There seems to be quite a few threads on her with heater issues and I seem to have one that is different then others I have read.
I bought my 2008 S-Type 4.2 back in the late fall and had some early issues with the engine not heating up and having no heat to the defrosters unless I revved the engine. This must have been an issue before because there was an invoice from Jaguar in the car for a thermostat change 2 weeks before I bought the car. The fellow I bought it from said Jag told him it just needed a thermostat and they changed it. They also told him there seemed to be little pieces of cork around the thermostat and they had removed them so it would be fine. When I could get no heat when it got a little colder I removed the thermostat my self and found the thermostat laying in pieces inside the housing. The little pieces of cork were the degraded black plastic that holds the thermostat in place so I effectively had no thermostat. As well my car does not have an auxiliary pump to move the coolant through the heater cores so the coolant was taking the path of least resistance which was not through the heaters with the big chunk of plastic missing near the path to the heaters. You will see the new thermostat housing has a lot more material which directs the coolant through the heater cores. I am really surprised that the Jaguar dealer did not know enough to replace that old thermostat housing. As well when I got that fixed it turner out that the Defrost door motor that can be replaced through the glove box was defective. I finally have defrosters for the Canadian winter and all is well.


Old thermostat was so degraded that coolant would not flow through heater cores or even hold the thermostat in place. The brown stuff is what the Jaguar dealer called cork.



Obviously new thermostat housing has more material.

 

I believe the 4.2 engine has an auxiliary coolant pump. Good call on the thermostat housing. Just don't change that piece but the whole housing.

 

The new housing included a new thermostat, temperature sensors and all gaskets. Replaced it all.

 

I thought it did too.

 

It's on the electrical guide, isn't it? Take a look, it even says where it is!

 

I did the same on mine too. I did replace with the metal unit. It was available so why not. That cross over pipe is still plastic but new is better. Congrats on your repair.