JeepMateo

Hey guys, I have a 13 jku rubicon with 4” lift on 37’s, winch, etc. been fighting an overheating issue for a few months now. Only overheats on high load on the freeway so going up a grade and going 75+ mph. Or going up a winding steep grade going 45+. Temps will creep all the way up to 250 in some instances. It never gets above normal operating temps while idling or when driving around town or when driving on flat freeway. I have been running 37’s with 3.73 gearing for about 1.5 years now. I think It started around 7 months ago or so around when I went from 37/12.5/17 general grabbers to 37/13.5/17 nitto extreme terrains with a lot more tread. I had replaced my t stat about 2 months before that. When I was driving up the mountain one day I noticed my temps started to rise and I could smell coolant smell. Went home and found my water pump had dried coolant around it. Had a friend come over and check it out and he found my radiator also had a small leak. I replaced the radiator and water pump and it was still overheating under high load. I put a vented hood on it and that didn’t help. I replaced the t stat again and new radiator cap and thought that had fixed it, nope. So then I figured it had to be the gearing was causing the engine to work too hard. So I just finished a gear swap to 4.88’s and I drive it on the freeway last night and hit a grade and it started to overheat again while going 75 and at like 5k rpms. I’m at the end of my rope with trying to figure out what it could be. Any body have any ideas? Could it be tires? I didn’t think it could be head gasket since it never gets hot any other time. Thanks for your help.

Mr.T

It sounds like the "usual suspects" (a good movie BTW) would be either there isn't enough airflow (from rad fins clogged, fan not going to high enough speed, etc) or a problem on the wet side such as the radiator is partially plugged, the thermostat isn't fully opening, water pump not pumping enough, etc..

Side note, I always boil a new thermostat in a pan of water, preferably with the old one, to make sure it opens.

You've replaced the radiator and t'stat, but I'd start with a visual of the radiator fins, and maybe just washing them out. If lots of fins are damaged and bent, the system has lost cooling. Hopefully the replacement rad is equivalent to the original, but don't rule out getting inferior parts. I suspect the cooling system on the Pentastar doesn't have any tolerance for loss of heat transfer at full load.

Next idle the engine and observe the fan operation. Restrict airflow with some cardboard to get the engine temperature up to ~225. It's a variable speed fan and should be increasing speed as the engine temp rises, ~225 it should sound pretty loud, pull the cardboard and the airflow should cool the temperature very rapidly AND the fan's air discharge should be very hot air.

Stay in touch,

JeepMateo

Thanks. I reason out airflow as I don’t think at 75+ mph that not enough air is getting through...however, I could be wrong...I have power washed the radiator area many times and no change.
I have confirmed fan operation by pulling over on the highway when the temp was up to 250 and hearing the fans going, they were loud.
also, when I get off the freeway or after I have crested a grade after the temp is high, the temp goes down pretty quickly ~15 secs or so.

Mr.T

What brands were the replacement radiator and thermostat? How do the rad fins look?

JeepMateo

replacement radiator is Mishimoto and T stat was from carquest. Fins look good from what I can tell...the condenser and oil cooler are in front of it. And it was put in about 5 months ago.

Mr.T

Tstat's are cheap, consider getting a mopar one and boiling both of them. Carquest is not my favorite brand, and it sounds like the problem started a couple months after installing it -- Maybe the symptoms were delayed due to colder ambient temps and/or slower winter driving

It's not tire size. The engine makes the same maximum power/heat regardless of tire size, and either you (with a manual) or the auto trans selects the gear ratio, and therefore the RPM to make that power and heat.

Mr.T

Here's a trick I came up with to check the thermostat opening temperature against the engine coolant sensor. It won't necessarily tell you which one is in error, but if they agree, chances are that the temp sensor, and at least the thermostat opening temperature are correct.

First step is find the spec for the tstat opening temperature, which looks like 203F (ref motorad cut sheet).

With a cold engine, startup, let it idle, and watch the engine coolant temperature digital readout (use a scan tool for OBDII engines without a digital dash readout like the 3.8L Jeep). When the tstat initially opens, cold water (from a cold radiator) will rush in and the temperature will drop a few degress. For example, at 204F (increasing) the temp suddenly decreases to 197F -- that's when the tstat opened and let in the cold water, which would be about perfect in this case.

G.A.H

This my sound weird but what coolant did you use ??
the same coolant that was in it or not ?
did you use a 50 50 mix or not ?
cause many times the problem is you have a lot of water in the coolant or you're using the wrong coolant and something is clogged up. Cause mixing coolants can cause a lot of problems.

tjkamp

2013s use OAT not HOAT. Had to argue and raise hell at the dealer, when I had them do a flush (clogged heater core).

JeepMateo

I used g30 coolant. Same purple stuff.