Ok, a few months ago I replaced the radiator, thermostat, hoses, and cap. Did a flush with Royal Flush too. During the cooler months, I thought it was running significantly cooler, and maybe under 195F, than that I am used to, but hey I just replaced a 17 year old radiator so I don't have a good memory how cool things were running then.

Earlier today in 80F weather, I noticed coolant temp running around what I was used to, the low side near 210F. Tonight I was out in 75F weather picking up a takeout order and sat idling. Got up to and a hair over 210F.

One suspicion is that new thermostat is partly open all the time, so in cooler weather it never gets up to or barely over 195F. Now in warmer weather, it gets up there and stays right around the middle mark. Also heard some talk around here that a bad cap could cause similar issues.

When I replaced everything, the fan clutch had resistance so ruling that out for now. Will try to remember to check it again in the morning.

Ideas?

 

Probably the thermostat, mine will rise to idle at 100C (212F) after about 5min, unless temps are winter cool.

I consider that normal..on a hot day the electric fan will cut in & out, so 105C at idle

moving down road, temp falls to about 90C

all within spec

some of our Aussie owners who live in very hot climate have been fitting the Grand Cherokee Viscous Fan, moves more air

 

Do you have a thermal IR gun?

It sounds like you have a good comprehension of what some of the culprits could be.

My Jeep, when I first got it used to run just on the "plus" side of the 210*. Because it's an 2001 & I had some money to throw at a 10+ year old Jeep at the time, I opted to switch out the crusty looking radiator with a 3 row radiator,and I proceeded to install a HESCO high flow waterpump, a HESCO high flow thermostat housing, silicone radiator hoses, some snake oil(??) (water wetter), distilled water mixed with the coolant, I don't remember what thermostat, but it had a steam hole in the top. Now the Jeep runs just on the "minus" side of 210*. Later on, I installed the Grand Cherokee 5.9 hood vents. I didn't notice an difference, except when parked. You could really notice the heat escaping from the hood.

 

Yes I have an IR gun, but I was probably hitting the wrong spots or something when I was checking things right after the install.

 

Is it a MOPAR stat?

 

No, it is a Murray (made in Israel) from O'Reilly. The local dealer had a 4 day delay for the $30something MOPAR one. But I still have the old one. I did test the new one and it appeared to work correctly. Got it when the one from Rock Auto failed test (they refunded right away).

Today will see what happens when I put the original cap back on. New cap on there now was from Rock Auto. And not a single parts store around me has a cap tester, nor do the shops that the parts stores referred me to.

 

Cap tester is part of the cooling system pressure kit......

 

86F outside, switched back to the old cap. The new one is AC Delco. Old one not sure. It is behaving more like I am used to and after driving around and getting all the fluids up to operating temperature (oil gauge responsive and in the 20s at idle), the coolant was staying around 200ish indicated. After an extended idle, still stayed on the cool side of 210.

Will see when the cooler weather comes back if this is the only thing or not.

 

Still on old cap.

Was upper 50s/lower 60s this AM and it seemed a little slow to warm up.

However, after the OAT was up in the mid 60s, driving around and a 1/2 hour idle, it did go up to 210 and stuck around there or just a touch over. Driving around 35 to 45 MPH it came down under 210 and stayed there, even during a shorter idle.

Seems like the old cap is working better than the new one. But still suspect of that thermostat.