Ok this is my first Jeep I've ever owned so bear with me. I purchased my 98 ZJ three or four days ago and the temp ran right at 210 while driving through town and around 213 or so driving on the highway. After I stop the coolant it bubbling in the coolant reservoir so I replaced the radiator cap and that helped a little but not completely. I got the A/C fixed yesterday and after about 10 minutes of stop and go traffic with the A/C on the temp started rising very fast. I shut off the A/C but it just kept climbing at that point so I had to pull over and let it cool off. Keep in mind that here in Tucson the temps yesterday were around 109 degrees and I was in traffic.

It has a brand new radiator and the T-Stat appears to be working but I purchased a new T-Stat anyway to be sure. I checked the fan clutch and it seems to feel right but I also picked up a new one as well since they are so cheap and I can just cross that off the list. If neither of these fix the problem where should I look? I'm kind of concerned about how the coolant bubbles after I turn off the vehicle I noticed that if I put about 1 or 2 lbs of pressure on the top of the cap it stops but the cap is a brand new 18lb Stant cap. Anyone have a thought on what could be wrong? Also I'd like to know why it runs a little hotter on the freeway than in town.

Engine is the 5.2L V8

The cooling system might need flushing out, or it could be the water pump. JMO

Yeah I was wondering about the water pump. The engine has 6,000 miles on it since it was rebuilt but I'm not sure if they replaced the pump or not. Do these have metal fins or plastic fins?

if its only doing it while sitting still i would rule out the water pump/ radiator. im going through a similar issue. check into a radiator fan relay switch. im gonna flush the system next

This vehicle has a mechanical fan not an electric fan so there is no relay. Also it is fine while sitting and in the city but starts to get a little warmer while on the highway.

New fan clutch fixed it. Moves about 10 times more air than the old one did.

Good. Hope that solves your problems on the freeway.

It should, the fan has to turn even at highway speeds to create the negative pressure behind the radiator that allows the air to flow through. Without the fan even at 65mph the air hits the front of the radiator and takes the path of least resistance which is around the radiator and not through it.

I'm aware of that, but if your cooling system is clogged up or your water pump is marginal, the extra air-flow would have little effect at highway speeds.

True the radiator is brand new I know that much and it looks to have a ton of flow through it when I had the cap off. But driving around the block a few times it was 107 degrees out and it was running about 10 cooler than with the old fan clutch so I'm gonna go out on a limb and say that was my problem. Gonna take it out later and I'll report back with how it does on the highway. Compared to my other cars this thing is dirt cheap to fix so I have no problem throwing money at it until it runs right. I'm just happy to have something that goes off the road.

I'm with you, and hope that was the problem.

Ok just got home from a 127 mile round trip that included 35 miles of stop and go traffic in 108 degree heat, 28 miles of uphill in 100+ heat all the way to 8,500 ft elevation and then 25 miles of off road down the back side of the mountain and then 39 miles of highway to get back home. Only time the gauge ever even hit 210 was going up the mountain in the 100 degree heat but once the temps outside dropped to the mid 90's it came back down to around 195 or so. So that being said I think I have my cooling issue fixed.

This was also my first off-roading trip in the Jeep and I'm very pleased with how it performed.

glad to hear you got her ironed out, I had to change my clutch a few weeks ago and also had to clean out the outside fins of radiator a bunch of dust, you should do this a couple times a year esp. if in dusty areas wheelin...

Yeah it sounds like you've got it fixed.