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Discussion Starter · #1 ·
I've got one of those OBD2 bluetooth sensors and a digital readout. I believe it to be highly accurate and very precise. On the highway it consistently reads 195.2 with a 195 deg thermostat installed. Recently I changed/flushed the coolant and installed a new thermostat. It now reads consistently at about 189.8 or about 5 deg F cooler. No doubt due to the new thermostat.
So here's the question, I know the manufacturers have gone to higher temp thermostats, at least partially because gas mileage increases with increased temp. Does 5 deg make a difference in gas mileage? Of what order of magnitude? Is it significant or so little as to be immeasurable?
This comes under the category of silly things I think about on a long drive .... :rolleyes:
 

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Listen to Carbuff2, or if it bothers you, put in another thermostat and wind up a 185 F.

The issue is not as bad as you think it is, as the engine sees the coolant temp as about 660 Rankine. And a 5 degree difference in Rankine is not very much.


The idea of running an engine 'hot' is to reduce heat losses to the cylinder walls, but an old engine is probably better off running a little cooler - less stress.

If I've made a mistake, I'm sure Jeepman and Levy will let you know.
 

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2005 Dodge Grand Caravan C/V
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Engines are NOT better off running significantly cooler, but 5 degrees is pretty much nothing. I wouldn't worry about it. And, you never know. There could be that much error in the sensor itself.
 

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What was the OE thermostat rated at? That would be the optimal one for fuel economy.
 

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my OEM thermostat was running the engine slightly cooler than normal. Replaced the thermostat and radiator and it now runs right near the middle. Often times a failing thermostat will have the van run cooler than normal and most won’t flinch since they think it’s a good thing that the engine runs cool.
 

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2005 Dodge Grand Caravan C/V
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I don't want my engine to run cooler than it should because in addition to suffering fuel economy, it also reduces heater output. Can't have that here in the Northeast.
 

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The 3.3L, 3.8L and 4.0L call for 195F, the 3.6L calls for 203F
 

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1997 Plymouth Grand Voyager Rallye
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They're not even intended to be that accurate. The FSM and Owners Manual state that the temperature gauge fluctuating between 1/4 to 1/2 when it's cold outside or 1/2 to 3/4 under stress is acceptable. The idiot gauge on the dash is far less accurate than 5° fluctuations.

Font Slope Line Parallel Pattern

Source: 2005 RG/RS SM; 2005 Chrysler RG-RS Platform Factory Service Manual; Property of CHRYSLER Corporation
 
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The 3 6L (5th Generation) has a 203 F thermostat. According to my EVIC, mine is running 185F/85C on the highway and secondary roads. Sensor must be in the wrong location. :)
 

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I wouldn't worry about a 5-degree difference, it won't hurt or effect anything. There are so many factors that affect the coolant temp: Outside temp, coolant/water mixture, are you using HVAC heating.

Edited: I forgot to add driving speed.
 

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I wouldn't worry about a 5-degree difference, it won't hurt or effect anything. There are so many factors that affect the coolant temp: Outside temp, coolant/water mixture, are you using HVAC heating.
Front heat is on. Shows the same in warmer weather. Below midway on the gauge as well. It will go up in slow moving traffic or very hot weather.
 

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The thermostat is a mechanical device, a quality made new thermostat will have a repeatable & relatively small hysteresis (open - closed range). A cheap thermostat may not be consistent when it comes to opening and closing temperatures and may change quickely over time.
I could not find specifications but I am guessing the actual temperature specifications for a quality thermostat may be as loose as ± 5° f (which is pretty good for a mechanical device). Then you need to consider the engine temperature measurement path that the OBD2 port reads, a car manufacturer is not going pay extra for a highly accurate measurement system when it does not need that type of performance, I am once again guessing that the internal temperature system may have a ± 3° f measurement tolerance. If you add these up you get a ± 8° tolerance, for a 195° thermostat that is 187° - 203°.
Don't get to hung up on the numbers because I made educated guess's (confidently pulled the numbers out of my butt).
What I am getting at is that the numbers you are measuring are so close to what you want I would be very happy...
 

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Front heat is on. Shows the same in warmer weather. Below midway on the gauge as well. It will go up in slow moving traffic or very hot weather.
What you describe is normal operation. To me everything seems to be working as it should.
 
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What you describe is normal operation. To me everything seems to be working as it should.
Yes and it will be interesting to see what others (5th Generation) are experiencing. Transmission temperatures don't run as warm as expected, the coolant neither.
 

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Discussion Starter · #16 ·
Just for the record, the minor difference never really "bothered" or worried me, but did interest me as a theoretical thing. Nonetheless i appreciate those who shared their views on the subject. May your minivans all reach the moon with no major repairs !!!:) Mine has 10,000 miles to go!
 
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