overheating in town

just put new motor in my 92' xj,4.0...new thermostat 195,new sensor and sender,water pump only had 6 days on it when motor spontaniosly combusted itself through the block. it runs great at 210 when on highway but when i am in town at a light or slow traffic in gets up to bout 230,240 really fast.drops down pretty fast once i get up to bout 30 mph.fan clutch is good as well. does the rubber mat under the motor contribute to air flow through radiator???if i put in nuetral at lite and rev to bout 1500 rpm it will stay in reasonable state but not 210. please help me figure this out before i crack a head!!!:shiver:

Maybe try flushing the whole system. Heater core, rad and block.
Sounds like maybe the radiator is plugged or crap somewhere else in the system. RPMs raise and flow increases just enough.

YOUR FAN CLUTCH HAS FAILED--REPLACE IT.

Had a similar problem with my '90. Radiator fins had rotted away. New radiator solved the problem.

From the 90 FSM:

"The cooling system must be in good condition prior to performing the test outlined below to ensure against excessively high coolant temperature.

CAUTION: Ensure that there is adequate fan blade clearance before drilling.
(1) Drill a 3.18 mm (1/8 in) diameter hole in the top center of the fan shroud.
(2) Insert a dial thermometer -18 degree to 105 degree C (0 degree to 220 degree F) with an 8 inch stem, or equivalent, through the hole in the shroud. Ensure that there is adequate clearance from the fan blades.
(3) Connect a tachometer and an engine ignition timing light (to be used as a strobe light).
(4) Block the air flow through the radiator by securing a sheet of plastic in front of the radiator (or air conditioning condenser). Use tape a the top to secure the plastic and ensure that the air flow is blocked.
(5) Ensure that the air conditioner, if equipped, is turned off.
WARNING: USE EXTREME CAUTION WHEN THE ENGINE IS OPERATING. DO NOT STAND IN DIRECT LINE WITH THE FAN. DO NOT PUT YOUR HANDS NEAR THE PULLEYS, BELTS, OR FAN. DO NOT WEAR LOOSE CLOTHING.
(6) Start the engine and operate at 2400 rpm with the timing light (strobe light) aimed at the fan blades. Within ten minutes the air temperature (indicated on the dial thermometer) should be 88 degrees C (190 degrees F). Satisfactory operation of the fan drive requires that it engage before or at 88 degree C (190 degree F). Engagement is distinguishable by a definite increase in the flow noise. The timing light will also indicate an increase in the speed of the fan.
(7) When the air temperature reaches 88 degrees C (190 degrees F) remove the plastic sheet. Satisfactory operation of the viscous fan requires the air temperature to drop 20 degrees F (11 degrees C) or more. A definite decrease of audible fan air flow noise should be noticed. Replace defective fan assemblies."

I had the same problem, mine was my electric fan motor. R2'd motor, ops check good IAW 99xj-sport

thanx for the advice.gonna try a full system flush and then go for the new fan clutch even though it is still tight when i spin it by hand...

Sometimes tough to test the fan clutch effectively. The symptom you describe of overheating at idle/lights though fits a failing fan clutch perfectly.

They are inexpensive and pretty easy to replace so I agree with Joe that you should replace it.

Rule of thumb, overheats in traffic, fan clutch, overheats on hiway it's the radiator.

ok got it.good rule of thumb.

First off, thanks for the advice because it helped me too! I am having the same problem, and I was leaning towards fan clutch but wasn't sure.

On a side note, 20-40 mph, cools GREAT. At a light, it will heat up to ~240* QUICK. As soon as i get going again, it goes down. However, it is cooler at 40 mph than it is at 70 mph. I flushed the radiator two weeks ago. New electric fan. I'm going to go ahead and change the fan clutch just in case, but I just wanted to roll that around yall's heads for a while.