Have you checked the coolant level?

 

 

If there are no external leaks from the system, it may be as simple as a stuck thermostat.

 

+1 on that.

Before convincing myself of one theory (worn water pump impeller), I'd highly suggest running some basic diagnostics.

Hook up a cooling system pressure tester. This the thing that looks like a bicycle pump with a gauge, and connects in place of the reservoir cap. You pump up the cooling system to the specified pressure and make sure the pressure holds. You also look for leaks. On my '02 V6, I've noticed cooling system behavior is very finicky if the pressure can't build up to normal. Heater performance drops off, which you've already experienced. What I found was a small leak near the reservoir, but because of the location, mostly air was escaping, with very little coolant loss. The symptoms were low heat and the coolant temp gauge above normal, although no actual overheat. No idea if this is your issue, but it's worth investigating. Some parts stores offer free loaner tools if you don't want to buy a pressure tester.

The other recommendation is to check coolant line temperatures with an infrared thermometer. A cheap one will set you back maybe $15. Keep a close eye on the car's temp gauge, and quickly check the coolant temps before and after the thermostat, and at the radiator inlet, too.

I'd check the serpentine belt tension, too. A slipping belt could cause your symptoms.

Also, when you said the car "started overheating", exactly what happened? Did you see steam venting under the hood? I'm wondering if you only had an indication problem, not an actual overheat. That wouldn't explain the lack of heat, though, but is still something to consider.

 

 

It's perfectly normal for the pipes to feel hard. What is normal pressure, isn't it 13 or 16 psi? That's a lot of pressure. If the pressure exceeded limits, the cap should have vented. Have the cap tested, too, just to be safe.

 

Maybe not common, but certainly not rare. See post #16 in my tale of woe, involving a new thermostat AND a new temperature sensor both bad from stock:

https://www.jaguarforums.com/forum/s...rature-186873/

 

 

 

Only the '99-'02 V6 engines had the CHT sensor. Your V8 has a conventional coolant temperature sensor, only one I think.

Re: "swaying towards a stuck thermostat". Be careful, don't just guess, but take some time for diagnosis. Originally you had yourself convinced the water pump was bad, but now the wind has changed direction and you're leaning towards a thermostat.

I'll admit I'm as guilty as the next guy, as if a part is inexpensive and easy to access, I'll usually just try it first and see what happens. But for something more involved, such as the water pump, I'd want to be positive before digging in too deep. The thermostat? Easier to change, but you'd still get your hands dirty, possibly for no good reason. Before digging in, pick up an inexpensive infrared thermometer and see what is really happening. You still don't know if you've got an actual overheat or just an indication issue. Or as in my recent foibles, a combination of both.

 

Don't forget that engine also has an auxillary coolant pump above the DCCV unit.

 

If you decide to replace the thermostat, you should also change the water outlet as well since they can only endure a finite number of cold/hot cycles before failing. The thermostat 'tower' is also made of plastic, but there is an aluminium version available from the aftermarket suppliers such as S.N.G. Barratt.

The water pump is relatively simple to replace once the water outlet is removed and the hoses out of the way.

 

I hope it's had its (timing chain) tensioners and related parts changed!!

 

Thats a good question if, as the OP believes, it could be the original pump!
Wouldn't think so though...............

 

OP mentioned upgrading the thermostat housing to aluminum. I'm personally leaning towards a failed water pump (broken fins) but could be thermostat related as well.

 

Yep, quite a while ago, Many years ago infact. It was replaced under jags service bulliten. Although this is irrelevant to my heating issue but thanks for th concern

 

This has already been changed sometime last year. My previous one which was plastic was leaking.

 

In a new fresh state of mind, I'll investigate and test the thermostat and see where that leads to. I'll again check the hoses. Nevertheless i'll be replacing the waterpump anyhow for peace of mind

 

May well be before they actually fixed things then

Worth checking.

You won't have a heating issue if the engine is written off...