After a thirtyish mile drive back home yesterday, I noticed that the temp gauge did not go all the way up to 210 like it usually did when I drove it in the summer. It was below 32F and at night, so would that be a factor? On the way there, I think it was normal temps, but I was driving up a hill. I also drove about 20 miles today, and it did the same thing. The outside temp was around 40F. Does this mean the thermostat is stuck open? What happens if I don't fix it?

 

An all iron engine can take a long while to warm up depending on ambient temp and load

The lowest temp around here is about 45-50F and in those cases I noticed my XJ doesnt get to 210

On older manually choked iron engines I noticed it can take 30mins to get to full temp

Having said all that, you still may have a stuck thermostat, which will cause excess fuel use, excess fuel washing the bores, and oil dilution

 

Excess fuel use is a problem I have. Although new spark plugs and wires seemed to make it better. As for excess fuel use, that would mean it is running rich right? Also, why would a stuck open thermostat cause more fuel to be used?

 

the computer is telling to inject more fuel to get engine hotter and up to proper operating temperature. But it can't due to over cooling.

 

That's partially correct. It's not adding fuel to get up to temp, it's adding fuel because cold engines don't run well when leaned out as well as a warm engine does. It adds fuel because it'll buck and sputter. Think of driving a carbureted vehicle without choke.

I did a check this morning. It was 37 degrees, I fired it up in the driveway and gave it about 30 seconds, then started driving. It was 5:30 when I started the engine, and by 5:34 it was fully warm. I consider fully warm when the scan tool reaches 190, since that's what the thermostat is set to. So 4 minutes of 30-45 mph driving had it at normal operating temp with sufficient heat to keep my skinny southern butt from shivering.

How hot (cold?) is it getting?

 

Job #1 Evil Goat is to "verify" what your actual coolant temperature is. Gotta have good data to start with as there can be some inaccuracy between the coolant temp sensor and your dash gauge. How is the heat in the cabin? Notice any difference?

Get an infrared point and shoot thermometer. If you don't have one, buy one....they are inexpensive and widely available (Walmart, Big Box Home stores, etc). Point it at the thermostat housing of a fully warmed up engine. Then you'll know. Post what you find here.

 

On your 91 the temp gauge reading comes off a small sender back drivers side...kind of hard to see. The temp sense to the computer, to adjust /schedule fuel flow still comes off the sensor in the thermostat housing. Sadly you cannot see or read that on your year. Use the remote temp guns or take the sensor out and drop the sensor end in a pot of boiling water...then read the resistance across the contacts...should be around 675 +/- 50 ish...thats for thermostat sensor. Thats the 210 range reading The 4.0 is very temp sensitive with regard to fuel scheduling. On my old 92 I jumpered in a resistor in parallel to the t stat sender to turn on the fan at 200 instead of 220. It tricked the computer into thinking 200 was 220 .Had a on/off switch for it. If i left it on by accident the truck would not start on cool/cold mornings till i turned it off. It leaned out the mix too much

 

With a Mopar thermostat in my Jeep 4.0s, I can start them, leave the house, and have heat in the cabin in 3 miles.

 

Good grief.
this is being way over thought.
take out the thermostat , stick it in a pan of cold water on the stove with a thermometer. Heat up the water and determine at what temp it opens.
if it was closed in the cold water and is opens at the rated temp it's OK. if not, it isn't OK, replace it.

 

Always keep this in mind with new parts: The CRAP factor

Chinese Replacement Auto Parts.

 

Yeah, I should have added to my post that if you need to replace it use the absolute cheapest one you can find and don't test it first. Then we can start a whole new thread.

I'm becoming convinced that the demise of vehicles produced after the start of closed loop technology ( O2 sensors/all the other sensors/"computers" ) will happen when decent replacement parts are no longer available .

 

After a 40 minute drive with 40 degree weather, the thermostat only reached 160-170, when going up a hill it looked to be 180-190. Since the blower motor is broken, I don't know how warm the heater is. There was warm air radiating from the vents though.

 

Yeah that's obviously not right. Time for a thermostat. If you don't know how old the hoses are, then it's time for a set of them as well. Do any cooling system maintenance at one time if you can. It minimizes the number of times you have to buy coolant.

 

Tomorrow, I will take the thermostat out and check it. How much coolant will come out when I remove the thermostat? And how would I put more coolant in? Though the radiator cap, or the reservoir? Would it have to be "burped" due to air in the cooling system?

 

You can drain it and collect via the radiator tap, or lower hose first, as it will lose coolant to the same level in the radiator as the thermostat

As coolant should be changed every 3 years at least anyway, it may be a good time to do that, if not you can re-use or top up