I have a 2000 Jeep Grand Cherokee Laredo. It was fine until I used the heat 2 times. Then my coolant was low in overflow & I added some (I could see some coolant leaking out under the drainplug & small hose under that has crimp, fluid around that too). Now fluid in overflow stays & trans over temp light comes on & temp goes up to 250 very fast.What could be wrong?

 

Welcome to the forum.

The coolant overflow bottle is used to maintain the fluid level within the engine and radiator to be 100% full, with zero air.

If the overflow bottle became low or empty and fluid was only added to that bottle, then there is probably air in the engine / radiator that must be corrected.

The resolution of air in the engine / radiator must be performed properly because of the risk of rapid / unexpected / violent discharge of hot antifreeze / steam that could cause scalding injuries.

The fluid level should never have gone low, so there is probably another problem too.

 

I would say you need to check out the overflow bottle for cracks and a leaking hose clamp or hose. Now is a good time to replace the radiator cap to eliminate that as a possibility also. You probably have air in the system that's causing hot spots and steam pockets.

There is a transmission cooler in the radiator and if the radiator is getting up to 250*, that trans fluid coming back to the transmission could be tripping the sensor.