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195F thermostat = 210F temperature?

M

maxbraketorque

NAXJA Forum User
Location
PDX
How is it that a 195F thermostat causes an XJ to run at 210F? I've now used a 180F thermostat, and OBD-II reports 195.8F after warmup. I find it puzzling that the reported running temperature is consistently 15F higher than the tstat set point. I've sent a lot of time logging engine temp on my other hobby car (not a Jeep) that has a 180F thermostat, and the logged temperature is almost always 180F.

I wonder a bit whether the temperature calibration is slightly off, but that seems pretty hard to believe. At this point, I'm guessing that although the coolant temp sensor is very close to the thermostat, it is not experiencing the full flow of water going through the tstat, and in fact runs a bit warmer. I need to do some measurements with my IR gun.
 
they all run hot. and it depends on where the temp sensor is located.. my Renix has one on side of engine black for the fuel injection for the computer and one on rear of the head. and some of these also have one in the radiator. i am thinking a Glowshift on the rad hose before it goes into the rad so i can read that. old gauges old sensors and old wiring is not to be give much trust. a separate modern gauge should tell you what you have. i assume we need to know the temp of coolant coming out of the engine not after it has been cooled down on rad exit side .https://youtu.be/X1J_SgpgV68
 
195* is the specified thermostat. 210* is the typical indication on the dashboard gauge, and on an IR temp gun pointed at the thermostat housing.

Running a lower temp thermostat confuses the PCM and often results in lower than normal mpg's.

The Jeep 4.0L is a 1960's design and well proven to run +300,000 miles with regular oil changes, normal preventative maintenance, and a 195* thermostat.
 
The thermostat sets the "floor" not the "ceiling".
 
Gunter said:
they all run hot. and it depends on where the temp sensor is located.. my Renix has one on side of engine black for the fuel injection for the computer and one on rear of the head. and some of these also have one in the radiator. i am thinking a Glowshift on the rad hose before it goes into the rad so i can read that. old gauges old sensors and old wiring is not to be give much trust. a separate modern gauge should tell you what you have. i assume we need to know the temp of coolant coming out of the engine not after it has been cooled down on rad exit side .https://youtu.be/X1J_SgpgV68
Click to expand...

I've got a 2000 XJ, so its OBD-II loggable, and the sensor is located in the tstat housing.

Tim_MN said:
195* is the specified thermostat. 210* is the typical indication on the dashboard gauge, and on an IR temp gun pointed at the thermostat housing.

Running a lower temp thermostat confuses the PCM and often results in lower than normal mpg's.

The Jeep 4.0L is a 1960's design and well proven to run +300,000 miles with regular oil changes, normal preventative maintenance, and a 195* thermostat.
Click to expand...

I don't mean for this thread to be a discussion on 180F tstat vs 195F tstat.

devildog0 said:
The thermostat sets the "floor" not the "ceiling".
Click to expand...

This puzzles me. Its not the floor value on my other car that I routinely monitor. Its definitely the set point. I need to connect my OBD-II logger to some more vehicles to see what I get.
 
Your 195 degree t stat is fully open at 195. Now the engine coolant temperature is dependent on the efficiency of the cooling system to regulate temperature. Most modern vehicles run right around 205 and are programmed to operate most efficiently there. This has all been driven by the demand for greater fuel economy. The faster and engine reached closed loop, the sooner fuel trims can be corrected.
 
I just put my IR gun on the tstat housing with the engine running after a 15 minute drive. The coolant temp sensor sits in the heater core flow, and if I put the IR gun on the heater core supply portion of the stat housing, I get a consistent 10F warmer than when pointing it to the radiator supply portion. So this is consistent with the reported coolant temp sensor value being higher than the tstat set point. Next, I'll see what I get after a longer drive.
 
Czjeeper said:
Your 195 degree t stat is fully open at 195. Now the engine coolant temperature is dependent on the efficiency of the cooling system to regulate temperature. Most modern vehicles run right around 205 and are programmed to operate most efficiently there. This has all been driven by the demand for greater fuel economy. The faster and engine reached closed loop, the sooner fuel trims can be corrected.
Click to expand...

I just did a survey of OEM tstat set points on modern (2017) vehicles, and the Japanese cars are almost all 180F. US brand cars are a mix of 180F and 195F with a few that have a set point higher than 200F. European cars vary from 190F to 226F. 2017 Jeeps are 195-208F.
 
maxbraketorque said:
I just did a survey of OEM tstat set points on modern (2017) vehicles, and the Japanese cars are almost all 180F. US brand cars are a mix of 180F and 195F with a few that have a set point higher than 200F. European cars vary from 190F to 226F. 2017 Jeeps are 195-208F.
Click to expand...


That may me full open temp of the t-stay. I’m talking about operating temp. Ive worked as an auto tech for the last 10 years and I my experience I have never seen a vehicle run at the t-stat temp. Thermal reading at an exterior cooling junction (such as the t-stat housing) is not always an accurate measure of the coolant temp.
 
Also, gm 3.8 cars from the same era have a 195 and they run 210-215 as well. When everything is working how it should these things run forever and get decent economy for what they are. Im going to call that a win, not going to lose sleep over why the gauge reads hotter than the therm. They all do it, it works, whats the problem?
 
I got some more measurements today, and the portion of the water neck near the coolant temp sensor definitely runs somewhat hotter than the portion directly in front of the stat. Seems to be about about 5-10 deg F hotter at the sensor location.
 
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