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#1
Old 06-12-2010, 08:49 AM
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Truck thermostat temp - pros and cons

Hi!
I'm hoping some of the Board's motor-heads will answer my question here...

I have a 2000 Dodge Ram with the 5.9L engine, and there is a small leak at the thermostat gasket. I've decided to go ahead and replace the thermostat since I'm taking the assembly apart anyway. I have 3 choices in thermostat:
195-degree(F) - this is the factory temp
180-degree
160-degree

I've read that there can be some advantages to the lower-temp thermostats in that the vehicle will run cooler, sooner, and it can help with preventing pre-ignition knock. First, are there caveats to running the lower-temp thermostat? Second, I live in Missouri now, where the winters can get brutally cold sometimes...should I stick with the factory thermostat so I get better heat in the winter? And would the 180-degree thermostat be a good "compromise" between the 160-degree and the 195-degree?
Thanks!!
#2
Old 06-12-2010, 04:15 PM
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I found this comment:
Quote:
A 195-degree thermostat that sticks open and reduces the engine coolant temperature to, say, 160 degrees, will allow fuel to condense on the cylinders due to cooler water jacket temperatures. Condensed fuel doesn’t burn properly and you pay a mileage penalty.
I assume the same problem may develop with a 160-degree thermostat.
#3
Old 06-12-2010, 04:28 PM
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I think mostly it's a holdover from the old days. Way back when, cooling systems weren't necessarily all that good so in the summer big V8's tended to overheat a lot. You could put a lower temp thermostat in during the summer which would ease the load on the cooling system a bit. When you worked the engine a bit hard it would still heat up, but since it started at a lower temperature it wouldn't overheat as quickly.

These days, a lot more engineering goes into the cooling systems and they work a lot better. Modern vehicles also have fancy shmancy computer controls and they work best when the engine reaches its factory spec temp.

Some people have the idea that running the engine cooler gives it more power. I'm a bit skeptical about this one, but I suspect that this is the reason they sell lower temp thermostats. People think it gives them more power, so they'll buy them even if they don't give more power and make the engine run a bit less efficiently.
#4
Old 06-13-2010, 04:12 AM
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Stick with the factory temp. An engine that never fully heats up is provided with excess fuel by the onboard computer. It can't all be combusted so mileage suffers, and the engine oil may be diluted with gasoline, which does not help with its longevity.
#5
Old 06-13-2010, 11:26 PM
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[and it can help with preventing pre-ignition knock.]

If you are having a lot of pre-ignition (on acceleration) you may have another problem. The gasket at the bottom of the intake manifold may be in bad order. A look into the intake will confrem this if you have oil in the intake.
#6
Old 06-13-2010, 11:38 PM
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Quote:
Originally Posted by Gary T View Post
Stick with the factory temp. An engine that never fully heats up is provided with excess fuel by the onboard computer. It can't all be combusted so mileage suffers, and the engine oil may be diluted with gasoline, which does not help with its longevity.
Seconded. I lost about 2mpg when I replaced the thermostat in a Jeep 4.0.
#7
Old 06-14-2010, 07:04 AM
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So the temperature rating of the thermostat means the temp where it tells the water pump to start pushing water through the engine, but the computer gets the actual engine temperature (from the thermostat? something else?) and uses that to adjust the fuel?
#8
Old 06-14-2010, 08:10 AM
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Quote:
Originally Posted by ZenBeam View Post
So the temperature rating of the thermostat means the temp where it tells the water pump to start pushing water through the engine
It doesn't really tell the water pump anything - it simply starts routing water through the radiator. Think of it as a thermally controlled valve.
#9
Old 06-14-2010, 09:41 AM
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Quote:
Originally Posted by ZenBeam View Post
So the temperature rating of the thermostat means the temp where it tells the water pump to start pushing water through the engine, but the computer gets the actual engine temperature (from the thermostat? something else?) and uses that to adjust the fuel?
An engine's thermostat is nothing like a building thermostat, which is an electrical device that allows user adjustment of the points at which a heater or A/C unit turns on and off, and often has a thermometer readout to boot. Rather, it's a mechanical device which helps regulate engine temperature by opening at a certain temperature point, thus routing the coolant (water/antifreeze mixture) through the radiator to prevent overheating, then closing at a lower point to prevent overcooling. It goes back and forth opening and closing to keep the temperature within a desired range. The opening and closing temperatures are designed into it, and are not adjustable. The water pump is always pumping (with the engine running), the thermostat just directs the coolant flow as described.

Engine temperature is read by a sending unit that operates a dash gauge (or warning light) and a separate sensor that informs the computer, or just a sensor that does both. Temperature sensors (and temperature sending units, if used) are located somewhere on the engine that allows access to the coolant in the engine's water jacket. They may or may not be near the thermostat.
#10
Old 06-14-2010, 10:16 AM
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Sometimes a picture is worth a thousand words.

Here is a typical thermostat:
http://2carpros.com/how_does_it_...tat_engine.jpg

This is a typical diagram of how and where they are attached to the engine:
https://w05.dealerconnect.chrysler.c...S/80add412.gif

The how stuff works page which shows a drawing of an open and a closed thermostat:
http://auto.howstuffworks.com/cooling-system8.htm
#11
Old 06-14-2010, 10:31 AM
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Thermostats prevent coolant from getting to an engine because an engine is best when warm (at its op temp).

Once warm, a thermostat pops open and lets coolant in, because we want to maintain warm (op temp) and not find out what hot/boiling is.

Cold is bad in long stretches for an engine. Hot is bad almost immediately. A thermostat get you out of COLD ASAP and hopefully keeps you from HOT all the time.

Cold engines wear down faster over months and years, get poorer mileage and probably spew more emissions.

Hot engines can developed warped heads and wind up in the junk yard.
#12
Old 06-14-2010, 10:43 AM
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Quote:
Originally Posted by Philster View Post
Thermostats prevent coolant from getting to an engine because an engine is best when warm (at its op temp).
Not quite. The thermostat prevents coolant from getting to the radiator, and thus cooled down, until it needs to be sent there. Coolant is always circulating through the engine itself.
#13
Old 06-14-2010, 01:03 PM
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Argh.... I know that. Point being that the engine is not getting a cool flow until it needs it.
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