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TJ cooling: A discussion II


co-de·pen·dent or co·de·pen·dent (kd-pndnt) adj.

1. Mutually dependent.

2. n.  One who is co-dependent or in a co-dependent relationship.

If there is anywhere in an auto that the word codependent can be used it is in the cooling system.

Cooling system factors:


Number of rows in radiator

Fan shroud


Number of fan blades

Pitch of fan blades

Ability of fan to pull air under heavy loads


Computer management system and how it responds to various temperatures


Transmission oil cooler in radiator

Speed at which air flows thru radiator, CFM

And I suspect there are a few others I have overlooked. [NOTE: I am not convinced that winches to include the new Warn PowerPlant have any impact upon the cooling of the vehicle]  

The entire system is codependent on each and every other component, alter one too much and the result is a failure to perform properly due to it being out of balance with itself.  Especially the fan and its linkages. Go to a multi-row radiator and expect for the fan to generally work harder in order to pull the air thru. More rows, greater resistance to air movement. Move too much air thru the radiator and there will not be enough heat transfer to effectively cool.  The air must have the proper amount of latency across the fins/rows. Move too slowly and the air heats too much and cannot accept any more heat transfer from the remaining rows it has to travel over.

1 row Vs 2 row Vs 3 row etc:

Simple question, complex answer.

Multiple rows bring with them their own bag of worms. The more rows the more turbulence in the air flow across them. This turbulence reduces the ability of the air to flow and:

1) extract heat from the rows

2) flow across the rows at a predetermined rate that makes calculating heat transfer and the resulting cooling predictable

3) it takes more fan "pull" [aka CFM]to drag the air across multiple rows

Cooling systems are very codependent on each individual component.

It is my suggestion that you examine your current system design and base your purchase upon that. That said your Jeep currently uses a single row rad design. And, you have modded it by adding the 7-blade fan/clutch setup.

What the 7-blade fan does is increase the CFM of air the fan will pull thus improving its cooling ability. [NOTE: This is NOT always the case. All too often more CFM leads to less cooling rather than more/better cooling. Why? In order for cooling to take place the air must have a degree of "latency" as it travels across the row(s). This latency is when the heat transfer takes place. Move the air too fast and it does not cool. Same as removing a thermostat. Take it out and most likely you will overheat, due to the fluid moving too quick thru the system.]

Like air slowing down as it passes thru the multiple rows, so does the coolant.  As it slows down or increases it latency then the air needs to speed up in order to extract the additional heat that this latency generates.  More rows, slower coolant flow, more air CFM required to cool.

In all likelihood you could go multiple rows if you have a fan that will pull more CFMs and probably overcome the turbulence factor.

On the other hand I see no reason to do so. The OEM rad works fine and with the addition of the fan/clutch upgrade will cool your rig with AC on rock crawling up a wash.


The cooling system is codependent and must ALWAYS remain in balance with itself or it fails to work.  Understanding this is why I chose to stay with a OEM cooling system.

When it comes down to it, it is just a question of cooling system balance.

Purging, 'burping' the air out of the radiator-heater-engine system:

The problem occurs when you get an air pocket in the system and as a result it will continually overheat.   

To that end here is how I learned to do it and know for a fact it works.  I grew up on a ranch and we had ranch trucks, tractors and various stuff that had engines and radiators.  Some of them would have the radiator drained as they sat up all winter.  This is how we did it:

1)  Remove the radiator cap on the radiator and inspect it for corrosion, proper functioning and the rubber seal.  Replace if needed, but leave off to purge.

2)  Start you engine and let it idle [caution:  do this with the engine at cold]

3)  Keep watching the radiator fill and add fluid as the fluid level drops.  'rev' the engine slowly up to about 2k rpm and let it slowly come back down to idle.  Do it too fast and it will cause the fluid to ebb and flow too quickly and overflow.  Also have the heater turned on max while doing this.

4)  Watching the radiator fill and the temp gage let the engine come to normal operating temp and insure by revving it a few times that all the air bubbles in the engine, hoses and heater have dissipated.  Generally revving the engine will purge all the air.

5)  Leaving the radiator cap off, now fill your plastic radiator reservoir aka 'thumper keg' with radiator fluid to the full mark.

6)  Replace radiator cap, turn off heater (if its summer) and test drive doing some high speed freeway driving.  Check the thumper keg when you return, add fluid if needed and check fluid again after the engine has had a complete cool down, add fluid if needed.

Flushing the radiator:

I always hesitate to recommend this due to so often all that happens is that you clog up your radiator more than it was.  That said if you chose to do it and take the attitude that 'I got nothing to lose' then you might win.  This then begs the question as to 'how do I flush it PROPERLY and reduce the chances of just clogging my radiator?'

Friend of mine, "urbex' posted this on a forum the other day and if anyone has the steps down then its him and I strongly recommend you follow his procedure:

"I figured at this point the radiator needed replacing, so I had nothing to lose by going full force on it. I pulled the radiator, and spent about 30 minutes with the garden hose rinsing the insides out. I rinsed it from both ends, angling the radiator every which which way when draining, flipped it over, etc. Basically made sure the flow caught every nook and cranny, and made sure that the water coming carried the crud with it. I kept this up until until no more crud came out of the radiator.

After, I laid the radiator flat on the driveway, with the outlets pointing up, and CAREFULLY filled the radiator with straight from the jug muriatic acid, and let it soak for 15 minutes. Drained it, rinsed it until the water ran clear and no more crud came out, then refilled the radiator with the remainder of the gallon of acid, and CAREFULLY swished it around for about 10 more minutes, and repeated the rinse until clear. Ran a baking soda/water mixture through the radiator several times to neutralize any remaining acid, and again rinsed real well.

Put everything back together, filled the system with an 80/20 mix of water/coolant, and it's been fine since. Runs a bit under 210 now on the freeway all day long.

I started pricing out new radiators, and found that I'd be looking at approx $200 for a standard replacement, and $300+ for a HD replacement. Considering I got the acid at Home Depot @$9 for two gallons, it was a cheap last ditch effort. I did a lot of research on it before I did it, and found good tidbits like how you absolutely can NOT do this on an aluminum radiator, as the acid eats away the aluminum. Likewise, you also can't run the acid through engines that have aluminum heads, blocks, intakes, water pumps, etc.

I also inspected my radiator for ANY signs of rot or corrosion on the outside, and found none. Had there been fin or cooling tube rot, or obvious corrosion on the outside, this method may very well have caused a leak in the radiator where there was none previously.

But at the end of it all, I was only out about $10, and a few hours of my time to get a properly working cooling system again."

Please note: Acid is considered hazardous materials, and thus you're not supposed to just dump the stuff down the driveway, down the drain, etc. It needs to be neutralized first (with the baking soda). The preferred way is to collect the acid drain in a large bucket or container, add baking soda to it until it stops fizzing.  Once neutralized, it's perfectly OK to dump it/drain it, whatever.  You'll also want to do this in a very well ventilated area, preferably outside with fans behind you. The fumes are very noxious.

Thanks urbex for the GREAT advice!

Click below for further reading, writeups and pics:

OEM 7 Blade Fan/HD Clutch Install>

Cold Air Fab Parts I/II

TJ Hood Vents Install

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