I have had and issue with my 99 TJ overheating at highway speeds... Mostly around 65- 70 mph. I have since replaced my rad, T-stat, water pump, upper/ lower hoses. Even still, it creeps up over my normal op temp at 210 and sometimes even redlines. The only things I havent messed with yet is the fan it self, the clutch, or shroud. I have a slight suspicion why... Recently I have had some mods ; bigger wheels, heaver bumpers, different gear ratios, and a winch. Im thinking all of these factors are playing a roll. The Jeep is heavier so the engine is working harder... And to add on to that, theres a big wall (winch) blocking air flow through the rad. ANY HELP is most appreciated!
I think yoj are right, adding equipment that blocks the air flow or adds to engine load probably was the straw that overheated the camel's back. Taking my winch cover off lowered my temp 10 degrees. Apparently there is not a lot of wiggle room in the Jeep cooling system.
Which radiator did you install?
Check that the lower radiator hose is not collapsing (even if new).
Just shine a flashlight on the hose and rev the engine, if it's collapsing, you'll see it.
It's possible that the new thermostat is defective — consider installing a Stant 195° thermostat.
Did you install a new radiator cap? Try a new 18lbs cap
X2, my question exactly. Many difficult to diagnose cooling issues are easily cured by simply going back to the OE radiator which is known to keep the engine cool in in the most difficult conditions. I've yet to read/hear of any reasonably priced aftermarket radiator in the past 19 years of owning a TJ and being in the various Jeep forums that would make me want to switch to it. I run plenty of aftermarket parts on my TJ but I wouldn't run anything but the OE Mopar radiator.
I posted this exact problem about a month ago. The only difference is that I had replaced the fan clutch assembly along with everything else. The reason I did not think it was an airflow problem is that at higher speeds it gets more air flow. Thus I believe it is a water flow problem.
To back up a little, about one year prior when I did the water pump, radiator, hoses, fan clutch etc I also was told to put in a 165 Tstat. Jeep still ran hotter than I wanted. About a month ago I changed back to a Stant 195 Tstat, and it actually made the problem worse.
Currently it is in the shop trying to find a solution...The only difference between mine and yours is that mine never redlined , it would go about halfway to the redline and no more. At lower speeds would immediately go back to 210 ish.
If they can not find a problem , I going to install a new factory radiator, removing the supposed upgraded radiator that I spend 450 bucks on.
After that I am clueless. I have tried to find a lower radiator hose with an internal coil spring, but have not been able to find one for a 99 TJ.
Okay lots of great advice guys. My rig is in the shop right now for a cracked manifold. Ill get back when I do some trouble shooting myself.
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