Coolant temp fluctuating

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I installed an aftermarket coolant temp gauge on my 2003 Toyota Echo since new. Recently I noticed the reading on the coolant garage would fluctuate up and down about 5 degrees Fahrenheit when the engine reached the optimal operating temp (I.e. Car has been driven for at least Half hour). It does not fluctuate when the temp is still continuing climbing.

What are the possible causes? I flushed the radiator by myself. Air in the cooling system? Early sign of a blown head gasket? The engine has 146K miles.
 
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A slight fluctuation is actually 100% normal. The thermostat is a dynamic device in most cars. It will begin to open at one temperature, and open slowly, depending on temperature until it reaches 100% open at a higher temp.

In my 2010 Fusion, the thermostat begins to open at around 186 degrees, and reaches 100% open @ about 208 degrees. Using the Torque Pro app on my phone, and a bluetooth OBD-II adapter, I regularly see the temperature going between about 194 and 205 degrees on a drive home from work, depending on how fast I'm going, if I'm on the highway, if I've stopped at a light on the offramp, etc..
 
Originally Posted By: Lubener
Yours is minor but large swings usually is due to air in the system.


When I drove on highway, the temp gauge reading would constantly fluctuating up and down in the range of 5F. But, when I checked the radiator when cool, it was always full.
 
Originally Posted By: sifan
Originally Posted By: Lubener
Yours is minor but large swings usually is due to air in the system.


When I drove on highway, the temp gauge reading would constantly fluctuating up and down in the range of 5F. But, when I checked the radiator when cool, it was always full.


Doesn't mean anything if the air collects at the highest point in the cooling system, many times behind the thermostat. Five degrees is not much. Try replacing the thermostat. I had a stat do that once.If its original, its due.
 
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Could be the sending unit. I had one go bad a year ago, temp gauge fluctuated up and down. Was cheap and easy to change on my Buick, not sure if that's the case for your Toyota though.
 
The problem is you installed an aftermarket temp gauge and now you're overanalyzing it. Happens a lot. Get a scan tool and watch all the PIDs on your car fluctuate...
 
This fluctuation is normal. In order for the flow through the cooling system to vary, the temp of the thermostat must change forcing it to change position. Same as a regular thermostat in your house. With cars, people don't understand this and would often take a car back to the dealer, while under warranty, when they see the temp gauge moving. The factory temp gauge/computer is programed to keep the needle stationary as long as the temp of the engine is within set limits, when in fact the temp is constantly moving slightly. Aftermarket temp gauges don't have this feature.
 
[/quote]Doesn't mean anything if the air collects at the highest point in the cooling system, many times behind the thermostat. Five degrees is not much. Try replacing the thermostat. I had a stat do that once.If its original, its due. [/quote]

The thermostat is still original. I will have a professional to replace it and flush the radiator to remove air. I noticed last summer the radiator fan had come on more often than prior summers.
 
Originally Posted By: carviewsonic
Could be the sending unit. I had one go bad a year ago, temp gauge fluctuated up and down. Was cheap and easy to change on my Buick, not sure if that's the case for your Toyota though.


It would be very easy to replace the sending unit since it was installed at the heater core hose with an adapter. I think I will replace the original thermostat first.
 
Originally Posted By: Ethan1
The problem is you installed an aftermarket temp gauge and now you're overanalyzing it. Happens a lot. Get a scan tool and watch all the PIDs on your car fluctuate...


The fluctuation did not occur when I first installed the aftermarket temp gauge when the car was new and years followed. It only started to occur last year.
 
Make sure there is no air. If it's not that, next thing to consider would be the thermostat starting to get sticky. Replace it.
 
Originally Posted By: mk378
Make sure there is no air. If it's not that, next thing to consider would be the thermostat starting to get sticky. Replace it.


How do you DIY in removing the air in the cooling system? When I flushed the radiator, I jacked up the front end, ran the engine to normal temp, let it cool and filled coolant, and repeated the whole process several times. Is there a sure way in removing air? ChrisFix on YouTube suggested to shake the front end several times in his video which is a much simpler way but I haven't tried his method. Is there a sure way to DIY in removing air?
 
To bleed the system, I run the engine with the radiator cap removed, or if the system is equipped with the pressure tank instead, remove the cap from the pressure tank. You will actually see the coolant level drop and bubbles come out to the top as the thermostat opens, just keep adding coolant and keep running the engine until the level no longer drops. Keeping the engine between 1500-2000RPM also helps.
Monitor the coolant level for few days afterwards in the radiator, for systems equipped with the radiator cap and overflow tank, or in the pressure tank.
 
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Originally Posted By: sifan


The fluctuation did not occur when I first installed the aftermarket temp gauge when the car was new and years followed. It only started to occur last year.


That wasn't clear but if it just recently started doing this check the temp with a scan tool and see if you get the same fluctuation. 5 degrees is well within normal in fact some electric fans don't come on till the temp is about 10 degrees more than thermostat temp and will drop quicky once the fans kick on.
 
While it wasn't clear that it now seems to be acting different, I honestly would consider the behavior you are seeing as normal.

Folks being overly worried that the temp gauge moved is one of the reasons why many auto makers have their gauge set up with a dead zone where the needle doesn't move much (or imperceptibly) within a typical range of normal operation. For certain, all of my newer fords have done that, which is noticeable when you use an aftermarket or scanner type gauge - fluctuation around 5-10 degrees in most cases normal as loads are added and dropped from the engine - the coolant lags a bit.
 
On the bleeding/burping of the system - when you have the front end raised up so that the overflow bottle is the high point in the system, give the throttle a few revs when the thermostat is open to help move things along. Remember that the water pump is generally linked to engine rpm so this helps the burping process.

And maybe you just need a new thermostat. Maybe the spring on it isn't as rigid as when new and now it fluctuates more frequently.
 
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