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Old July 15th, 2019, 15:04
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NXJ4CB NXJ4CB is offline
NAXJA Forum User
Join Date: Oct 2007
Location: San Diego
Posts: 12
Overheating solved


Bear with me, I rarely post. I usually just read threads for the wealth of knowledge here.

I believe I have stumbled across the cause of our engines running hot. Like so many others I've tried everything. Here is my setup and after I'll explain my theory:

2000 XJ 4.0, 33's, 4.10 gears

CSF 3 row radiator.
Flowkooler Highflow waterpump.
Hesco High Flow thermostat housing.
180 Deg. Stant superstat thermostat (drilled with 3/16 hole).

13 inch Spal pusher fan in front of the condenser passenger side.
Ford Explorer aux trans cooler in front of the condenser driver side plumbed in series after the radiator.

I, like many others have gone down this rabbit hole for years and was discussing it yet again with a co-worker who figured it out. He determined that with this setup that the majority of coolant is passing through my heater core and not the radiator. After calling him insane, I tested this theory by installing a valve in the thermostat housing outlet to heater core hose and shut it off.

My temps immediately dropped from 220 at idle with both fans running to 185. So, in effect i blocked the heater core and forced all of the fluid to flow through the radiator.

Today it's 96 degrees and I drove it around town in traffic with the temps hovering between 180 and 190 only because I was running errands. After each restart the temps were high, above 210 like we are used to with heat soak but driving it for a minute or so the temps would go right back to 180 0r 185. Temp monitoring done with a scan tool.

I believe the cause for many of us who opted for "high performance" parts and were left scratching our heads are those very parts. Either the design of the water pump forces too much fluid through the heater, the thermostat outlet to the radiator is too large now or the three row radiator has too much resistance to flow.

In any case my overheating is solved. I hope this helps someone if not more. This theory can be tested on your rig with a simple valve in the closed position. Also, when I re open the valve it goes right back to overheating. The major clue was, in desperation I turned on the heat to max and my temps were fine after that. I wondered how could it be that my heater core was better at cooling than my friggin radiator?

Thanks for reading,

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