2000 Jeep Cherokee 4.0L overheating

[ickman]

2000 Jeep Cherokee 4.0L overheating

For most of the time the engine runs between 210 and 213 degrees, but then it will suddenly increase to about 245. If I start or keep the Jeep moving it will cool down in a few minutes, then all is fine for a while. It will do this either in idle or while running.

I have removed the thermostat and ran it last night and it still happened.

The lower radiator hose does not seem to be compressed or pinched.

When I tested the Jeep last night the fan did not loose speed. It was producing just as much air as usual.

I can't tell whether there is water coming out of the water pump weep hole, but is it safe to bet that it is probably the pump?

Thanks

[fredjacksonsan]

I'd think that it's the radiator. A slight reduction in flow will cause overheating.

[ickman]

OK, here is the latest. After replacing the radiator, then the water pump, and the thermostat it will still overheat.

It overheats in a very strange way. It can be fine, then (only while idling) inside of two seconds the gauge will go to 240/250 and not come back down until you start moving again. Then it comes down in about 30 seconds or so.

Last night I tested this by running the car up to normal operating temperature then let it idle with the AC on high. Could it be that having the AC running on idle for so long is causing it?? I haven't tried to duplicate it with the AC off.

What have I missed?

Thanks.

[Saudade]

Could also be a bad sending unit or may a short in the wire to to the gage.

Use a small mirror to look under the pump but if you have a new pump, I wouldn't suspect this yet.

Did you flush the system when you replaced the radiator? Could be some other blockage.

Do you still have the thermostat out?

Try running it with the heat on.

Of course, make sure your coolant level is topped off.

[fredjacksonsan]

Do you have an electric fan only? Is it coming on? Have you tried to get any bubbles out of the system?

[sector95]

Under what conditions will the tempurature spike (long idle, hi-way speeds, stop & go, A/C on or off, etc)???

Sounds like a tempurature sensor gone bonkers or possibly a bad ground. Me, myself and I would R&R the temp sensor..... see what happens. Also, check the fan cluctch.... with the engine good and hot, shut it off, pop the hood and try to spin the fan.... it should have resistance. If not (as in it spins easily) then the fan clutch is probably not engaging fully if at all under hot conditions.

mike

[ickman]

Well, the water pump was the root of the whole problem. That and there was air in the cooling system. It's really amazing how little air it takes to cause the system to overheat.

Thanks for all the pointers.