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I read in the Motor Oil University article on Bob the Oil Guy's websites that:

The automotive designers usually call for their engines to run at 212°F ( 100°C ) oil and water temperature with an oil thickness of 10. ... The engine is designed to run at 212°F at all external temperatures from Alaska to Florida.

I installed a 82.5°C thermostat on my 97 Mazda 323 BA 1.8L BP DOHC. The radiator was replaced by the previous owner and is not OEM. I'm using a premixed glycol-ethylene coolant rated between -10°C to 118°C which specifically says on the bottle not to add water. The service manual calls for 65% water to 35% coolant in environments over -15°C.

After all those details, here's my question:

When I hook up to the ECU, I find that most of the time the engine temperature stays between 85°C to 92°C. Rarely, when standing in traffic for a long time, the temp will get up to 97°C, at which point the ECU turns on the fan and the temp rapidly returns to the previously mentioned range.

Is this normal? If not, any idea's?

So far I haven't found anything regarding normal operating temps in the service manual, although that doesn't mean it's not there of course.

  • Looking at the quote from Bob the oil guy leaves me shaking my head slightly ... Oil runs way hotter than the coolant does, which is the nature of the beast considering what it has to do and where it does it at ... and a lot of engines today run hotter than 212°F. There's some good information on that site, but like all oil commentary, there are some problems with their facts. – Pᴀᴜʟsᴛᴇʀ2 Feb 20 '15 at 0:07
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Yes, it's normal. Maybe just a tiny bit on the cool side. But if you're in traffic a lot that cooler thermostat is a good thing. All the cars I've owned would run between 85 and 95 degrees Celsius. 85 at highway speeds, then creeping up to 95 or 96 when in slow traffic, at which point the fans kick in.

Pro tip: the hotter an engine runs, the more efficiently it burns evil hydrocarbons.

If your car is running too cold for your liking (or according to the opinion of a good mechanic), there are basically three things you could do:

  1. install a hotter thermostat
  2. drain your oil and fill up with a more appropriate option (ask your mechanic)
  3. take the car to a tuner to have them lean out the air/fuel ratio slightly and play with the timing.

The third option will increase the temperatures inside the combustion chamber and exhaust and also provide slightly better fuel economy/performance. Of the three though, #1 and #2 are your best options.

  • So why not use a hotter thermostat, Especially since even a 30 weight oil won't reach the right viscosity at those temperatures. – Robert S. Barnes Feb 19 '15 at 10:55
  • By the way I do almost exclusively city driving about 50 kilometers a day and the fan comes on maybe once a week. – Robert S. Barnes Feb 19 '15 at 11:02
  • In that case an 87 degree thermostat is probably better. For reference: I had a highly tuned Astra Turbo Coupe and it had a tiny radiator and either an 87 or 91 degree thermostat and it never overheated. It just stayed around 95 or so. – Captain Kenpachi Feb 19 '15 at 11:13
  • I just noticed that the parts listed on autozone for the Protoge with the same engine are 192F which is 88.88C and I just found a data sheet from autodata.com which says the initial opening temp for the thermostat should be between 84C to 89C. – Robert S. Barnes Feb 19 '15 at 13:53
  • That goes for most cars. – Captain Kenpachi Feb 20 '15 at 7:17

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