overheating

so my 1990 4.0 gets hot when it idles, up around 230-240. but when i drive it gets down to normal temp, around 180-190. and the second i start slowing down again, it gets really hot again. so what could be the problem?

Do you have electric or mechanical cooling fan? If electric, it may not be coming on.

Also, on a 15 year old vehicle, you may have just enough restriction in the radiator to reduce your cooling effectiveness at lower speeds.

you were right about the electric fan. it wasnt coming on when it was supposed to, around 210. but the motor works because it comes on when i turn on the A/C. so what could be stopping it from coming on when it gets hot?

There is a relay that turns on the cooling fan, do a search in this forum with relay and cooling and you should find the description.

IIRC, it's located near the passenger side headlight and is an easy repair. Of course until you can find the time to fix it, just have the AC on, so the fan is turning.....

There is also a temp sensor mounted in the radiator (driver side) that turns on the fan when the coolant gets too hot. If you disconnect it, you can jumper the harness side of it and it should turn on the fan.

You can also use an ohm meter across the sensor side of the connector. When cold, it should read open (infinity ohms). When hot, it should read a low resistance. It essentially completes the circuit the the fan.

Also, check your coolant level. If it's low, it may not be reaching the sensor

is it possible that the water pump isnt working well?

If the water pump was out, it would overheat all the time, not just when stopped or slow.

so the water pump is good, its got a new thermostat, its got all new coolant, and the fan relay is good since it turns on with the A/C. so i guess its wiring somewhere?

Try reading these:

http://www.automotiveforums.com/vbulletin/showthread.php?t=545509&highlight=relay

http://www.automotiveforums.com/vbulletin/showthread.php?t=572555&highlight=relay

http://www.automotiveforums.com/vbulletin/showthread.php?t=576908

i figured out the problem, it was the temp sensor on the radiator that was bad, and there was some air in the system. so now the fan works and its all great.

Hey,
I'm having the same problem with my '93 xj. Could it be the mechanical fan clutch? It runs a little below normal operating temp (170-185) when moving (ie. highway) - but when I stop and sit for a while it shoots up and over normal op temps (210-230). The lower temps is what really throws me - I just recently put in a new 190 degree thermostat. I also have a new 2 core radiator. As far as the electric fan - it does't kick on until temps are about 230. Any ideas??

Thanks