Hi, anyone else running the 180* thermostat instead of the 195*?

My 4.0 is running great with a 195 t-stat and the needle is mid-scale, except when I sit and let it idle for 15 -20 minutes.
Then, the temp starts climbing. I've let it go to ~75% (on the 'L' in Normal) When I start driving, it settles back down to 50%.
Though we've had a few 100* days, "Summer" isnt really here yet. I don't want to be second guessing my cooling system next month.

When I bought it several months ago, it had a 180* t-stat in it. (And it was missing the fan shroud.)
I did not notice any problems. The gauge was steady at ~ 35 -40% Of course, ambient temps were lower too,

Since then, I've replaced the belt, pump, fan clutch, thermostat, and shroud. As I would expect, the gauge is now steady at 50%
City / hiway driving - no problem. But if I sit and let it idle with AC on, gauge starts climbing.

Should I put a 180 back in it?

Someone else suggested it was my fan clutch and I should just replace that and leave the t-stat alone.

What do you folks think?

 

ThermoStat rating only dictates lower temp when coolant flow starts; it does not control how hot coolant will get above that.
That's controlled by capacity & health of your Radiator\Pump\FanClutch.
If the Radiator\Pump\FanClutch are 1994 originals or haven't been flushed\changed in several years,
they're are more likely to be the cause of temp rising when your sitting stopped in heat\traffic.
Radiators clog internally starting at the bottom with calcite+rust & clog externally with road debris, slowly loosing cooling capacity.
Pump impellers+bearings just wear out & stop pushing coolant as it should.
FanClutch wears out, doesn't fully engage Fan & stop pushing air as it should.
How old are these items?
With AZ temps, I'd do everything to insure cooling is tip top; overheating a V6~4.0L~OHV will cost you much more than these items.
If your old Radiator is 1Row PlasticTanked, consider a 2or3Row AllAluminum replacement.
If your old Radiator is 2Row PlasticTanked, consider a 3or4Row AllAluminum replacement.

Click for info on swapping 2RowPlasticTanked~4RowAllAluminum Radiator & 192f~183f TStat in a V6~4.0L~SOHC; simular but different.

Click for 1994 Ranger Radiator vendor search.

 

You're right: the thermostat should only affect the lower threshold temperature and wouldnt prevent temperature rise.
As previously mentioned, most of the cooling system components were replaced - except the radiator.
It seemed to be in good shape so I flushed it, along with the rest of the system, and put it back in.
I don't have a lot of experience with viscous coupled fans so it's hard for me to tell if it's doing what it is supposed to do.
Initially, I have to assume it is working correctly since it is new. But considering the symptom of only overheating at idle, it does seem suspect.
Im not sure about the radiator cap. My MO would be to replace it when doing all the other components but I'd have to dig out receipts to make sure.
Otherwise, there's not much left to replace.

 

If you don't know age of RadiatorCap, best to replace that too.
For the FanClutch:
~ With engine OFF before a cold start, Fan should be stiff to rotate on Clutch by hand.
~ Start+Run engine for ~30sec, then shut off; Fan should rotate much easier by hand.
~ After driving for 30min or longer at operating temp, shut off engine; Fan should again be stiff to rotate by hand.
If any of above fail, then FanClutch is bad.

 

You need a fan shroud or it will over heat when stopped or driving slowly, the shroud makes the fan PULL air THRU the radiator, no shroud and fan pulls air from the sides, inside engine bay, and NOT thru the radiator, so coolant gets hotter
When moving the vehicle speed provides air flow thru the radiator, so it cools down again

There are two different fan shrouds, single core(manual trans) and Dual core(automatics), its because dual core is thicker so shroud is thinner
manual trans WITH AC can also have a dual core
GET A FAN SHROUD

Also check fan clutch
Common symptom of failing fan clutch is overheating at idle or driving slowly, for the same reason as above, lack of air flow thru radiator
When engine has sat over night, so stone cold, open the hood and spin the fan, should be hard to spin, "locked"
Start engine and then shut it off after 30 seconds or so
Spin fan again, should spin easily

Drive it for at least 20 min like out on errands and then back home so engine/radiator are warned up
Open the hood and spin fan again, should now be "locked" hard to spin, if not replace fan clutch its bad