I have a small overheating problem. Car is a 94' miata with no modifications.

The needle goes up only to the 3/4 mark
Ambient Temperature around 35C(95F) In the city (slow traffic)
Only overheats after 15 minutes of running at normal temperature and with A/C on. If you turn A/C off will lower temperature SLIGHTLY
On the Road: Over heats when you turn A/C on or when you go up a hill (5000+ RPM) It returns to normal temperature if you turn A/C off or you drive around 4000 RPM in 5th gear

Coolant level is OK
Coolant color and mixture is OK
Fans turn on OK and at the right time
Just got my A/C recharged
Just got new oil (Castrol GTX3 10-40)

My guesses:
1. Radiators are dirty outside (bugs and dirt) not letting enough air flow to pass through
2. Fans turn too slow (broken motor or bearings ??)
3. A/C recently recharged thus getting hotter in A/C's condensator than before
4. Oil changed recently to semi-synthetic
5. Design flaw in the miata (A/C's condensator in front of engine's radiator) OK in cooler temps but not in spanish weather .
6. Water pump

Any suggestions ? (6/00)

I have experienced the same sort of thing in similar conditions - (before I fitted my baffle shroud - see below) You may find the temperature will fall significantly if you pop the headlamps up (at speed anyway - won't help in the city)

I don't think you have any faults, but it is a bit worrying.

Two suggestions:

1) Is the baffle shroud in place? (Covering the gap in front of the top of the radiator) Or have you even got one? If not, I suggest you make one - I have, but I've not been able to test it (due lack of suitable conditions) properly yet. But I am told it can make a big difference, and I have not had any sign of gauge rising with air con on, high speed up hill at over 30 deg C. with it fitted. I need 40 deg ambient - must come to Spain (next month maybe!).

Extra shrouding to make sure that all the air that comes through the mouth goes through the radiator may be a good idea (and you have got the gravel tray fitted haven't you?).

2) Get a 160deg (F) thermostat. The normal thermostat begins to open at 80.5-83.5 deg C (177-182 F) and is fully open at 95 deg C (203 F). I think the car runs with the thermostat just open (ie at about 85 deg C) in normal conditions (say below 28 C ambient, no air con, modest speed up hills) but will go up to thermostat well open (say 93 deg C) in conditions you describe - and I think the gauge will read about 3/4 then. (But I know a lot of people think the gauge reads the same over a big temperature range.)

But I suppose that if the thermostat is nearly fully open anyway, a cooler one will make little or no difference. You can get a 160 one from various miata.net suppliers (including Roebuck Mazda) (6/00)

Hmm - it looks nice and will certainly keep stray rocks and wildlife out. But I also think it will break up the air flow substantially, so reduce cooling, and that could well explain some overheating on long fast runs on hot days.

I have a Moss grill, and have never experienced overheating in the UK (even without the baffle shroud) Any grill is going to reduce airflow to some extent - look at garden windbreak - the holes are a lot more than the solid bits, yet it breaks up (modulates) the wind so that your plants survive the winter (from observation of gardening wife! :-) ) But I did experience overheating without the shroud in S France - way hotter weather than it ever gets in UK.

I think a baffle shroud (or a different grill!) might help. See http://www.teammiata.com/proR-cov.htm for a picture of the pukka thing - some cars have them as standard - if yours hasn't you could make one from sheet ally (like mine) or sheet plastic. Easy if you make a cardboard template first - and they're easier to fit and remove if you make it in two parts with a split in the middle - helps to get it past the bonnet catch handle. You need spacers under the shroud where the bolts are - they are in sort of recesses. You'll probably need longer M6 (10mm spanner) bolts too. (6/00)

As some of you may of noticed at Jurby last week my car overheated on the track. I replaced the thermostat and it's been driving OK since, however I had another track day at Kirkiston yesterday and after 3 laps of really going for it the car overheated again. You could let it cool for 10 minutes and the temp guage would stay about halfway until the 3rd lap, then quickly rise to H. I was up against Caterams & AC Cobras so I was really, really going for it :)

In my first track day I did a few months ago the car was fine, but three things have changed.
1) I replaced the Standard Air Filter with a Cone Filter. This is quite close to the Manifold, so maybe it is sucking in hotter air?
2) I advanced the timing to 14 degrees.
3) I fitted letters to the front grill to act as a registration (much to the disgust of Old Bill).

I guess they could be restricting airflow. Picture of modification at http://members.tripod.co.uk/stripy_miata/plate.htm

Does any of these factors explain why the car is more prone to heating up, or is it a combination of all three? Can anybody suggest a fix? On the day some guys suggested wiring the AC fan to come on when the other fan did. Has anybody done this? Also, one of the Cobra guys suggested an additive to the cooling system to stop the water overheating. I can't remember the make. Lastly, another guy suggested draining the system to get the crud out and refilling with less antifreeze. Any advice would be gratefully received (7/00)

Probably talking about Redline water wetter - meant to stop overheating a bit. Less antifreeze will also do this but you need to top up in winter again. I'd guess it's your numbers on the grill as 14 deg timing shouldn't make much difference from what I've read. Have you fitted a cover fover the top of the rad to make sure all the air getting into the grill goes to the rad's fins??

