazda Miata has started overheating recently in stop-start traffic/idle. I don’t know much about mechanical questions, but I can tell you what it has had done: It recently had a new water pump, master cylinder, clutch kit, valve cover gasket set, thermostat, thermostat gasket, timing belt component kit, and rear main bearing seal set, and radiator fan motor.

There is no steam/smoke associated with it overheating, and seems to be maintaining most of the coolant (she said there may be slight loss, but not significant). The overflow cap for the cooling reservoir is cracked.

Can anyone shed some light on likely causes, and approx cost to repair the causes?


I forgot to include that as soon as the car is moving normally (not stop/start, just driving), it begins cooling down rapidly. It goes from the needle all the way to “H” to below the half-way mark in about 1-2 minutes.

Given that it happens at idle, and resolves itself when the car is in motion, it sounds like the rad. fan motor isn’t “kicking on” when it’s supposed to.

The good news is that it’s easy to test: in a well-ventilated area, start the car and allow it to warm up to (and past) normal operating temp. If the fan never comes on, that’s the problem.

It could be anything from failure to plug the new fan in, to a bad fan, to a bad coolant temp sensor.

P.S.: 1. Does the overheat problem predate the thermostat/fan motor replacement, or is that about when the problem started?
2. Is the “check engine” light on? If so, what are the codes?

It was overheating before the thermostat/motor replacement, it kept running out of coolant and overheating, which is why she had brought it to the mechanic and why they replaced those parts, that was in May. She has only noticed it overheating tonight, and one time a couple weeks ago since then.

And nope, no idiot lights are on. Nothing happens other than the needle hits the “H” and it smells rather, well, hot.

I haven’t had a chance to check the fan yet, but will tomorrow. Thanks.

I see they replaced the fan motor. . .Does that mean they only replaced the motor and not the fan as well (this would be weird, since it’s a lot easier to just swap the whole part rather than disassembling and reassembling them). Anyway, meanjoe is right - it’s most likely a fan issue. Here’s another possibility on top of what he said:

I’ve seen people hook fans up wrong. It’s direct current, so if you get the plug backwards, the fan will spin backwards, which means it would be blowing hot engine air over the radiator. Once you speed up, the fan would be overpowered by the air movement and things would cool down.

I did the test for the radiator motor. I parked the car, and turned the AC off so its fan wasn’t running, within 1 minute the thermostat was maxed out. The fan never came on; BUT, as soon as I turned the AC on to see if the AC fan would be enough to drop the temperature on its own, both fans kicked on. If I am correct in my reading of how the Miata works, one fan is supposed to be for the radiator and should kick on automatically, while the other is strictly for the AC, so turning the AC on should have no bearing on the second fan, right?

Does any of this give any more ideas as to what the problem could be? I am wondering thermostat, but am still curious why both fans kick in when the AC is turned on?

Also, is it likely this was caused when they did the previous repair? And if so, what is the norm in terms of what the mechanic should be responsible for?

It sounds like there is a fault in the fan relay. There are two circuits in the fan relay, one for the radiator fan only and one for radiator fan plus AC fan. See the info here. You can replace the relay yourself, it just plugs into an electrical panel under the hood.

It is also possible that the temp sensor that is supposed to tell the fan to come on was not connected correctly. If a new relay does not fix the problem, check the temp sensor connections per the diagram above or ask your mechanic to check.

If it’s a bad relay then it’s not the mechanic’s fault. If the sensor is not connected then it might be the mechanic’s fault but 5 months might be too long to be able to prove it. It should be a quick fix though if that’s the problem.

If the fan were running backward it would cool at idle and at higway speed. It would not cool at some point in between.On a car I experienced this with it was 45-50 mph. I would check for a non operating fan. Bad fan motor, bad relay or bad wiring.

Thank you very much for that link and the information. Do you happen to know about how much to expect to pay to fix either of those possibilities?

No, it would fail to cool at idle because it would be sucking hot air from over the engine across the radiator.

Google says a relay for the Miata is about $40. If the temp sensor is disconnected you might get billed for half an hour of shop time to diagnose it and fix it. If the temp sensor is failed, maybe more. It also depends what kind of warranty you got on the last job.

Okay, just figured out the problem. The wire from the Water Thermoswitch had broken off right at the connector. I will post a picture of it in just a minute, is there a way to repair this? the wire broke off right at the plug so soldering it would be next to impossible since there is no exposed wire.

I managed to take that apart and found it was a little connecter inside it basically, and the plastic holding that connector is no longer really there, can that whole connection be replaced without running all new wires? It really just needs the wiring connection, and the plastic connection housing/plug piece, if those can be purchased.

Here is the connector where the wire broke off, the wire basically plugs in it seems. Can this part be purchased alone? And the wire “plug” that connects to it? It looks like a simple crimp on type connection for the wire that then just plugs into this housing.

How many wires went into this connector? (Looks like 1.)

What I’d do (assuming we’re only talking about 1 or 2 wires) is cut the connector off on the other side. Buy generic connectors at the local auto store; feed bare wire into both male and female ends and connect.

Wrap in electrical tape or otherwise protect from the elements, make certain the wire isn’t under tension, and you’re good to go!

You can get a variety of crimp-on wire connectors at stores like Lowes/Home Depot, Radio Shack, and auto parts stores. I’m sure you can find something that will work.

There was only one wire. And I’m not sure what you mean by “cut the connector off on the other side”? I figured I could buy the generic connecter that plugs into that piece, it’s one I recognize from my job so I probably already have it somewhere. The plastic housing that I posted though is damaged on the inside, some of the pegs that keep it aligned are gone, can that part be bought separately (the whole piece I posted that is)?

Thanks, that is what I am thinking. I am mostly concerned about getting the plastic piece I pictured I think, which also needs replacing. Can that piece be purchased seperate from a whole wire harness? just the plug like piece?

Just a tip: often when replacing a connector such as this it’s easier to crimp a wire to the replacement connector and then splice the wire/connector assembly to the (cut) end of the old wire and then tie it to the bundle. Often there’s not sufficient extra wire length to do otherwise. When I splice wires like this I like to use heat shrink insulation over the splice.

Great job with the troubleshooting, by the way.