Sounds a lot like a bad Coolant Temperature Sensor to me. What year XJ? My 89 does this and it's the sensor.

 

Picked up an IR temp gun today so I'll give that try in the morn after she's good and cold and goes through a full warmup. Before I drove it in to town blocked about 1/2 the rad with a piece of cardboard and that cut the time to full warmup dramatically!

Rad is full best as I can tell. I'll make sure to check again and let it run a bit with the cap off. It has pulled so water from the overflow so I know all that is functioning properly. Maybe I'll pull the water temp sender too and see if it bubbles. Funny thing is after the water pump change the system burped itself in one little drive down the interstate. I don't understand how I could have trapped air by pulling the t-stat. I drained more water out of the system when I changed the water pump. I've changed water pumps and t-stats and 3 of the last 5 DD I've owned and never ran into this before.

I'll check the temp with the gun and water level again tomorrow and let you guys know.

Thanks, Jason

 

Any time coolant is drained, air enters the system. If the rad cap is not functioning correctly, the coolant recovery system (open system) will not function (burp) correctly.

 

When I first moved to MO mine did that in the winter, thought it was the thermostat too. Wound up changing it 3 or 4 times in 2 months, realized I was chasing ghost and gave up. The only bad thing I have seen this happen with is a cracked head in/on/or near the #1 cylinder specifically. Mine happened in the chamber between the valves, drove for almost 2 years like that, no choice at the time. I do not recommend driving on it for that long. The other was a friends, his cracked in the exhaust jacket. The tattletale for mine was a rusted and wet spark plug, My friends was a O2 sensor that kept going bad.

 

I'm just gonna say this...most of y'all got the nail on the head with the tstat opens slowly, but its not a cold shock. Remember your water pump is pumping at a decent rate (I don't know the rate but I know its decent) so even that tiny opening is letting a lot of coolant in. Also remember -

The temp gauge is just that- a gauge. Its relatively accurate yes but not spot on. Is it going from 210 to 150? No. Probably more like 195 to 165. To be honest I wouldn't worry. When I'm cruising the interstate mine says I'm running at 150 but I know the tstat doesn't open till 195 so I know I'm probably closer to 190 then 150. I wouldn't be worried brother

Also, the 'extreme xj cooling" comment, I got a buddy running 3 fans all three pushing 400 com over his 3 row aluminum, its pretty extreme if he decides to run all 3 at a once

 

Is there a way to clean the gauge sensor? Because mine goes the other way. It'll be below the 210 mark and then when I stop it likes to jump up to above the 210. Once I start moving again it drops below the 210 marm again..

 

Your problem may be deeper than a gauge sensor.

Yours may be indicating what is actually going on.

 

my neighbor had a similar problem with his 96 dodge cummins diesel. the coolant would slowly rise to normal operating temp then drop fast to 150 degrees. this cycle would continue all day (not like yours, only a few cycles then normal). he also had no heat. he replaced the heater core to fix the heater issue first. this fixed the heat and also fixed the coolant temp issue. does your heater work well? might be beneficial to flush the rad with some cleaner.

 

http://jeep-xj.info/HowtoRadiatorRestrictor.htm

Edit: wanted to clarify something before a war broke out on restricters as I saw on several different threads.

I am not nominating this idea as a means of a solution to an overheating problem of any sort. If you're overheating, it isn't because your coolant is moving too quickly through the rad. That's a totally different problem with a different solution.

I am bringing this solution up as a means of fixing 'over cooling', which is what I experience and plan to fix today. I know I will be disagreed with, but I believe the issue here is as follows:

The engine starts and goes through its normal warm up cycle, tstat doing its job of staying closed. After running for a just a moment, the average driver goes ahead and drives off. While driving, the engine is still reaching operating temp, but the already cold coolant in the rad is being further cooled by air rushing through the moving radiator. At OT, the tstat does its job of beginning to react with heat and opening. All high pressure/heat in the block is immediately drawn by physics to the low pressure/cold of the radiator, resulting in an overcompensated exchange of heat before the tstat can react and seal off again, resulting in a drop from 195-200* to 150-160*. This cycle repeats until temperatures stabilize enough so that the tstat can react accordingly to the varying temps in how far it opens. The expansion and contraction of the wax element isn't super fast enough to deal with the first few initial heat exchanges that result in the temp drop. Enter a restricter, which controls this flow of coolant to a degree to give the wax element time to react to extreme changes. Granted during summer I would probably remove my restricter, but during winter I doubt the tstat ever has to be wide open and the cooling system being pushed to its max.

Again just my humble opinion. No expert here.

 

Very detailed response. I just disagree its "normal". None of my 3 vehicles do this. Only time any of my previous 35+ cars did this (yo-yo on the temp guage) was a '98 suburban and the coolant was low. I had just bought it, and it did it on the way home. I flushed it professionally and all was good.

My bet is it's either the guage or air in the lines.

 

Here is a wrench in your theory? I have a 180* stat with a drilled 1/8" hole. So, coolant is always flowing at all times. Mine, works normal. Slowly builds up to 185* temp and maintains there. Never drops.

2nd, why is only his vehicle doing this? None of my other 3 vehicles do this. Nissan, Chevy, nor my Jeep. Something isn't right. Again, I say it's air in the lines, or temp guage is bad. Respectfully.

 

Quote: "...but the already cold coolant in the rad is being further cooled by air rushing through the moving radiator."

The coolant, at rest in the radiator, will not cool below outside ambient air temperature whether or not air is flowing through the radiator fins.

 

Do any of those vehicle have high performing cooling components? Mine didn't do this until I replaced the rad last year with a 2 core unit and have the cooling system a complete overhaul.

Like I said, it was a humble opinion. I acknowledge not all cars do it. My Kia not my wife's car does it. My old focus didn't. All I know is my Xj does and I have burped the system of all air, rad cap works, everything is new, stat is new a reputable brand... Never gets above 200 though even in summer.

 

Quote: "Never gets above 200 though even in summer."

A lot of people on this forum wish they had that problem.

 

There r 2 coolant flow "loops" in virtually all automotive cooling systems. Coolant flows thru the "bypass loop" the moment the motor is started and continues to flow thru the "bypass loop" as long as the motor is running. Coolant ALSO flows more/less thru the "radiator loop" when the t-stat begins to open. The entire cooling system becomes pressurized as coolant begins to expand from heat......even the radiator becomes pressurized well before the stat open.