Running hot while moving, fine at idle??
 

So recently I have noticed an over heating problem. The vehicle is a 91 xj, 4.0, csf 3 row rad, new water pump, thermostat, dirtbound triple electric fans, Dakota digital fan controller. Now onto the issue.
I live in vegas so daytime Temps are over 100 everyday. Since I've owned the jeep it would always start running hot with ac on so no big deal. Recently I was cruising around 45mph and it started going over the halfway point so I payed attention and it just kept creeping up. At this point it had a stock aluminum with plastic tank radiator that was 3 years old, along with water pump and thermostat that was replaced three years ago when the new motor went in. BUT, it has had the electric fan setup going on the second summer so I know that's not the issue. I replaced the water pump and t Stat right away and the old ones seemed to be okay actually. I let it run with the AC all the way up for a good 20 minutes in the middle of the day and it never got over the halfway mark.
From there I took it for a test drive, ac still on, for about 20 15 minutes where it then began to run hot again. Not as bad mind you. So, I pulled the trigger and bought a 3 row csf radiator and installed it. Again started it up while I filled with coolant had the AC on and wouldn't go over halfway. The last couple errands I have driven it and it does perfectly fine stopped, doesn't go over 210, but as soon as I start cruising along it starts heating up.
This brings me to my new dillema. When hot I opened the hood and manually did the throttle while checking to see if lower radiator hose would collapse and it never budged.
I did however in the spring replace my old "modified" catalytic converter with a performance curve one. While under the jeep today I noticed the cat is discolored as a piece of steel would be after it had heat on it. It started as a galvanized color. I have a 99 intake, APN header, and flowmaster cat back. Is it possible the cat is restrictions the exhaust flow and causing the motor to heat up while under power (more rpms than idle)?
Sorry so much reading to do but I wanted to thoroughly explain the situation

Cats normally run in the neighborhood of eight hundred to a thousand degrees or so. I think that might discolor it a bit.

Oh okay. Never knew that! So where should I look next?

I would make sure that all of the fans are pulling air through the radiator. Wile moving if the fans are rotating the wrong way they will pushing the air against the air coming through the radiator wile driving. I have done it a long time ago with a mechanical fan. I have also read some where that the 3 row radiators could have a tendency to heat soak wile the air is moving through it. The first row does a lot of the work and the other 2 get hot air dumped on them reducing their efficiency. Essentially turning the 3 row into a single row with a smaller core then stock. The standard single row radiator core is 1.25 thick all of which is doing the cooling. Now make the radiator thicker and the tubes smaller and the efficiency will go down. I found a picture of a standard OEM radiator next to a 2 row radiator from an XJ.

https://cimg5.ibsrv.net/gimg/www.che...4088c854b2.jpg

here is a link to the site I found it on. Hope this helps.

http://www.xjtalk.com/showthread.php?t=7847

I'll check it out! Also the fans haven't been touched since they were put in so I don't think they would reverse. But good knowledge thank you

More rows = better cooling is FALSE.

There is much more to a radiator's design than just the number of rows.


It really sounds to me like it's an airflow problem. One reason for a fan clutch on a mechanical fan is to allow the fan to "get out of the way" of the airflow at higher speeds.

I'm wondering if something like that is going on with our electric fans, that maybe they aren't allowing full airflow when you are moving.

Why is it just becoming an issue 2 years into daily driving the jeep? I put them in before last summer. Wheeled and daily drove it everyday since then and just since the heat started again and the only thing different is a catalytic converter.

Attachment 290018
Here is what it looks like now. Haha pay no attention to the crap welds

I understand what Blue Ridge is saying. if the shroud that has the three fans mounted on it does not allow for faster air to flow through the radiator when moving, then the air would become stagnant and over heated inside the shroud with no were to go. Take a picture of the shroud. You may need to add some hinged flaps that open when diving and close when sitting idle. If the cat was plugged you would notice a significant power loss.

Hmmm. Good point.

It may be that your cat is somehow managing to cause the engine to run hotter when you are at speed.

Where is your O2 sensor? I don't see one, but then, I don't know where it should be on a 91. Mine (97) is at the rear of the cat.

Mine I right after the header, or even on the header down pipe. Can't remember exactly but somewhere in that area

Spitballing a few questions in seemingly random directions:

I'm not sure I understand. Do you still have a mechanical fan with a viscous fan clutch turning on the engine, or did you replace it with an electric fan mod?

If you still have the mechanical fan, do you still have the fan shroud in place?

Did you flush out the system thoroughly when you replaced the water pump?

If you have an automatic, how's your transmission behaving? Do you have a transmission cooler?

Have you looked at the plugs recently? Any chance it's running lean?

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I have replaced all of the cooling system including the coolant temperature sensor. Here is the readings from an infrared temp gauge

Attachment 290166



Attachment 290168



Attachment 290167

These are the readings I got while that is what the interior gauge says...

those readings are normal.

200 degrees for a 4.0 is normal and its happy place really.Its a cast iron engine they all run hot to use but its normal to them.