I have been having this issue for awhile now.... since i redid my coolant system and it has been bothering me.... but i tried to trouble shoot it on my own... I have a mopar t-stat before anyone suggests that I burped the system it doesn't run hot it runs to cold... if i do not have the heat on it runs at about 200 degrees or so... If I have the heat on it runs at about 150 or the first white line under 210 not sure what temp line that is... and then evently it will go back up to 200 or so... but takes a long time... and this only happens when I have the heat on... any thoughts on this??

 

this is perfectly normal, the heat for your heating system comes from the engine, so while the heat is running it is taking some of the heat created from your engine. so since you using the heat from the engine, the engine temp decreases

 

It is normal. That is why when your vehicle start to over heat it is a good idea to turn on the heater to bring down the engine temp.

 

Hm.. I thought that expect it never used to do that until after I redid my coolant system.... That's why I question it... But if its normal that's great news I guess my coolant system was that old being steady all the time no matter what

 

Thank you everyone

 

it is also winter, since the air is cold it cools the engine faster, and you use the heat more in the winter. it is nothing to be alarmed about

 

Yup I know but even last winter it never moved... I'm just not worried about it anymore... Thank you for your help

 

Dropping from 200 to 150 when the heater is turned on ain't normal......dropping a few degrees when heater is on....yes, 50 degrees....no. Need to verify both numbers.

 

What's the best way to verify both numbers?

 

Infrared thermometer. That does sound like a big difference, but unless it stays too cold in the winter then i personally i wouldnt worry.

 

same here I would not concern myself over this.....I wish mine would run that cool, have over size rad, 2 fans and still hovers at 215 all the ****ing time.....LOL

 

your heat comes from the engine coolant, heater coil is like a little radiator so naturally coolant temp is gonna drop

 

My 90' does the same thing just replace all hoses, mopar t-stat, and a new housing. I believe the reason is because the temp sending unit is at the back of the head so the cool coolant hits that before the t-stat, once it hits the stat it closes and starts heating up, and so on until all the coolant is the same temp. But I could be wrong. Hope what I said makes sense

 

FWIW, the ScanGauge in our XJ floats 175-185, give or take, freezing ambient, heater blasting (plenty of heat), 180 stat.

If coolant temp is really dropping to 150 with a 195 stat, something ain't right and needs to be looked at. Rising to 200 with a 195 stat in cold weather and running the heater sounds about right, but not dropping to 150.

 

Sounds like your cooling system is running very efficiently, then.

Turning on the heater will, effectively, add another small radiator to your system, increasing heat rejection capacity. This is an old trick for staving off overheats in traffic - turn the heater full on and run the windows down.