Big radiators aren’t cheap so I’d rather not spend $400-$700 just to see if my problem gets resolved. I do see the radiator works, but just not well enough for my setup. But how do we know that the radiator is faulty vs simply inadequate? Measuring upper/lower hose temperatures shows a 20° drop. That may not be enough, but is that faulty or simply all these short radiators can do?

 

Please let me know if you find a fan that provides better air flow. I currently don’t have any complaints about the fan shroud as the fan sits in quite a ways with the thicker radiator.

 

Everyone has an infatuation with "big radiators", but I'm not at all convinced that they do any good in an XJ. With the "natural convection" oriented cooling system we have (water pump is a fan), there comes a point where a larger radiator is detrimental. Fast moving water moves heat a whole lot better than slow moving water. The last thing you want is for the water to move so slow that it becomes laminar. Very little heat transfer because there is very little mixing. The faster the fluid is flowing, the more turbulent the flow, the more mixing that occurs, the more heat that is exchanged...

The ability for air to flow through the radiator is another thing to consider. More rows equals more restriction. The XJ body doesn't seem like the best design for channeling air into the radiator, and since the splash guard is missing on most of our XJs, with it being lifted, I suspect there's a point where flow through a multi-core radiator is actually reduced because it's easier for the air to flow over/under rather than thru the radiator.

That's a whole lot to say that I've had great luck with a cheap, parts store "2-row equivalent" radiator. I've pulled a ~3500# pontoon boat up and over a couple PA mountains many times in 95+F heat and while the transmission does get a bit warm by the time I make it to he top of the driveway, the engine coolant temp stops rising once the electric fan kicks on. If I was a betting man, I'd bet you that a $100 parts store hunk of junk will fix your issue...

 

I did remove the winch back in 6/2022 and it made no difference. What leads you to think it’s the radiator so we’re not shotgunning?[/QUOTE]

I have never seen any one do such a thorough job testing a cooling system as you. I believe sediment from your 4.0 has settled into your radiator before you installed your stroker and now with the higher demand for cooling the stroker ,the radiator can't keep up.

 

I have a Champion 2 row radiator with the 1998 4cly Heavy Duty fan clutch and it does cool good but if it is over 100 and I'm pushing it off road it will still get to about 225 then if i put it in neutral and bring the RPM up to about 2000 it will cool right down , so if I just had a little more air flow I think it would fix that . I have been looking for a better belt driven fan , they designed the fan in the XJ about the early 80's and the fan they make now are much better at pulling air and are lighter . also the fan shroud should caver at lest haft the fan blade so it can pull more air though the radiator .

 

Interesting points and there seems to be a big debate on the interwebs of whether the coolant should flow faster (high flow water pumps, tstats, housings) or slower to let the coolant absorb more heat from the engine and release more heat in the radiator. Your statement about turbulent flow dissipating heat better is something I haven't heard before and need to research to better understand. Thanks for the information. I've also come across this video which makes mention that too thick of a radiator can be detrimental if you don't have enough airflow through it. Given how poorly air does flow in our typical applications (lifted XJ), strengthens your point about sticking with a thinner radiator.


Thanks for the comment and your view. I always want to know why something works or does not work and the effect of each change. Having semi-controlled tests helps provide that data. When the stroker was installed, I did run a bunch of distilled water through the radiator to get as much junk out. It's possible there is still some left -- mostly dirt. After one dusty wheeling trip post-install I did notice dirt sentiment at the bottom of the reservoir and filler neck and did see dirt in the coolant when drained during one of the many times in my tests. I was thinking about putting a fuel filter inline between the radiator and reservoir to help keep the dirt from cycling through the radiator, but several folks have tried it with poor results (filter disintegration issues).

 

There has been a lot of talk here about "speeding up the water flow." Keep in mind, the water needs to remain in the radiator long enough for heat transfer to the atmosphere. It seems odd that a 20% increase in engine displacement has resulted in so many cooling problems. I guess maybe our cooling systems are marginal when new? There does seem to be a lot of discussion about overheating problems on the web. Thankfully I haven't experienced that, yet...................Outstanding testing/documentation! As an old engineer friend of mine always told me, opinions have little value, only the facts can demonstrate truth.............

 

That video is true if your radiator is to thick the last row will not cool the is all ready hot I only have a 2 row each tube is .75 thick and the core is 1.75 thick . I have noticed that if I have the hood open it does cool down but I don't want to cut the hood . The Radiator is to short and long if you could make the radiator go all the way down to the botton of the frame it would have a much larger face and the a one row would be good but to do that you would have to cut pockets in the frame and then it would be weaken the front bumper mounts and you would have to get a custom radiator and I have done that on other cars I had , once I did that they never ran hot even when at Bull Head city in the summer and the temp was over 100 and A/C on . On the XJ it looks like the do not get good air flow that's way people cut louvers in the hood . but if you up the air flow with a better fan that would also fix it . If I had a shop to work at I would take apart the front end and try cutting the frame to make the pockets for a larger radiator but the steering box is also in the way where I work now I am not aloud to work on my cars there and my home is a condo so I can only do small work .

