I'm no engineer, obviously, but maybe, just maybe, when the coolant spends less time in the engine, it can spend less time in the radiator, if that makes sense. And that could mean the coolant isn't as hot when going through the radiator so it doesn't need to spend as much time in the radiator. Or everything I said could be a bunch of nonsense. But then again, wouldn't the thermostat need to be a high flow thermostat for this theory to actually work? The stock 195 thermostat is probably made for a stock water pump. However you put your finger over the nozzle of a hose and it the water pressure increases and the water travels further, right? So the coolant in the engine is traveling at a faster rate, therefor extracting heat at a faster rate. But I think the rest of the cooling system, at least the radiator, would need to also be a performance radiator (or anything better than a stock radiator) in order to use a faster flowing water pump to its fullest extent, or maybe not?

Your water hose explaination makes for a good example.The water from your finger over the hose outlet forward is moving faster due to your finger restricting flow in the hose creating the pressure you feel against your finger.. So actually the water in the hose has been slowed down. We used different size restrictor plates in our small block late model to control flow rate and thereby controlling coolant temp in the engine.

Interesting conversation.

It could also be if the coolant spends less time in the engine, it has less time to absorb the engine's heat.

Yeah with the garden hose analogy, the overall flow rate (gpm) will be reduced but the overall pressure head in the hose will increase because of the 'finger' restriction.
The increased water velocity at the 'finger' restriction is representative of the increased pressure in the entire length of the hose.
Of course an active running pumping source is required for all this to happen.

I'd think the hot uhh i mean cool set up would be;
the stock pump,
higher flow thermostat with maybe greater 'close' reactive lag (if it exists),
larger or more efficient radiator,
and of course a fan clutch with a higher % thermal lockup spec like the now infamous 272310.

Were an equation short of turning this conversation into a Bill Nye episode!

I read under the the review of the GMB high flow water pump that it helps at lower Rpms while off roading, which makes sense since cooling tends to be an issue at lower speeds so getting the coolant to the radiator faster makes sense when paired with a 3 core radiator. What we are talking about (finger on the hose) might be for higher rpm. So people who don't do a lot of off roading but do a lot of highway driving would have the best efficiency of cooling with a stock water pump, while the people who do less highway driving and more off road slow speed driving can benefit from a high flow water pump?

Just PM'd Bill Nye. He says we shouldn't be driving off road. Something about ruining fossils or some dribble.

Anyways I believe the most inefficient part of our cooling system is the radiator. There's just not enough room for a more efficient one. Room for a wider one for more cooling area and/or a thicker one for either one larger passage or two or more of the existing size ones. Only thing to do is get more air flow through the existing one.

Tell you what though. I'd say if your OEM cooling system is at it's optimum working condition that 99+% of the time it'll do just fine. The less than 1% would be extreme conditions and then I don't believe your engine would be in any danger of damage. JMHO.

I'll have to toss the water pump selection issue around some before deciding.

The more research I do the more convinced I am that a stock pump is ideal unless you're racing and need more hp, then go for a high flow water pump. In a nutshell, any stock 4.0 should do just fine with a stock water pump, and a good one, along with a 195 stat and stock radiator and trans cooler.

Other than the 272310 mystery fan and a B&M trans cooler my system is stock. I don't count my elimination of the HCV as improving cooling. So now in the hottest weather I can leave it idle for I don't know how long with the A/C on and I can creep around our farm roads and skid trails with no issues. Working your way over rocks and similar situations may make a difference in some way I don't know.

For what they had to work with I think the engineers did a good job .

You might have a point there! Its the idle scenario where myself and it seems a lot of others have cooling problems.

By the way who is this Bill Nye guy?

I disagree that the engineers did a good job at least on the 2k and 2001s.
Seems they did everything possible to get those 4.0s to run hotter for the EPA tree-hugging bureaucrats from the previous regime.

Speaking of tranny coolers, i think that has a negative effect on cooling also as the idle air flow from the mech. fan has that one big and hot obstruction in the way for coolant radiator efficiency.

Anyhow planning on installing the 272310 clutch tomorrow barring no more sudden catastrophes pop up in the meantime.
Anxious to see if really will make a difference.

Being a self-proclaimed pessimist, i seldom get disappointed when things don't work out as planned, just PO'd.

Mounted to the floorboards of a TJ
Under a Ford Mustang
Under a wrangler
Haha I grew up watching Bill Nye's science shows.

Anywhom, the trans cooler doesn't NEED to be placed right behind the grill, just an area that has air flowing by it. Hell, if you get fancy enough, you could make some ducting from the front fascia to wherever you find space for the trans cooler. Haven't seen it done, but I bet it's possible if you're that worried about adding more heat producing things in front of your radiator. Or just do what is shown in the pictures and install it under the Jeep somewhere with an electric fan on it. Maybe tuck it out of the way in the trans tunnel with that fan since a lot of air moves through the trans tunnel.

Ahh, Bill Nye the science guy! Still never heard of him... til now.

The biggest obstruction in front of the engine coolant radiator is the AC radiator and right in the path of the mech. fan air flow.

Maybe one day when solid state thermoelectric chip efficiencies get high enough that technology will replace the clunky freon/pump based AC systems we have today.
No moving parts, a large heat sink to dissipate the cabin and any device heat and a higher output alternator would be needed to power that type of solid state AC system.

But i digress, that won't help me today with my XJ's idle cooling issues.

There is going to be some trade off, but I believe it's minimal if you don't go overboard with the size of the cooler. I have a Hayden 677 that is recommended for vehicles up to 14000 GVW, and it only covers about 1/4 the area in front of the main fan shroud. There's been no noticeable increase in water temperature during operation or at idle, even when pulling a 2800 lb trailer, but I have seen a reduction in transmission temperature. Keep in mind that, though it is obstructing some air and increasing temperature of the air pulled through the radiator, you are also somewhat reducing the heat load in the factory heat exchanger inside the radiator by lowering overall transmission temperatures with the aux cooler. That's the trade off and, for my uses, it seems to have been worth it to reduce the transmission temperature. If I had idle cooling issues though, I probably wouldn't add one unless it was the remote type with its own fan.

Subbed because xj idles too damn hot

Looking back at my post where i initially mentioned the tranny cooler, my wording wasn't exactly what i meant. Kind of confused the AC and tranny coolers, by bad.

I meant to say the AC radiator which is a much larger form factor than the tranny cooler is the main obstruction in front of the engine radiator.
And if the AC is on, its a hot obstruction at idle.
The perfect storm for cooling issues at idle.

I about half way convinced myself the answer is more mech. fan generated air flow at idle considering there is absolutely no cooling issues once my XJ is rolling AC on or not.
If the 272310 don't work out then its; all efans to replace the mech. fan, better more tight fitting shroud or even a flex fan as a last resort?

I installed a flex fan on my old '64 Plymouth's souped up 413 engine which had minor cooling issues after the engine re-build. It worked as intended but that old Plymouth was a 4 speed manual on the floor with no AC.

I hate it when the coolant gets hot enough for the PCM to turn on the efan.
What concerns me is right before that happens under prolonged idle conditions the engine starts stumbling.
The reason i always try to beat the PCM with the efan over ride switch soon as i see the temp creeping up to or at the 210 mark.

Edit:
Forgot to mention my XJ came stock with a tranny cooler.
Its mounted in front of the AC radiator.
I think it was part of the UpCountry package.