I tried doing some research first, but everything I found was different from what I am experiencing.

My XJ tends to overhead (or so I think) only after idling stationary for extended periods of time. Several times now, when either stuck in bad traffic due to an accident on the freeway or being stuck in a drive through fast food place behind a long line. After about 10 or more minutes of idling, I will hear some gurgling in the coolant over flow bottle and then some steam will start coming out from under the hood. The steam and cooling begins to spray out from underneath the cap. At this point the temperature gauge is still reading under the halfway mark. I pop the hood to release some hot air and shut the engine off. If I cycle the key back to "on" the temp gauge jumps way up, almost to the red.

Now, once I get moving again, the immediately comes back down. It never gets hot in city driving, never gets on on the freeway. The temp always seems to be great 95% of the time. Fans are working, plenty of coolant flow, the radiator IS NEW ALONG WITH A NEW CAP, etc..
This only happens every once in a while during long idles.

I'm starting to think my radiator cap is not the greatest, as the cooling is coming past it when it gets hot enough, but there is no coolant spraying from anywhere else. Also, I wonder if my temp sensor is wacky due to it reading ok even when it starts to "boil over", then cycling the key seems to reset it and the gauge jumps up right away.

I'm kind of at a loss here. Should I try a new cap with higher "lbs" rating? The current cap is a new replacement Murray from O'reileys.

 

coupe things off the top of my head..

bad rad cap? if it isn't holding the proper pressure it will boil over at a lower temp.

fan clutch bad? not locking up and pulling air as it should.

water pump on the way out?



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Overheating at idle only is a sign of poor airflow. Most likely the mechanical fan clutch needs to be replaced. A contributing factor could be the air passages through condenser and radiator being clogged with bugs/debris.
Also, the fan shroud needs to be in place for proper airflow.
Radiator caps are designed to vent at specific pressure, around 18psi.

 

Thanks fro the replies.
Shroud is in place and its a new clean radiator. I have blown out the condenser several times with a pressure washer to keep it free of bugs and debris.
Perhaps I will try a fan clutch. I have read some of the other jeep fan clutches are an up grade, ZJ I think?
Also thinking about wiring my A/C aux fan to come on even with the A/C off, that my help too.

 

the stock setup is plenty enough to cool the jeep properly, so getting the ZJ fan clutch or wiring the e-fan to always be on is only putting a band-aid over the problem. first of i'd get a new fan clutch, cheap and easy to change. then i'd make sure your e-fan is working properly, which means it kicks on at 217-218 degrees. if it doesn't look into replacing the coolant temperature sensor which is located in the thermostat housing.

 

Awesome, thanks for the tips. I was thinking about replacing the temp sensor anyway, as it is cheap and easy. I'll order a new stock fan clutch as well.