1999 4.0.
I'm looking at my XJ, along with other XJ's and it shows the upper heater hose (that goes straight into the thermostat housing) going into the top hole on the heater core, and the lower heater hose (the one that goes into a black tube which is screwed into the thermostat) is going into the bottom hole on the heater core. I'm looking at diagrams online and they show them being switched.





So, which way is the right way to route these hoses? Does it matter which way the coolant flows into the heater core?

 

Aren't both of those diagrams the same? They both show the lower hose at the firewall as the supply line.

 

The hoses are two different diameters so you should be able to tell the correct routing from that.

 

the diagrams are the same but my engine bay and other engine bays I've seen online don't match it. My biggest concern is plugging into the heater core opposite the direction of flow.

 

Nothing bad will happen if the flow is reversed, however the different hose sizes will make it obvious if you try to hook them up backwards.

 

The upper hose on the heater core should be connected to the water pump inlet. Not totally sure where your confusion comes from, both those diagrams show this being the case. Also, I just overhauled the cooling system on my '96 this past weekend and that's how it hooked up (2nd diagram looks like it directly from my manual). Drove 400 miles since, everything works.

 

So glad I found this in a Google search! I am about to do a radiator/thermostat replacement. When I do the flush, if I get clean water right away after getting the coolant out, is a chemical flush advised?

And here is a picture of my heater valve. Top line from firewall goes to bottom of valve, lower line from firewall goes to upper valve fitting.


Please ignore the yellow paint pen on the oil related parts. Seemed like a good idea in theory that did not survive contact with reality.