The Bonehead Rule - 1994 Mazda MX-5 Miata Long-Term Road Test

1994 Mazda MX-5 Miata Long-Term Road Test

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1994 Mazda MX-5 Miata: The Bonehead Rule

September 06, 2010


Recall a while back Project Miata's cooling fan was operating erratically. It's now fixed (I'm pretty sure).

The short explanation: replacing the coolant thermosensor at the back of the head did the trick, as expected.

For those curious / bored readers, the resolution entailed some other cooling system shenanigans as well. Okay, the whole process sort of turned into a goat rodeo. Click on to read the long and twisted saga.

After replacing the relay, my next plan of action was to replace the thermosensor. I should have kept my eye on the ball and just done that. Instead, I got distracted.

A friend had a fancypants aluminum race radiator left over from a customer's Miata project car. It was removed simply because the customer is one of those bigger-is-better guys that always wants the latest thing. Whatever, he didn't need it and gave it to me. Used for only a short time, the radiator was in surprisingly good shape, plus was made by a reputable big-name manufacturer and not some chintzy Chinese off brand.

Perfect, I thought. We'll be adding power, so some extra thermal capacity is a must. I threw it in Project Miata and even sealed it up nice with a bunch of foam strips so that all incoming air goes through the radiator (and fans) rather than around it.

The next day while commuting to the office, the temp gauge started to climb.

Now, if you're familiar with factory temp gauges, you know that they are essentially worthless. They're highly nonlinear, meaning they sit in one happy place over an absurdly wide temperature range. This is because most automotive consumers suck -- when they see a temp gauge move around, they freak out.

In response, manufacturers have over the years "deadened" their temp gauges so that their needles hardly budge even as coolant temp varies widely. This comforts the lowest common denominator customer into thinking that everything is just peachy but makes for a less-than-useful gauge. This, of course, sort of defeats the purpose of the gauge.

To wit:

temp gauge.jpg

temp gauge.jpg

"Your engine is stone cold." "Your engine is about to blow up."

Okay, slight exaggeration, but the point is if the needle in a Miata moves off this 11 o'clock happy place over to 1 or even 12 o'clock, it means that things are getting crazy hot -- I confirmed this years ago by comparing to an actual, real temp gauge with actual, real numbers.

blasted the heater to bleed off some heat from the cooling system and starting driving like a sissy. The needle recovered.

One of the golden rules of working on cars is if something goes wrong immediately after you made a repair, it's certain to be related to the work you just performed. I call this The Bonehead Rule. In this case, the hot running was very likely related to the radiator swap -- it may be a big 'ol air bubble trapped in the cooling system, or a coolant leak somewhere or the radiator cap wasn't seating correctly in the new radiator which would mean it wouldn't hold pressure and would puke coolant into the overflow bottle.

However, none of those things applied -- burping it did nothing, all coolant was present and accounted for, the fill level was right where it was before and the cap was holding pressure.

Huh. Well, I guess our fan issue is getting worse, then, I thought. Better jump on that.

So I did. But before I did, I let the car idle in my driveway to test the theory that the erratic fan would cause a thermal runaway. Oddly enough, the fan -- being erratic -- didn't operate erratically at all during this test. It switched on and ran without a hiccup. Yet the needle marched over towards the hot zone of 1 o'clock. Now this is weird.

t-stat oblique.jpg Some probing with an infrared thermometer suggested that the thermostat wasn't consistently opening fully. The notion that another component not directly related to the work I'd just done could fail so coincidentally went against my better judgment (I'm an adherent to The Bonehead Rule), but thermostats can fail in many ways, and this is one of them.

I'd never replaced the t-stat in this car, and it's always a good idea to do so in an old car, especially one with an unknown service history. Besides, it's dead-easy to change a Miata t-stat. Less than a twenty-minute job start to finish, without trying hard.

t-sensor coils.jpg

I did so, and also changed the thermosensor.
This guy is located waaaay at
the back of the head, buried beneath the ignition coils and crank angle sensor.

At some point I'll do a coolant reroute, but that's a topic and installation that will wait for another day. All in time.

t-sensor back of head arrow.jpg

Once those bits are out of the way, it's an easy 19mm zippywrench to change the thermosensor.

All buttoned up. Warm up the engine. T-stat opens nicely and we're getting more heat at the top radiator tank now. Good. That's progress. Fan comes on smoothly, no erratic nonsense. Great.

Now let's see if it can actually control the coolant temperature..... and that's a big no. Temp gauge still climbs at idle. Even with an additional squirrel-cage fan blowing air into the car's mouth, the car's cooling fan never switches off -- it simply can't shed enough heat to bring coolant temps in check.

Yeah, remember that bitchin' aluminum radiator? Not so bitchin' after all. It was used, so it may not be representative of new. Perhaps internal passages were plugged. Dunno. Couldn't see anything awry peering into the top tank but who knows. Whatever the case, it sucks.

Re-swapped in the original old stock radiator (it's the slightly larger one from an automatic Miata) and everything works as expected -- the fan can now actually bring down coolant temp, and it switches off after it has done so. Finally, a normally-operating cooling system.

It's still woefully undersized for the demands of future actual power, but it's good to know we've got the basics of our cooling system sorted out.

rad in trash 2.jpg As for that race radiator, it's found a proper home.

-Jason Kavanagh, Engineering Editor

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