I finally found the answer

Well after years of fighting an overheating jeep and replacing everything including the motor I believe I have finally found the problem.  As I have stated before I live in Dallas, Texas.  It only get to freezing temperatures a few days a year.  It got over 100 degrees last summer for several months.  If you live in the southern United States and have an automatic AW4 transmission and have tried everything under the sun to keep your jeep cool, you probably have the same issue as me.

Here it comes.  I have been running a B&M transmission cooler inline with my radiator "cooler" thinking that two things cooling the transmission are better than one, Right?  WRONG!!!!
The transmission lines going to your radiator are not designed to cool your transmission.  WHAT THE FU**....They are designed to warm up your transmission in very cold climates.  I LIVE IN DALLAS so I dont give a Sh** about warming up anything in this damn jeep.

I have a heat creeping problem on very hot days with the AC running.  There is something called "heat saturation"  This is when metal stuff like radiators cannot cool and begin to get hotter and hotter because there in not enough cold air passing through it.  Turning off the AC will bring it back to operating temps.  This tells me that the small load of the AC running is pushing the engine temps over their limit.  This tells me that my jeep is walking a fine line between good and bad.  This also tells me that a small change in load, airflow or coolant temperature should make all the difference that I need.

My transmission runs at about 160 degrees when air flow is good but gets up to 220 and slowly creeps higher when sitting in traffic or idling in a parking lot in the hot ass summer months.  This is about the same temperature as my engine...coincidence?

No, its not a coincidence because both the engine coolant and the transmission fluid cross pass each other in the radiator.  The engine wants to run at 210 degrees and the transmission wants to run at about 160 degrees.  Why would I want hot AW4 transmission fluid running across my engine coolant?  They find thermal equilibrium at 205 degrees and then both slowly creep up because of heat saturation.  Both the engine and tranny begin to overheat when sitting too long.

The answer, I believe has always been that I should have been running the transmission cooler in bypass of the radiator.  The B&M tranny cooler should "want" to keep the transmission at ambient temperature of 110 degrees instead of the 210 degrees that the engine "wants" to maintain.  With the transmission cooler bypassing  the radiator the engine coolant will have less thermal input and should help with the heat saturation problem.

I will put a new B&M tranny cooler in front of my condenser on the passenger side of the radiator since this side has shit loads of air flow provided by the mechanical fan.  (by the way, there is NO ELECTRIC FAN THAT CAN MATCH THE CFM AIRFLOW OF THE MECHANICAL FAN ON A XJ)  If you are having an overheating problem like me, spend it on a quality tranny cooler and bypass the radiator instead of a $400 triple electric fan setup.

I will post more with the results.....

Update for summer 2012

The specs on my Jeep today (30 Aug 2012) are

Make: Jeep
Model: Cherokee Sport (XJ)
Engine: 4.0L inline 6
Transmission: AW-4 Automatic (4 speed with overdrive)
Transfer Case: New Process 231J with SYE
Lift Kit: Rubicon Express/TNT Customs with ACOS -  7" total lift
Tires: BFGoodrich KM2 - 35x12.50x15
Wheels: Cragar 15x8 - 3.75backspacing with 5x4.5 bolt pattern

My XJ has 253500 miles on the odometer.

At ambient air temperatures below 100 degrees the temperature gauge reads 212 degrees and has no problems at all. When the ambient air temperature gets above 110 degrees and my air conditioner is turned on the temperature gauge slowly creeps up and over 210 to 220 then spikes up to 250 and then the "Check Gauge" red warning light turns on.


The old Jeep has over 250,000 miles on the odometer and about 15,000 on the new motor.  I only had two days this summer that the old XJ ran too hot and I had to turn off the A/C in stop and go traffic.  The new motor did help quite a bit with the overheating but it seems the big culprit has been the electric fan.  It was still functional but not turning as fast as it could.   I replaced it with a new electric fan and that has made a large difference.  Like I said it still overheats on days around 110 and above and in stop and go traffic with the A/C running but all I have to do on those few days is turn off the A/C.  I'm looking into the Triple Threat fan with aluminum shroud as my next upgrade for this issue.  I will be interested in seeing if that cures this nagging problem for good.

It may be the head gasket leaking

This is my old original motor that has a suspect leaking head gasket. Since it has over 200,000 miles I figured I should just go ahead and replace the whole motor and that should take care of my overheating problem, right ?

This is my new ATK motor that I purchased from gearhead jeep. These guys make remanufactured motors for JeepSpeed off road racing.

These were some pictures taken when I pulled the old motor out of the vehicle. Talk about shadetree mechanic work. This is it.

This is my new ATK engine in the crate. One issue I had with ATK was that the motor was delivered without the blue straps attached and the motor had fallen over on its side. The crate was destroyed on one side. This picture was taken after I repositioned the motor into the crate and strapped it back down.

Overall the motor looks fine and should run great. This motor replacement did not correct my overheating problem. It wasn't a head gasket problem.

Exhaust pipe crushed

I purchased a new down pipe from NAPA auto parts to replace my suspect doenpipe which was chushed by a rock on one of my off-road trips. This is the new pipe next to the original down pipe.

New exhaust down pipe.

Ready to install.

Original down pipe with a large dent on the side.

In this shot you can see that the dent has restricted the flow of the exhaust gasses out of the engine. At this point in my quest to correct my overheating jeep this seems to be the culprit of my problem.

Another shot of the dent on the original pipe.

Aluminum Radiator

This is the aluminum radiator that I purchased last year when my XJ first started to overheat. It is still in fantastic condition and I plan to use it again.

The welds on this radiator have never leaked. Not too shabby for a $150 aluminum radiator that I purchased new on Ebay. Although it is made in China.

Here is a decent photo of the welds found on my old aluminum radiator.

This is the drain cock on the lower-passenger side of all Jeep XJ radiators.

You can see on this radiator that it is a single core. I was surprised to find a large single core instead of the more common 3 core radiators that most jeep guys use. This is the primary reason I switched to the GCI 3 core which is currently installed in my Cherokee.

Stock Lower Radiator Hose

After consulting the guys at Flowkooler I decided to purchase a stock lower radiator hose which included a spring inserted into the hose.

This spring is designed to ensure the hose does not collapse under the suction caused by the high flow water pump.

The spring runs from one side of the hose to the other. Some people claim that all new hoses have a construction internally which stiffen the rubber eliminating the requirement for a spring. I dont care what "people say" If the engineers at Jeep still use them ang the guys at Flowkooler say that I need one then my jeep will have one in order to eliminate any possible causes of my overheating problem.

This is the new hose and FlowKooler water pump. I plan to have this installation completed in the next week.

Some New Parts

This is the flowkooler water pump. It is a counter clock wise (CCW) rotation pump for my inline 6 motor. It is designed to make the biggest difference below 1500 rpm.

This plate that is soldered to the rotor blades on the backside is the only thing I can find that differentiate this water pump from any other pump from autozone or O'Reilly's.

You can see in this photo that the plate is very thin.