 

There is zero basis for this in physics. It is completely and entirely an internet myth and a very wrong one at that. You will not find a degreed Mechanical Engineer anywhere on this planet that will agree with the statement.

As stated above, cooling is maximized by increasing the flow rate, causing the flow to be more turbulent and thus more efficient. The ABSOLUTE LAST THING you ever want is for the fluid to move slowly, allowing a thick boundary layer to form.

Edit: I'll add that it's my understanding that this myth began with flat head fords, or something, where installing a washer into the upper radiator hose lowered the engine temperature by "slowing down the coolant". Actually, it's performing the same function as the thermostat is in the Jeep 4.0; providing a restriction to allow additional pressure to build in the engine.

 

Thanks for all of the input and information sharing. To summarize the input thus far:

@awg suggests running the test with the heater on high as that could suggest an issue with the radiator.

@nowhereman98 thinks I have too much sediment in the existing radiator over the last six years thereby diminishing the efficiency of the radiator.

@cruiser54 suggests testing the radiator cap to ensure it's indeed holding 16psi.

@tech thinks it's the fan setup and not enough airflow through the radiator.

@Jim Malcolm suggests the lack of thermostat is detrimental to heat transfer (creates laminar flow instead of turbulent flow) and thinks the Mopar tow radiator is too thick and a thinner single row/core will provide the necessary cooling.


Technically in my last test with the HD fan clutch, I introduced two changes -- the fan clutch and the lack of thermostat. Jim suggests the lack of thermostat is also detrimental to heat transfer. In the next day or two, I plan to test the radiator cap, flush the Supercool out, put the OEM 195º thermostat back in, and put Prestone 50/50 antifreeze/coolant + WaterWetter in (that coolant seems to have netted better results than the Supercool). Then I'll conduct two tests -- one with A/C in this configuration and one with the heater. The next steps will be determined after seeing the test results and weather forecast as Summer is coming to a close.

 

Thanks Jim for the passionate defense of your position. So, it is your opinion that accelerating the water flow is better than allowing the required transfer time for the heat load to be transferred from the engine to the coolant and then to the atmosphere? If the convection time is of little consequence, then why not just run the water through a large pipe to speed it up even more? I don't think I ever stated that the water should be run slowly, just that one needs to account for the designed period that the water needs to flow through the radiator for proper heat transfer.
I understand and agree with your position of pressurizing the cooling system to minimize disturbed contact areas between the coolant and the heated surface. I have seen engines operated without a thermostat overheat badly. This could be coolant contact failure, or it could be coolant flowing too fast to allow for transfer time. Even an open thermostat provides some flow restriction to keep the coolant "pressurized." I didn't have the requisite instrumentation to verify what the actual problem was, but I've seen it more than once. I have seen even more disastrous results in HD diesels with flexing cylinder liners getting eroded through from coolant not maintaining constant contact with the liners. CAT has a special additive that prevents this aeration.
My point in all of this is, that the OEM engineers spend a lot of time designing theses systems, so before we do any modifications we should be able to validate/demonstrate how the tested change actually improves the system. On a gasoline engine 60-80% of the thermal load generated is not used to create operating power, but is actually transferred to the cooling system/atmosphere. That is why a Hyundai radiator will not work in a Class VIII truck. We had the same issues in designing coolers for our marine engines in the Navy. That was actually easier, as we had an entire ocean to utilize! Even at 120 GPM, we still had to apply the correct size coolers to remove the generated heat load.
Not trying to start a pissing contest, but I tend to get a little concerned when folks speak in definitives that allow no room for other solutions. I wish engineering problems were solved this easily, but in 55 years of wrenching, it never seems to be so easily solved.

 

Man, WillyXJ, I wish you could learn how to organize your thoughts!! LOL.

Begin with the cheap stuff first.

 

Note to self, don't make posts to the internet at 12:40 am while waiting for pain pills to kick back in so I can go back to sleep. Being in no better spot now than I was then, .... There's a lot of good info out there debunking the bad; one just needs to have the desire...

 

caps are cheap, I did have that problem once

stock cooling systems are cheap and easy to fit, and should work, I would be inclined to fit a stock rad, WP and thermostat

The electric fan should cut in and keep the temp below 225C, when mine idles or works hard in summer thats the way it operates

my previous XJ would sometimes overheat

maybe the engine block passages get somewhat blocked with dirt & scale with age, like on other old engines I have seen

I have the opposite problem atm, my '96 XJ runs too cool it never gets above 180F with new stock rad, but its just gone winter here, ambient temps still in the 60s, but I am suspecting a failed thermostat. It uses more fuel running this cool

 

You can try putting spacers on the hood lifting up the rear of the hood and see if it helps , it is cheap to try and see if the added air flow helps ,