Hey guys I'm scratching my head on this. History:

I replaced my water pump in an emergency situation on a cross country trip. Upon returning to home I changed the t-stat with a new mopar and replaced the fan clutch. Now that it's cold in Indiana (in the 20s now) the jeep heats up quickly, but the second it hits 210 the temp plumits to 150! It slowly builds back up to 210 then plumps to 160, slowly builds to 210 then plumps to 175. This process continues for 15miles! Until it will finally run a solid 210 at all speeds. I don't like the idea of shock cooling my block! I'm running a Duralast water pump, Mopar T-stat, and switched back to my old Mopar fan clutch and still the same deal.

It seems to me that the t-stat is snapping open and shut. Not a slow smooth regulated motion. I'm also wondering if there is some air trapped somewhere that can't be burped out. Anyone have any ideas? I think this is an actual event not a gauge issue as the heater output gets weak too when this happens.

Thanks, Jason

 

What you are experiencing is normal, I'm afraid.

You're going to get that bounce until the coolant temperature stabilizes.

It's not hurting your block.

 

It's normal now that it is getting cold out. The rest of the coolant is just warming up to normal operating temp. especially since you put a lot of new cooling parts. You have nothing to worry about.

 

Ok, I'll beg to differ and say 210 to 150 is NOT normal, even in cool ambient temp. I would suggest checkin 3 things.......verify coolant is full (motor cold), verify temp sending unit electrical connection and last, but not least, verify t-stat operation in a pot of water on top of the stove using a cooking thermometer. Coolant is constantly flowing by temp sending unit and beside the t-stat the moment the motor starts. With cool ambient temps, the stat will be only slightly opening and slowly closing......not enough to actually be causing a 60* swing.

 

Yeah I know it has to be the stat but that's what really has me puzzled. It didn't do this with the old stat, but it took the jeep forever to warm up even with ambient temps in the low 100s. After replacing the waterpump it started to run hotter so I thought the clutch fan and tstat should be done as well. This mopar t-stat is actually the 3rd one I've installed with this project. I used a Stant at first, then Duralast, and finally Mopar. All three stats did this same thing. I would put the old Mopar stat back in but it took the thing forever to warm up and started to run warm. I threw the old Mopar stat in some hot water with the Stant and I noticed that the Stant open twice as far as the old Mopar one. This makes since why it would case such a big temp drop. All 3 stats have given me the same result, so I replaced the new clutch fan with my old one, and still the same. I guess cooler is better than hotter (it does get up to temp and stay there, just takes forever on these cold days). Maybe I'll just throw some cardboard in front of the rad for the winter unless someone has any other ideas. I had a 98 Cherokee in the past and it didn't do this in the winter. It appears I just have a ridiculously efficient cooling system.

Thanks, Jason

 

It's not normal. Sorry. Maybe air in the lines. Leave radiator cap off, and let it idle for awhile and see if you get any bubbles.

 

If all 3 stats do the same thing, I'd say it's NOT the stats.....check the other 2 suggestions. Ain't no such thing as a ridiculously efficient XJ cooling system.

 

Agree, my jeep runs cooler than most (180*). So, OP can run cooler. But, 150* means the t-stat isn't even open. Has the OP taken a meat thermometer and tested the coolant? Take off the radiator cap after warm, and test it. See what readings your getting. Just to compare against guage.

 

Minimum actual coolant temp should be at/very close to the t-stat rating.

 

His is, once it warms up fully.

 

It shouldn't yoyo.

 

Well, is it really? Maybe he needs to get an infrared checker and monitor the engine/thermostat area during warmup to see what actually is happening.

But he did say the heater output changes, too.

 

Wouldn't one assume coolant heat exchange speeds up as temperature rises, so by the time a tstat would 'open slowly', the temp has risen slightly higher and therefore makes the wax expand faster and open more so? The sudden rush of colder radiator coolant that has just been sitting in the rad cooling and cooling would have an instant and dramatic cooling effect before the tstat could react. Like was mentioned, it just takes time for temps to stabilize and the initial shock properties to dissipate.

Edit: this is why some people install and restricter of sorts in the upper rad hose to make this stabilization faster and the initial shock less.

 

.....that's why I suggested to check coolant level. The coolant entry point to the heater core is up high (front of head) and the coolant temp sensors are also up high (front/rear of head).... thus coolant level must be full for good heater and gauge performance.

 

There's not a sudden rush of colder coolant. A properly functioning stat does not snap open or closed, it opens very slowly and only slightly at first....thus allowing colder coolant from the rad to very slowly be mixed with the coolant that's been circulating from the block, to the head, to the bypass loop, to the water pump and back to the block the moment the motor was started. When coolant temp stabilizes between the motor and rad, the stat "hovers" in an infinite number of more/less open positions between full closed/full open.