Temperature gauge fluctuations in a 2006 Scion TC?
September 24, 2014 7:17 AM   Subscribe

Recently replaced the thermostat on my 2006 Scion TC, now the heat gauge is fluctuating again within the first 15 minutes of my commute, after which is settles. Am I going crazy? Is my car going to explode?

My 2006 Scion TC overheated about two weeks ago due to a locked thermostat. Thermostat was replaced, there were no cracks or leaks, car was running perfectly. Yesterday I noticed my temp gauge is flirting with that red zone again. It has not spiked or gone out of the normal range, and once I've been driving for about 15 minutes, it settles into the happy place. But that first fifteen minutes sends me into anxiety city. Am I being neurotic? Is this just because I have a working thermostat again? The first time it happened I pulled over as soon as I could and there were no leaks, the engine was not overly hot to the touch, and the heat was working well. The engine does not appear to be struggling at speed or while idling (any more than it ever has). I will have time to take it to the mechanic on Saturday, but until then I have a lot of commuting to do, and cannot afford to miss work. You are not my mechanic, but I could really use some peace of mind that my little car is not going to explode over the next few days. Or, if it is, what I can do about it.
posted by picklesthezombie to Travel & Transportation (9 answers total) 1 user marked this as a favorite
 
Am I being neurotic?

Absolutely not. It could be a just a bad thermostat - but it could be much more serious. Overheating a modern engine is a serious thing that can lead to other problems - like a cracked or warped head.

It sounds like you have that going on, maybe. I think you are smart to keep a close eye on it.

The good news is that it (almost certainly) won't blow up and die in the next few days. I've sometimes gotten weeks or months out of a car with a faulty head seal.
posted by Pogo_Fuzzybutt at 7:25 AM on September 24, 2014


Not a mechanic here, but that's kind of what thermostats are designed to do. They react to a rising temperature to open up and let the coolant/water circulate, which then reduces the internal temperature.
posted by uncaken at 7:53 AM on September 24, 2014


Not a mechanic here, but that's kind of what thermostats are designed to do. They react to a rising temperature to open up and let the coolant/water circulate, which then reduces the internal temperature.

Yes, but what the OP describes (temps dropping into normal range only after flirting with the red zone) is not the way it should work at all. Automotive thermostats are mechanical and react incrementally, gradually allowing more coolant flow as the engine heats up. They are not binary on/off devices like the thermostat that controls a furnace or air conditioner.

OP, did traffic conditions change later in your commute? In stop-and-go traffic your cooling system is dependent on a fan to pull air through the radiator. If that fan fails then the car will tend to overheat when it's not moving much but be fine when driving at speed.

Did you replace the thermostat yourself, or was this done professionally? If the latter, did they pressure-test the system or just check visually for external leaks and send you on your way?
posted by jon1270 at 8:18 AM on September 24, 2014


Response by poster: @jon1270, it was done professionally and they did a pressure test. My commute begins in the city (10min), transitions to highway (20 min), then is city driving again (20 min). The fluctuations don't return when I go back to stop and go, which is one of the reasons I'm confused. If it were a fan issue, wouldn't the fluctuations return when I resumed city driving?
posted by picklesthezombie at 8:37 AM on September 24, 2014


If it were a fan issue, wouldn't the fluctuations return when I resumed city driving?

The Scion has an electric fan, so it could be an intermittent failure.
posted by Pogo_Fuzzybutt at 8:49 AM on September 24, 2014


Just guessing but especially since you recently had things open if this isn't a bad head gasket you could have air in your cooling system. I don't know how hard it is to bleed your car's system (some cars are very hard, some are easy) but you should make sure to check your coolant expansion tank every morning to make sure it is sufficiently full.
posted by Mitheral at 9:25 AM on September 24, 2014 [1 favorite]


The first thing I'd do is determine of the radiator was bled properly, and redo that if needed. An air pocket in the cooling system is one possible cause of this behavior, and is the easiest to fix.
posted by zippy at 11:30 AM on September 24, 2014


This post, which is from the internet, so take it for what it's worth, suggests bleeding the radiator on a Scion TC is not hard. Steps 15 and 16 are the ones for bleeding (and topping off) the radiator.
posted by zippy at 11:33 AM on September 24, 2014


Response by poster: For the record, I brought it to the mechanic yesterday. Turns out the water pump needed replacing and there was a small pinhole in the hose that was leaking coolant. Currently being fixed. Thanks everyone!
posted by picklesthezombie at 5:18 AM on September 26, 2014 [1 favorite]


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