Got truck last November, had over heat problems, replaced factory stat with 180 F. Fixed a lot of other things that were the core of the problem. Read about how MPG is cut by lower temps. Changed stat to a new one (Autozone - because their computer said OEM is 192 - but stat is stamped 195). Truck runs down the road at 167 - 172 F, 60 mph. At idle, with AC on, outdoor temp 80 degrees, goes from 174 to 207 in 20 minutes.

Is this stat stuck open? Shouldn't your driving temp be pretty close to the stat temp? Note that at 207 F, the guage is low. Imagine what it is when it reaches above midpoint.

 

Yes your t-stat is bad, I also have a 195*F t-stat and my truck runs right at 196* going down the road at any speed, sitting in traffic on a 90* day with the a/c on it will creep upto 200* or so but never over 205* no matter how long it idles.
Replace your t-stat.
And to add, do not boil your t-stat in a pot of water to check it, you will ruin it.
A new one is $10, just buy it and put it in.

 

Thanks, Spike.

This one was brand new in the box from Auto Zone about 3 hours ago. Now that I know their computer description and actual stock are not the same, will go to another supplier (who said 195 was OEM, not 192 like Auto Zone). You had mentioned before about running at 195 - 196, so that is why I wanted to get away from the 180. Getting just a hair under 14 mpg on flat ground, hoping 195 will bring that up slightly.

Now here is a thought - if you have an overheating incident, might that also stress or damage the T-stat, just like boiling? So over time, or with overheating problems, stat could change performance.

 

Yes overheating can damage the t-stat, just like boiling it.
I'm talking about severe everheating mind you not mild.

If your cooling system is in good order your engine should run right at the t-stat temp assuming you have the correct temp in the truck.
I have heard that 180*F is the OEM temp and I have also heard that 195*F is the OEM temp.
I personally like the hotter one because it gives me better heat in the winter, if I lived in Death Valley I would use the colder one.
I think you could go either way, you dont get very cold in the winter and your winters are short.
Winter here lasts 9 months, then we have 3 months of bad sledding and road construction.

 

180 stat was working OK. Put in a salvage OEM, but it appeared to be stuck open. Bought new one from Auto Zone (Duralast), installed, behaved like stuck open. Went to another supplier, bought a Beck/Arnley. When I took off the Duralast, I saw that the shade tree fool (that be me) allowed the t-stat to drop down 1/4 inch, leaving a huge arc for water to bypass the stat, just like "stuck open". The gasket held everything together.

Went ahead and installed the Beck/Arnley, which came with gasket, despite notes to contrary in Advance Auto's computer. Same performace, and when I tore it down I found I had still let it slip 1/8th of an inch out of the hole.

Went to store again, got Auto Zone #12994, water outlet gasket, fits GMC V-8 61 - 81, Jeep V-8 68-71, and Rover v8 68 - 82. Fits the Disco 1 as far as I can tell. This time I took off the upper hose and just had to dance the water outlet around, seems like I got it.

Comes up to 189 in about 5 minutes from cranking while driving 55 - 60. Did see it go to 194, then it dropped back and never came back over 189. Stayed 187 - 189 on a 20 mile drive up to 65 mph. Got back to the shade tree and left it at idle with AC on and it reached 201, then dropped back down to 194. Did not have my IR thermometer to check readings. I suppose my T-stat could be a little off, and my two-prong coolant temp sensor could be a little off. But since I was running around 174 with the 180, and this one is a 195, that reads 189, that is still 15 degrees warmer, just like it should be. Thanks for the help. We get below freezing 3 days a year. With all your potholes, must have to change shocks every other oil change.

 

If your coolant temp sensor is covered in scale/lime/calcium/whatever it will read cooler than what the engine actually is.
This is because the scale acts like insulation, so it might not be a bad idea to remove your temp sensor and check its condition, if it is covered in crap you can clean it with CLR, Virginia, etc.
It could also just be on its way out too.

 

Yep, good idea to check because I had radiator rodded out. If coolant temp is higher than the ECU and guage think it is, then I could have more of a problem.

 

But I am better that your coolant temp is fine, just your scanner is reading low because of the build up on the sensor.
The clutch fan should be kicking in at idle around 205-210, you said yours kicked in at 201, so I am better it is kicking in at the right temp but your scanner is reading wrong because of the sensor.
Remember that the ECU and the dash gauge use separate sensors.
I'd worry more about the ECU sensor than the dash one.

 

Pulled out the 2 wire coolant sensor, was pleased that very little coolant spurts out when cold. It was a brass rod, did not have any build up I could see, cleaned it with a wire brush for good measure and re-installed. Idled with AC on for 30 minutes, came up to 185, outdoor temp was in low 90's. So maybe it is just calibration of items. I'll pick up another to play with, and also report back on MPG change, if any.

 

With temp staying 185-187 when driving I decided to try another sensor. Got one from a donor vehicle, it did have some light scale build up, cleaned that off. So that means my original sensor was probably replaced by PO because it was shiny brass. With new sensor in drove 30 miles. temp seemed to stay 189-192, and at idle eased up to 201-205 after ten minutes or so. Will report back on mpg. Suspect sensors vary a good bit from one to another. But at 200 degrees, 5 degree change would only be 2.5%.