I would frist replace the Thermostat here is a good one for $13.54 there is a link to a place you can get a good part from
I as well want to say I agree with the person that posted water pump I would check the thermostat frist as it sounds like it is locked closed if you replace bolth and still have a problem it could be a hole in a hose, bad radater cap, cloged radater. I hope I was of help and it's a cheep fix.
Part Number: ST13008Manufacturer Number: 13008
Our Price: $13.54
You Save: $2.44
STANT THERMOSTAT -- 180%XB0 F, A Direct Fit OE Replacement Engine Coolant Thermostat, Made Of High Strength Stainless Steel Assembly, Offers Engineered Springs For Smooth Performance, Comes With Self-Cleaning And Self-Aligning Thermostat Valve
Fit Note: Alternate Temperature
Fits the following 1999 Jeep Grand Cherokee submodels:
1999:Laredo; 6 Cyl 4.0L; "GAS, FI, VIN S"
Sunday, January 31st, 2010 AT 4:53 PM