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4.0 ZJ Cooling System Upgrade Questions/Discussion

Old 08-29-2018, 10:45 PM
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Hatsuwr's Avatar
Join Date: Aug 2016
Location: Maryland
Posts: 205
Year: 1996
Model: Grand Cherokee
Engine: 4.0 I6
Default 4.0 ZJ Cooling System Upgrade Questions/Discussion

Hi everyone, just been thinking about going through my cooling system and potentially making some upgrades.

To address the 'If it ain't broke...' concerns - I bought this thing 20 years old and wouldn't mind doing some preemptive repairs. Plus, it's fun.

That said, here's what I've picked up. Not really looking for anything specific here, just discussing the cooling system and how to potentially improve it.


Coolant has four jobs that I can think of: transfer heat well, not freeze, not boil, and prevent corrosion. It's hard to go wrong with any popular recommended coolant, but I still want to look into this a bit more. I've started a spreadsheet comparing the components of many popular coolants here:
Let me know if you want to see any added to it, or have suggestions/input to improve it. I've emailed all the manufacturers to get better info since that is all pulled from SDS's.

Corrosion protection completely depends upon the materials that the coolant contacts. Obviously we have the cast iron block, aluminum radiator, and stainless + brass thermostat. Anyone know more about what other components are made of?

Assuming an ethylene glycol base (which transfers heat better than the less toxic propylene glycol), freezing/boiling will depend on the antifreeze to water ratio. A higher ratio of antifreeze will give better protection from extreme temperatures and corrosion, but will reduce heat transfer rate. 1:1 is always a safe bet, but I think an ideal would be as close to the maximum corrosion inhibition ratio as possible while avoiding temperature-related issues. That ratio will depend upon the specific antifreeze used.

The only coolant that really stands out (not necessarily in a good or bad way) from the rest seems to be Evans Waterless Coolant. Still reading up on that. Any experiences?

Coolant Additives

I have generally heard these are not recommended, but I will be looking more into it, and doing a comparison like the antifreezes above. A couple off the top of my head are Purple Ice and Water Wetter. Any suggestions for ones to compare?


I've heard a lot of people upgrading from the OE radiator, but none have really stood out to me. My only personal experience is with an OE style Spectra (CU1396) which has been great. Any recommendations?

Properly functioning radiator cap is more important than a lot of people give it credit for. The cap needs to pressurize the system, vent gas, and draw in coolant from the reservoir when the system cools. I have a Stant 10331 with the safety level which, besides the safety aspect, is useful for manually purging air from the system.

Water Pump

This won't be a popular opinion, but I really want to install an electric water pump. My main motivation is reliability. The electric pump itself would be less reliable than the belt driven-one, but the belt drives 7 different pulleys. Any one of them fails and you're either getting towed or slowly limping while risking heat damage. I'm fine with losing power steering, alternator, A/C, and the fan, but not the water pump.

Unfortunately, I've never even heard of this on a ZJ. I have an ACDelco 252-279 OE style pump in mine, with a Fel-Pro 35582 gasket that I trust a bit more than the included one.

I've heard many concerns that high-flow pumps will actually cause a decrease in performance by not allowing the coolant to proper contact heating/cooling surfaces. While the general principle is possible, I don't think it comes into play at the rates we are looking at. Higher flow pumps should increase cooling capacity. However, if temperatures aren't rising much above the thermostat setting anyway, a high-flow pump that uses more engine power is essentially an expensive mpg drain.


A low temperature thermostat won't help with overheating. A 160 and 195 will both fully open before overheating occurs. A lower temperature thermostat can potentially disrupt the computer's engine management, reduce contaminant burn off, increase engine wear, and reduce fuel economy. A thermostat's primary purpose is not to help engine cooling, but to keep it from cooling too much below proper operating temperature.

I'd be surprised if there wasn't something that benefits from a slightly lower thermostat temperature and would be curious if you all have thoughts on that. As it is, I have a Stant 45359 195 that works well. Fel-Pro 35024 gasket fits the cover.


Stock is probably going to always be the most reliable here.

Any upgrades I've seen have always just been electric. Plenty of used conversions out there. The Flex-a-lite 180 Black Magic seems to be one of the best aftermarket options from what I've read.

To really get all the benefit of an electric fan though, I'd want to see variable speed and good response to coolant temperatures. I haven't been able to find much about more advanced controls for electric fans. Anyone have experience here?


Not much to say here. I'd like the cap to seal a bit better to keep dust from entering, but don't want to mess with venting and coolant return. It's a very minor point, but I might try securing some filter foam to the cap.

Coolant Filter

I have a post on this somewhere, but I installed a Luber-Finer LFW4685 coolant filter on my heater hose. I figure it will reduce wear (especially on the pump), catch dust that the reservoir lets in, and reduce silicate deposits from the coolant. Next time I do anything to the cooling system I'll cut the filter open and see what it caught!

I drilled a small bypass hole into the housing since I found I wasn't getting as much flow to the heater core as I wanted without it.

I might also throw a small magnet in the system somewhere, like there is in the transmission oil pan. I don't expect it to catch much of anything in the cooling system, but am curious and have too many magnets.


Not a whole lot to say here either... I replaced my heater hoses with 5/8" silicone tubing since I had to cut my old ones off to remove them.

Also replaced that ridiculous bent metal heater return tube with a straight 3/8" MNPT x 5/8" barb fitting that can be removed without having to remove the water pump.

Replaced the worm gear clamps with constant tension clamps.

Last edited by Hatsuwr; 08-29-2018 at 10:48 PM.
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