99 Overheating

The engine in my 99 is running a bit hot. The crappy OEM temp gauge doesn't budge. However, after driving for 30 minutes, the engine is radiating a ton of heat.

The radiator is the original (116,000 miles) as far as I know, so it's probably due for replacement anyway.

I replaced the thermostat with a generic from AutoZone. I hear the aftermarket ones are slightly different and not as good as the Mazda, but this was a stop-gap.

I replaced the radiator fluid with 70/30 mix, along with a new cap.

Neither change had any effect.

The fans are both running; I'm planning on checking the amps on the fans next.

Also, the water pump is not leaking, nor is it making any grinding noise. Anything else to look for regarding?

Anything else I'm missing?
:confused:

I'd start with the radiator.

What color is the radiator? Brown?

How's the cat?

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Originally Posted by goofygrin

What color is the radiator? Brown?

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Yes, it's the faded brown / army green plastic.

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Originally Posted by goofygrin

How's the cat?

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No, it's the CAR that's causing problems, not the CAT.
Rim-shot! Tip your waitresses.

Seriously, I hadn't considered that. I recently had TDR put on a racing beat header, w/ roadster sport midpipe & exhaust. The baffles make a terrible rambling sound but I haven't had the chance to bring it back in. If the cat seemed suspicious, I would have thought Gary would have mentioned something (like my motor mounts were shot), but maybe not. I'm not sure if they did a back pressure test or not.

Got your email. Give us a call and we can set a time to bring it in.

Definitely check the fans. Feel the airflow to see if it is significant. I've seen fans spin but the motors are weak enough to stop them with your finger. We can wire the fans to run at the same time. We do this for a lot of customers.

The Roadster Sport mid-pipe has a built-in cat so it should be good. Keep in mind the header is unshielded and will give off more heat than the stock one which is shielded. Not sure if our header heat shield will conflict with the autocross rules but it will lower engine bay temps. We can also pressure test your cooling system to look for trouble. A newer radiator may help. Will a radiator with a larger tank conflict? We have the PWR all aluminum radiators that keep supercharged track cars cool in August so I imagine they would work great for short autocross runs on a NA* car. *(Normally Aspirated)

Also, are you running the belly pan? It makes a significant difference in directing airflow through the engine bay. We have found if you modify them by cutting the sides, it allows hot air to escape the engine bay easier. Again not sure how nit-picky the rules are which is why I didn't suggest it at the time but normally recommend it to our customers.

FWIW on my black NA, the engine bay gets hot as hell in the summer. I've had my fair share of overheating problems, but when everything's running fine the damn thing gets hotter than any other car i've driven, i've heard it's not all that uncommon for darker colored Miatas in Texas heat. I assume your green NB might suffer the same fate, especially with an aftermarket header. I'm not a mechanic or a tech by any means but I would assume that if the temp gauge is staying put that you should be fine, unless your engine is pinging or you think your gauge is not working properly.

That being said I did have a slow fan once and it caused all kinds of problems and was virtually undetectable unless you knew what to look for.

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Originally Posted by Jiggerachi

I would assume that if the temp gauge is staying put that you should be fine, unless your engine is pinging or you think your gauge is not working properly.

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Not even the slightest bit true. The stock gauge will read the same from 160° to 220°F. By the time your gauge reads "slightly hotter than usual" you are about 240°. By the time you get to the end of the range shown on the gauge (just before the H tick mark), you are at 260°F. EDIT: That's for an NA, but I'd imagine NBs are similar - I know they have an equally large dead spot, but the actual range might be different.

The crappy part is that I don't know if you can mod an NB gauge to read actual temps like you can an NA gauge. I'm sure you can, in fact it has to be possible, but I don't know anyone that has ever done it. I also don't really use the internet much more than local and formerly-local forums though, so maybe it's common knowledge now.

^Wow! Good to know. I had no idea the dead spot was that large. I might start thinking about modifying my gauge...

When you want to get into it, I have pics from one of each gauge type I've done. I know most of the write-ups online are dead for the most part, so if you need fill-in info, let me know. Or if you don't feel like doing it yourself, I'll gladly help. Actually, I should still have the email I sent of how to do a 1.8l gauge to someone here - I could just forward that if needed.

For reference:
http://www.guapozx.com/users/dogpile/deadspot1.JPG
99° 120° 139° 159° 174° 200° 220° 240° 260° 270°

You'll notice 160-220 is where everyone says their needle stays during normal operation. Each gauge will differ slightly with regard to where it rests in that spot: fluctuations in that limited sweep range are more from current changes than temp changes. Notice where the needle is pointing when you would consider the car "running slightly hot." All the customers that have come to me over the years will point out the one that shows 240°F. Hello, way hotter than you thought!

I don't remember where I got that from back in 2003, but props to whomever documented it. It also got a bit distorted when I enlarged it, but you can still make out the numbers.

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Originally Posted by TexasStig

Yes, it's the faded brown / army green plastic.

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Sounds like it's about time to replace the radiator. My NB's radiator top had turned brown/green by the time the upper hose once again needed replacement. Sure enough, the upper inlet came right off with the hose. The healthiest of tanks will be black, at least they have been on the new OE radiators I've used.

I was told by a Spec Miata guy (forgot his name) at Hallett this year that the NB temp gauge is worse than the NA. He said it is not much better than the 'dumb' oil pressure gauge pm the NB! Mazda might as well put a light there!

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Originally Posted by TexasStig

I replaced the radiator fluid with 70/30 mix :confused:

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I thought the 50/50 mix is best around here!?

Agreed on the color meaning it's time to replace the radiator. Also, if you have AC, make sure to spray the condenser with a high pressure nozzle on your hose. Try to spray from teh engine bay towards the front of the car (so opposite of the airflow direction). You'd be surprised at how much crap gets in there.

Sorry about the brief threadjack too.

50/50 is for where it actually gets cold. Around here, you can get by with a lot less antifreeze, especially if your Miata is a fair weather car.

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Originally Posted by gabkwong

I thought the 50/50 mix is best around here!?

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Yea, the '70' in that equation is the water... don't reverse the ratio. Water is actually a better cooling agent than the antifreeze itself. Just don't drive to Chicago this winter for a visit.