This week I did my first longer trip in my XJ with our pop-up camper in tow. It was mostly great, but the Jeep ran noticeably hotter than normal.

It usually runs just below 210, going to just above 210 in traffic. The electric fan always manages to keep this in check and it never gets warmer than that.
This trip, driving up and down hills with the trailer, the gauge got as hot as one bar above 210, and only went down to 210 or below when coasting down a long hill for an extended period. It was running hotter when moving (at around 60mph) than when slowly crawling in traffic.

The outside temperature was only around 75 degrees.
The trailer only weighs about 1700lbs and is in good condition (rolls easily). I lock it in 3rd gear on hills.

The Jeep's cooling system is in good condition:
-New(ish) Spectra Premium radiator, in good condition.
-New fan clutch. Roars when engine gets a bit warm.
-Electric fan comes on as it should, plus I've wired it so I can turn it on manually.
-New AC Delco water pump.
-New upper and lower radiator hoses.
-New Prestone coolant/distilled water. 50/50 mix.
-Mopar thermostat, tested good.

Any ideas why it gets hot, and what I can do to help it?

This is a stock 1998 4.0 XJ. AW4 automatic, 235/75R15 tires, stock height. Currently on around 189k miles.

 

I was going to ask if you had it in 3rd or D, but then I saw you kept it in 3 on the hills to keep the TC locked. It is staying locked, right? An unlocked TC will certainly dump a lot of heat into the cooling system via the transmission cooler, but that doesn't seem to be the issue here. From personal experience, my Cherokee in 3 with the pedal to the floor will keep the TC locked at any speed above about 70 mph (30" tires with 3.55 gearing). I've had it pinned like this for a minute or two up long highway hills in warm weather, and the temps stayed right where they should. This is with a factory radiator.

I mention this because I wonder if your Spectra radiator isn't quite as effective as the factory one? Or maybe your water passages in the engine have insulating rust in them? At highway speeds, airflow shouldn't be a factor.

How clean is the cooling system? Did you flush the engine thoroughly with water when you replaced those parts? Did you do a chemical flush or use anything that converts rust like Evapo-Rust Thermocure?

 

How is oil pressure?
What engine speed when in 3rd pulling a hill? Consider 2nd gear if speed allows it.
has accuracy of temp gage been confirmed?
Belt, is she slipping?
how is the water pump? Impeller condition?
You running AC or not when this happens?


Consider a tranny cooler is recommended when towing, in fact the xj if equiped with tow package has a tranny cooler.

 

Thanks both for the help. I'll try to answer all the points raised, but tell me if I miss something:

-Oil pressure is fine. Gets down to first bar on the gauge when (very) hot idling in Drive. Pressure is great when moving.
-Engine speed pulling a hill is somewhere around 2800rpm from memory. I use 3rd to avoid screaming up at very high RPM in 2nd.
-No evidence of belt slipping.
-Water pump is brand new last year.
-Not running AC (doesn't work).

A few other points I wanted to mention:
-This XJ has the tow package, so has the factory transmission cooler.
-I thoroughly flushed the cooling system when I replaced the water pump, but only with water. I've never tried a chemical flush.

I've had issues with it running hot ever since I properly overheated it about 8 years/75,000 miles ago. I clogged the factory radiator with thick mud and it overheated. I replaced the radiator, hoses, water pump, thermostat at that time. I'm on my second radiator since then, and have been through a few water pumps and thermostats. It has always struggled a little bit to manage its temperature, but never actually overheated since.
This recent problem seems a little worse than normal, but then it's the first long journey I've done whilst towing.

 

Do you normally tow and find this temperature to be oddly high for towing, or is towing not done before?

Maybe you should be in second gear and go slower when towing? Of course downshift manually, dont keep her in drive, else the converter will slip and over heat.

 

I tow occasionally, and rarely for this long. I've only seen my Jeep get to this temperature twice in the past 17 years. It doesn't get anywhere near this hot usually, even off-road.

The trouble here is that I'm doing highway speeds, and will cause problems if I slow down further. That means I'd be screaming at very high RPM in second gear.
I've realized though, I think my torque converter might not have been locking up as it should. I've had some trouble with the brake light switch, and I'm now wondering if this was preventing lock-up. I was putting in third, but if it weren't locking up I guess that would cause this heat.

 

this may not help much but try removing the check valve in the transmission cooler line. Just bypass is with hose and clamps. I just did mine, completely plugged. You won't miss the check valve, no issues. Your tranny just might run cooler and shift better.

 

Likely not related but I didn't see it mentioned. Did you replace your radiator cap? Often overlooked but an important component in the cooling system.

 

Good pick up , TJ!

 

Interesting points, thanks everyone!

The radiator cap is seems to be good - I forgot to mention it but I've replaced it several times in the time I've had this XJ. There's no sign of it leaking and the overflow bottle seems to be being used as it's supposed to.

I'd never heard about the check valve in the transmission cooler line - I'll look that up!

 

Now, mine is the ZJ and Im not sure yours has one but,,//
I can tell you, I deleted mine a few days ago now. Only wish Id done it 10 years ago. Single biggest performance boost I ever did for it.You can just feel it, immediately. Yes, you have to put it in N to fill the converter in the morning (or just replace it to fix this) but thats no big deal. I think every one out there must be just as plugged up as mine was. All I did was get a 3/8 hose barb and some worm gear clamps just cut it out and put it in. Hasnt missed a shift yet and feels strong all the way up. Huge.