Miata Cooling System

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Normal temperature gauge indication puts the needle just slightly to the left of center for the 1.6 and just slightly to the right of center for 1.8 engines. It is quite common for the needle to climb quite high (without overheating) on a hot day, though there are some steps you can take to reduce the possibility of overheating and keep that needle close to normal.

Note: The stock water temp gauge on the Miata in a way is linear. It's just that between 180°-210°F, it will stick to that 11AM position. If you ever climb over 210°F, you will notice the needle will climb up slowly. At about 210°-220°F, it will sit at the 12 oclock position. At 230°F, you can see the gauge start to move to the 1/2 oclock position. Many here will probably never see it move, but if it does, be sure you stop and cool the car. (Thanks to Tom Mak.)

Water is a better thermal conductor than anti-freeze, however you need the anti-freeze, even in hot weather, for its anti-corrosive and lubricating properties to keep your cooling system running efficiently. If your mixture has more than 50% coolant, you're cooling ability is substantially reduced. You can even go down to about 30% in the summer to get more cooling efficiency from the added water.

With the engine cold, fill the overflow reservoir to the "Full" line with your coolant mix. Do not fill it while the engine is hot. This will cause extremely cold mixture to be sucked back into the radiator as the engine cools, possibly causing thermal shock. In addition, you won't have the proper coolant level. Add coolant to a cold engine.

Thermostat's, over time, have a tendency to freeze up. They may freeze in the open position, which will result in slower engine warm-up and, in winter, lack of heater output. Or they may freeze in the closed position, causing your engine to overheat. We recommend replacement every few years - they're cheap.

Note from Magnus Bo Bojesen:

We agree.

Your cooling system is a closed system, designed to build up pressure as the temperature rises. The higher pressure raises the boiling point of the coolant mixture. (Remember college physics?) When the temperature/pressure rises to the proper level, the valve in the radiator cap (yep, they have a valve - ever wonder what that spring is for?) causes the cap to open and the coolant to flow out to the overflow reservoir. As the coolant flows out, the pressure drops and the valve in the cap closes. As the engine cools, the valve opens in the other direction and the pressure drop in the radiator sucks the coolant back into the system, re-establishing equilibrium. Similar in operation to a thermostat, the cap can freeze up causing pressure problems in your cooling system, leading to an increased possibility of overheating. One symptom of a bad cap would be hearing a sound like a coffee percolator when you shut down. They are cheap enough that you should just plan on replacing yours every year or so. NAPA sells a "Stant" cap (part #703-1406) with a higher pressure rating than stock for about $8.95.

In addition to the above, if you're still having trouble, you can try some of these methods of gaining a bit more from your cooling system.

Water Wetter is a product designed to maximize the efficiency of the coolant by reducing the surface tension of the coolant. Without going into all sorts of physics, basically it makes the coolant transfer heat to the radiator fins a bit better. Does it help? We think so. It isn't a cure-all, but it helps.

If your fans aren't working, you'll overheat. Period. Check under the hood and see if the fan on the driver's side is running after the engine gets good and hot. If not, you have an electrical problem. It may be as simple as a fuse or an unplugged connector. Or it could be a bad sensor or broken wiring. The passenger's side fan only comes on when the A/C is running. Check it and be sure it operates properly.

You can try wiring your fans so both fans come on at the same time. This will only help if you don't use the A/C and will give you maximum fan cooling. It takes just a bit of electrical knowledge.

When a water pump craps out, it usually leaks. If its almost time to replace your timing belt (every 60,000 miles), you should change your water pump anyway.

From Scott Gomez:


Random and intermittent occurrences of very brief overheating or near overheating getting closer together over time. Mileage was approx. 148,000 to 155,000 miles during this period. Engine temp would climb rapidly about 5 minutes after a cold start, regardless of ambient temp. or engine load, but *not* with the engine just idling to "test". A small amount of coolant would briefly be sprayed out of the thermostat cap/housing junction, even with a new gasket and properly torqued bolts. Almost immediately on reaching some point at or very near the "hot" end of the gauge, temp would suddenly and rapidly drop to the normal range. Total cycle time, normal to hot to normal again: approximately 2-3 minutes, tops. After the one occurrence, the engine would not overheat regardless of ambient temp. or engine load unless completely cooled again (as from being parked over night, or all day at work).

Rad and heater core were less than 2 years old. Both clear and flowing fine. Coolant was 50/50 anti-freeze and distilled water. The problem continued after a water pump replacement and pressure check done as part of normal maintenance when the timing belt was replaced. The rad cap and thermostat were replaced during troubleshooting *without* curing the problem. Hoses were fine and there were no leaks during normal operation, except for the brief coolant spurt noted in symptoms when the problem occurred.


I'd seen it mentioned in messages on Miata. net that there's sometimes gunk in the thermostat housing that can cause weird cooling problems should it block the bypass port there. There was no blockage at that point in my car. However, based on what I found in troubleshooting my Miata, one should also check to be sure that there is good flow through the bypass line from the thermostat housing to the water pump inlet.

First gen. Miatas (1.6L engine) have a 90-degree metal barb for connection of the rubber bypass line at the water pump inlet. It's a true 90-degree bend, rather than a radiused one as it probably should be (due to space constraints, I'm sure).

To check, remove the rubber bypass hose from the the thermostat housing to the water pump inlet. It runs almost vertically from the housing to the inlet. Coolant should flow freely from the barb on the water pump inlet once the hose is removed.

Mine proved clogged with bits of rubber--I suspect from a previous hose or water pump replacement or just accumulated over time. I cleared it using a straightened piece of coat-hanger as a "broach." The symptoms have not recurred after this barb was cleared.

I expect the problem occurs as follows:

>From a cold engine, without sufficient flow through the bypass line, the thermostat doesn't heat up properly--in tandem with the coolant. The coolant, however, heats up normally. When the engine finally reaches a near overheat or overheat condition there's enough pressure against the still closed thermostat to cause the brief leakage at the housing gasket. Then, the thermostat finally pops open from being heated by conduction through the housing itself, and the sudden resumption of proper flow rapidly cools the engine again.

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7 June, 2006