Cooling System Upgrades
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Is your Jeep 4.0 engine getting hot under the collar?

The Jeep 4.0 engine is designed to run at 195-210*F under normal conditions. Under certain conditions, the coolant temperature can rise to 220*F and this is normal:

1. Very hot weather (>100*F)
2. Idling or crawling in slow traffic with A/C off
3. Scaling long inclines
4. Towing

The engine should NEVER RUN AT ABOVE 220*F if the cooling system is working properly. The coolant temperature is not permitted to rise above 220*F by the cooling fans.
A stock Jeep Cherokee XJ 4.0L engine has two cooling fans. The main fan is a viscous clutch fan that is mounted to an engine-driven idler pulley. When the engine is cold, the fan freewheels and has little cooling effect. When the engine is hot, the clutch engages and the fan spins at a speed proportional to engine speed and keeps the engine cool.
The auxiliary fan is electric. It is triggered by a relay either when the A/C is switched on or when the coolant temperature reaches 220*F (105*C). It switches off again either when the A/C is switched off or when the coolant temperature has fallen back to 200*F (93*C).
The stock thermostat is rated to open at 195*F. This ensures a fast warm up from a cold start. A coolant temperature of 195*F is optimum on a stock engine for emissions control, fuel economy and engine longevity.
Many Jeep 4.0 owners encounter overheating problems particularly in hot summer weather. This is usually due to a defective cooling system component. There are, however, some deficiencies in the normal stock cooling system that can be overcome to ensure that the engine runs in its normal operating temperature range.
Here's an outline of some easy upgrades that can be performed by any competent DIY mechanic with some simple hand tools.
Modine heavy-duty 2-core radiator

The factory plastic 1-core radiator isn't up to the job of properly cooling the 4.0 engine in hot conditions. The factory 2-core heavy duty radiator fitted to vehicles with the optional towing package is much better, and it'll even keep a 4.6L stroker engine cool. The ultimate solution is to fit a Modine heavy-duty 2-core radiator. The Modine radiator is available for both the closed ('87-'90) and open ('91-'00) cooling systems the Jeep Cherokee has used over the years. You need to specify the model, year, engine type and transmission type when ordering.
Flowkooler high-flow water pump/ Robertshaw thermostat

Flowkooler water pumps solve low-speed heating problems by doubling the water flow rate at low RPM and idle. At 3,500 RPM the flow rate returns to normal to prevent too much water flow at high speed.
All pumps are new and not rebuilt and are covered by a lifetime warranty.
I have installed the Flowkooler water pump #1747 in my 4.0 engine. Coolant temperatures during highway cruising fell by 15-20*F. The rate of coolant temperature rise when scaling long inclines was also reduced. I highly recommend this upgrade.
Flowkooler also recommend the Robertshaw balanced high-flow thermostat. The ROBERTSHAW SERIES NO. 330 THERMOSTAT fits the Jeep 4.0 engine. Heat ranges available are: 160*F 180*F 195*F 205*F. The 195* thermostat is recommended for stock engines, and the 180* thermostat is recommended for modified engines with performance engine-control chips to prevent pinging.


Dual electric fan conversion

There are two problems with the stock clutch fan. The first is that when the engine is idling, the engine temperature reaches 220*F after a few minutes and the electric fan is triggered to switch on. Clearly the clutch fan isn't up to the job of keeping the engine cool enough by itself. The second is that when the clutch fan does engage, it makes a lot of noise and imposes a lot of drag on the engine (it weighs a hefty 5lb). This costs horsepower due to the engine driving the fan. Gas mileage is also reduced particularly in conditions where the clutch fan is most likely to engage i.e. city driving.

An easy way to remedy these problems is to replace the clutch fan with an electric fan. I have done this conversion with great success on my Jeep by mounting a 14" High Performance 1500-1800cfm electric fan to the stock clutch fan shroud. For more details, click on the image to the left or go to Dino's dual electric fan conversion.

The alternative fan conversion is the Flexalite 110 electric fan. This article describes how this was done on another 4.0 XJ.