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My mx-3 has never had problems with overheating and during the four years I've owned it the engine temp. gauge has always remained in the same exact place and the needle has never budged from the middle of the indicator between C and H.

Now when I drive it, at any random time regardless of trip length, the needle will rise about 3/4 of the way to the H level and then all of a sudden drop back to normal for the rest of the trip, even if I keep revving hard. It may or may not go back up, but it never seems to actually overheat and the coolant overflow tank remains at its normal levels.

Is this a problem with my thermostat or can someone suggest other possibilities?

check your cooling fan,water pump,and your thermostat

That is totally typical of a "sticky" thermostat. It doesn't want to move, and it takes a bigger temperature difference to make it do so. One day it won't be enough and it will stick for good - probably closed.

Fit a new one - quickly

Listen to Ian for he is wise... Your thermostat is flaking out and in effect it's warning you to take action NOW. Ignoring this may leave you stranded soon with an overheated engine and even worse, extreme overheating could blow a head gasket and warp your cylinder head. It's a $5 part and a thirty minute bit of work (if you are taking your time). If you find that a couple different temp. ratings are available for your car's replacement thermostat, choose the one with the lowest temp rating for ensuring coolest running engine temps-always a good thing IMO.

I forgot to mention this, but the thermostat I have currently is only 2 years old; I had it replaced when I broke a belt and pulley and my car overheated that one single time.

Should a thermostat last more than 2 years (less than 15,000 miles in those two years)? If so, what could prematurely cause one to malfunction?

Also, as far as damage to the engine goes, the car is not actually overheating. The needle goes 3/4 of the way up and then comes back down. Can this still damage the engine?

Originally posted by Sandman223:
Should a thermostat last more than 2 years (less than 15,000 miles in those two years)? If so, what could prematurely cause one to malfunction?

The Mean Time Between Failures (MTBF) for thermostats is probably of the order of 10-15 years, some last longer, some fail really young - but very few. I'm afraid you got one of the few.

Why do they fail? They're a crude, cheap device, easy to change, and it's not worth the investment to improve the design. You wouldn't use one of them in a spacecraft!

<small>[ May 31, 2004, 07:18 AM: Message edited by: IanL ]</small>

How can I make sure that the thermostat is faulty and that the engine is not actually overheating and the thermostat is simply reacting?

Would the temp gauge stay near the H if this was the case? I'm sorry about the stupid questions, I'm a newbie when it comes to car trouble.

Think of it this way, when the thermostat operated properly before, your temp gauge indicator would move up (as the coolant got hotter) then, as the thermostat opened, allowing coolant to circulate from the engine thru the radiator and back, the gauge indicator would stop rising. When a thermostat opens, it opens all the way and stays wide open as long as the engine is at operating temperature.

As you said yourself, "during the four years I've owned it the engine temp. gauge has always remained in the same exact place and the needle has never budged from the middle of the indicator between C and H."

Your current overheating problem is occurring because even with the engine already at full operating temp, the faulty thermostat is now partially or fully closing when it should not be doing so. Temp reading climbs too high and comes back down when the stat opens back up. This should not be happening in the first place and is caused by a faulty thermostat which will worsen & soon stop opening at all.

That will cause severe overheating and serious damage to your engine. Only way to test whether the current thermostat is faulty is to install a replacement and see if it happens then. G'luck.

Absolutely correct. The engine IS overheating, because the thermostat is not opening sufficiently to allow enough coolant flow. So far, when the coolant's excessive heat has reached a temperature to force the reluctant themostat open, that temperature has not been high enough to kill your engine. Tomorrow, that may change

I'd say the probability of a new thermostat curing the problem is 99.99%.

The 0.01% is that the new thermostat is faulty.

<small>[ May 31, 2004, 04:12 PM: Message edited by: IanL ]</small>