Test old cap at store b4 buying new one . Also check new one .

 

Too bad I can't verify this myself, because here I will never see sub zero temps, but I find it hard to believe that colder outside temps would overcome the enormous heat produced by the engine to the extend that the underhood temp actually would prevent the thermostat to open....

WP creating water flow is belt driven as far as I know, and if anything it would be related to turning speed of the pulley...not outside temp. The WP will turn the same speed regardless outside temp. Maybe you drive slower in colder weather, or am I missing something here!?

 

We live in a desert, never seen sub-zero temps here either and it's highly unlikely we will. However, we usually see some mid-teens January/February and temps cold enough that require the use of the heater from say December thru March. This year, we had snow the end of April.

Seeing is believing......we use a ScanGauge, which is a digital scan device that plugs into the ALDL port under the dash and it displays in 1* increments the info it receives from the vehicle ECM (computer). We run a 180 t-stat (which we tested and verified open temp prior to installation) and on those really cold days (cold to us) and just about any day we use the heater, the ScanGauge will float between say 176 and 183. If u have observed a 180 rated stat, u will see that it appears closed or only the slightest sliver open AT 180. At any stats rated temp, whatever that may be, flow thru the radiator is virtually nil/none. However, flow thru the motor (bypass loop/heater core/block/head) can be slow to rapid, depending on the water pump speed. Again, during cold weather operation the stat will cycle between closed and only slightly open and when the stat does slightly open, it's only open for a brief amount of time as really cold coolant in the rad will quickly close the stat after circulating thru the motor.

I've mentioned numerous times in my posts that "the t-stat hoovers in an infinite number of positions between full closed/full open. Obviously the flow thru the radiator becomes less and less the closer the stat moves to the closed position and during winter driving, using the heater (which removes a substantial amount of heat from the coolant), the stat will spend most of it's time closed or only slightly open and slightly open only briefly. The ScanGauge proves this by coolant temps "hoovering" at 176-183 using a 180 t-stat.



Quote from my previous post:

"In freezing ambient temps yes, there will be a significant difference in radiator inlet/outlet temps due mainly to very slow coolant flow. In 3 digit ambient temp, stat wide open, no, there will be very little difference in rad inlet/outlet temp due to very rapid coolant flow. When the thermostat is wide open, coolant flow is at max (water pump speed). The reason the stat is wide open is, the cooling system has been maxed out by the driving conditions."

I should have said "due mainly to very slow coolant flow thru the radiator". My bad......however, in winter driving conditions, and using the heater, coolant flow is very slow/only a trickle thru the rad OR momentarily not at all thru the rad. The t-stat controls flow thru the rad until the stat reaches it's wide open position. Only when the stat is wide open, is coolant flow speed determined by water pump speed.

In winter driving, the stat is mostly closed.....in summer (3 digit ambient) driving, the stat is mostly wide open......at least the XJ stat is mostly wide open during the summer due to the ridiculously small radiator.

 

Radiator cap fixed it, and the shutting down was what I originally thought, CPS. Thank you for all of ya'lls input it was a big help.