Can't you fit a lower temp thremostat as well, although it won't stop overheating - it will just take longer to get up to those temps. (7/00)

Your 3 mods won't really make the car prone to heating, although the third might well do. Though not much, I'd have thought.

How many miles has the car done ? It might be that the coolant is so full of crap that its affecting the efficiency of the cooling system. I'd firstly try flushing it out - best if you can is to remove the rad and reverse flush it - that is put water in from the top and let it come out of the bottom (i.e. opposite to what it does in real life). You might also wish to get some flushing additive - Holts do one called "Radflush" - guess you can get it across the water there. The additive the guys were going on about is called a "water wetter" - Redline make it. It basically increases the boiling point of the coolant in your system. It also works - I've some friends who use it in Caterham's, specifically for track days.

Finally, how about removing your thermostat altogether ? I remember on one of my dad's old Rovers, we resorted to this because the car used to overheat in heavy traffic, despite flushing and then REPLACING the rad on it. It worked a treat - there was coolant constantly going around the engine, so it kept the temp down just that extra bit. For the odd track day, that extra bit of circulation might give you the edge ? To be honest, I'm not sure how much of a job this would be on a '5 - although on my dad's old motor it was 3 bolts, and there it was. (7/00)

Finally, how about removing your thermostat altogether?

Unfortunately (under 'everyday' driving) this kills fuel economy because the engine takes longer to warm up so runs richer for longer. (7/00)

I think that might be the answer, the "water" is a brown colour and foams like Fairly Liquid! Probably hasn't been flushed since it was made 60,000 miles and 9 years ago.

If that's the case I fear for your rad as it is probably well-clogged with limescale... Definitely worth trying a chemical flush at same time as replacing coolant. I used Holts two-part - time-consuming but thorough. (7/00)

I tried all sorts with my V8, what worked in the end for me was the big expensive rad from dealer alternative, but you shouldn't have to go that far. I found that quite large areas of my original rad weren't working - cold air coming through, so I guess these things are prone to clogging up. It's relatively easy to pop the rad in and out (I became expert at it) (mind you mine doesn't have the fans bolted to it) get it flushed out properly - I tried to use bath taps (far from ideal & didn't work) & holts radflush also did nothing. guess you need to use a fire hose or something to back flush it effectively.

Good luck with it anyway, symptoms sound just like mine was. The chemical mentioned was redline water wetter which sounds like credible kit & I'll get some soon, mostly for corrosion protection - theory it must be that whats blocking the waterways in the rad. "breathe through " numberplate - I would think you'll get lots more restriction from the mesh than the letters ignition advance would be infinitesimal in its effect - if it does make the car more efficient then there should be more of the energy going in appearing as mechanical work therefore less as heat - I know well dodgy theory...

Seen some other replies now so I'll add me 2p worth extra flaps in bonnet catch area & gravel tray components - made no difference to my setup - but I've left them in

holts radflush was utterly useless - maybe the 2part one someone mentioned is a bit more viscious

antifreeze conc. made no discernable difference

I put in some bonza bonnet vents - made no difference

if you reverse flush the rad in situ you'll spray filth all over the engine! plus getting a big hose to the bottom rad pipe also sounds difficult. BTW the water wetter stuff doesn't alter the BP or temp. of the coolant, but reduces actual engine temp. by making better thermal contact coolant to metal - thermostat determines coolant temperature (if everything else is working......) (7/00)

found that quite large areas of my original rad weren't working - cold air coming through, so I guess these things are prone to clogging up.

And therein lies the problem. All the extra kit (shrouds, louvres, water wetter, etc) won't help if your radiator is basically not working. So if that's the case, get it cleaned (properly) or get an exchange one. Anything else is just bodging (eg removing thermostat) and likely to damage your engine one way or another. Bob's case is different - big engine, big fuel consumption, big heat - need big radiator. The rest of you just need the standard radiator working properly (forced induction engines might need a bigger rad for hot track use) - with a bit of help from shrouds for extreme use (sustained 100 mph in over 35 deg C)

If you maintain the cooling system properly, radiators don't get clogged up. Always replace coolant every two years, and use the proper amount of antifreeze - 30% minimum, 50% for preference. If the coolant is rusty coloured when you buy the car, or goes rusty looking very quickly, flush it and flush it and flush it and change the coolant every three months until it's clean. Your coolant should be blue-green (for most brands anyway). Anything else, especially rust colour, calls for action! (7/00)

Removing thermostats needs a bit of care if you are racing the engine - otherwise the flow pattern in the head gets screwed up and you get differential cooling problems (I'm told!)

If you do it you need a thin blanking plate with a hole in it that is not too big. This creates enough turbulence in the right place but not as much flow resistance as the thermostat. (7/00)

Just to add my few pennies worth.... My ex-rad had done just under 100K miles and at the last service the coolant was flushed and replaced - closely following this, it generated a leak which only leaked under cold conditions ( about 1 litre a week). It wasn't until I got fed up with replacing coolant and removed the radiator did I manage to find where it was coming from. The coolant was coming from there the metal meets the plastic on the base of the radiator - obviously the seal had gone. Replaced with a roadster radiator and all is now fine.

Might be coincidence that the leak generated after a flush but something to at least look for, especially in the older radiators. Second hand units are relatively cheap! (7/00)