1999 Cadillac Deville Coolant Temperature

Tiny
MARKVELLUTATO
  • MEMBER
  • 1999 CADILLAC DEVILLE
  • 135,000 MILES
Today I had a new water pump and thermostat installed in my 1999 Deville. For the four years that I've had this car, my coolant temperature reading on my dash has always been between 192 and 196. After they put in the new water pump and thermostat today, I took my car for a long drive and noticed that the coolant temperature now fluctuates between 202 and 205. Is something wrong or is this ok? What is causing the coolant to be hotter?
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Monday, November 25th, 2013 AT 4:08 PM

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Tiny
CARADIODOC
  • EXPERT
That's due to normal tolerances of the thermostats and nothing to worry about. Gauges are notoriously inaccurate anyway. That's why most of them have a "C" and "H", and a red warning area, but no numbers. Many GM vehicles run a lot hotter than 205 degrees. On a lot of front-wheel-drive cars the electric radiator fan doesn't even kick in until 220 degrees.

Also, when the water pump is removed, much of the coolant is lost and has to be replaced. Changing the ratio of antifreeze to water will change how well the mixture holds and carries away heat. Water holds more BTUs of heat and transfers that heat to the radiator better, but antifreeze contains the desirable additives like corrosion inhibitors and water pump lubricant. Some mechanics go a little heavy on the antifreeze because that's what you're spending your money for, but then the heat doesn't travel as well, so the temperature of the coolant could rise a fuzz. Some mechanics go heavy on the water, thinking they're saving you money. Heat will be removed to the radiator more efficiently and the engine will be a little cooler.

Regardless of the coolant mixture, it's still up to the thermostat to regulate the temperature, and 205 degrees is perfectly fine. Real low engine temperature promotes incomplete combustion and increased emissions, so logic would dictate higher temperatures, to a point, would result in lower emissions. I don't know if that's an accurate parallel to draw, but I still wouldn't be concerned.

What I would be more interested in is the freeze point of the coolant. There's always some old coolant left in the engine when the system is drained, and you don't know what the ratio of that old stuff was. Some mechanics pre-mix the new stuff with water at the desired 50 / 50 ratio, but it will still be wrong if the old stuff left in there was wrong. I used to pour in the antifreeze first, add the water next, run the engine a while to mix it thoroughly, then test the freeze point to know whether to fill the reservoir with water or antifreeze to get to the proper mixture. Adding more antifreeze won't lower the freeze point very much beyond -50 degrees so there's no advantage to doing that. Once the magic ratio is reached, a higher percentage of antifreeze will actually raise the freeze point. Straight antifreeze freezes at around -10 degrees. It's when it's mixed with water that the freeze point gets lower. If you have any concerns at all, stop in by your mechanic after a few days and have him double-check the freeze point. By that time what is in the reservoir will have mixed completely and you'll get an accurate reading. I often told my customers I left a little room in the reservoir so I could add what was needed to fine tune the mixture later.
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Monday, November 25th, 2013 AT 5:00 PM

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