why will a jeep idle setting still and not get hot ....but when driven it gets very hot???

Answer 1

Dirt clogged between the radiator and A/C condensor, or a blown head gasket will also cause those symptoms...

Answer 2

Have you drained and refilled the cooling system. I am assuming a 4 liter. You MUST bleed the air from the head. Easiest is the heater hose near the thermostat. At idle the engine produces little heat, but alot under load. Radiator clogged either inside or bugs on the fins or fins bent. A stuck closed thermostate could cause this as some water does flow through the heater and back to the radiator, but not enough for the heat load during driving.

Answer 3

Lots of reasons.

Get a Haynes or Chilton's book at the parts store and check the following

Most common is low fluid or malfunctioning thermostat.

Coolant is full (DO NOT check hot, only COLD!)
Fan belt is tight
Thermostat is working
Fan clutch is working
Air flow through the radiator (no mud clogging the cooling fins)
Radiator is clean (fins don't have calcium build-up)
Leaking head gasket/cracked head (leaking exhaust into cooling system)

Unlikely, but it happened to me, your impeller is broken and not spinning. That's a "Everything else is fine"

In this case, temps were great idling for 40 minutes, but immediately climbed as soon as I started to drive down the road.

Eric D

Answer 4

increases in water pressure can cause this. eg blockage in water system. the main cause in most cars is the thermostat and radiator. alternative to above is your timing is out. if a car runs to far advanced it can run hot and over heat under general running conditions

Answer 5

Stuck thermostat, or plugged radiator.