Hi all,

I've been wrestling with my XJ8 and heat for a while now. It's super cold outside, and finally warmed up enough (30s-40s F) to look into this some more. I took it into a shop and they've checked it out and tried bleeding the cooling system, 'flushing' the system, and evidently backflushed the core but I'm not seeing results...

How strong is the heat supposed to be on an XJ8 in cold climates? I'm expecting this thing to at least get to the point where I want to turn the heat down...but I've never felt that - only the need to turn the heat UP more. I have it on Hi and if I'm idling in the driveway it can start to get pretty warm... and if I'm on the highway it works fairly well.

But otherwise, in city driving, i almost always feel this cool breeze from the left and right vents when it's on auto...and if I take it off auto and off AC, and to have it just blow on the front, it sometimes starts to get cool when the car's not moving. It's all over the place, but not hot, ever. Engine's warming right up, so that part is ok.

I'm not getting codes with the dealer tools, not getting any codes with the HVAC system's built in diagnostic. Is there any other metrics that are published out there on what temp things should be? Do the non return valves get clogged up... my pump seems to be ok. I'm literally at the point of wanting to start throwing parts at this, in desperation. Usually the shop handles that desire on it's own, but they just keep turning the car back to me thinking it's ok...

As a comparison, I have a land rover discovery which is 80% windows and that heat is unbelievably warm. Thus, I know i haven't reached some pointin my life where i've failed to detect proper temperature, which I keep considering every time I pick up the Jag from a few days in the shop for this.

Any info is appreciated. I've seen a lot of good diagrams and such, have the tools lined up, just trying to figure out how to approach this logically, as I can't find consistency to the problem..

Thanks,
Pete

 

I've been dealing with a similar issue, sporadic heat not working. First time it happened I was driving along and the heat went down from pleasantly warm to not heated at all. It's happened occasionally a few times since then, can't find an obvious cause for it to happen. That being said, I haven't had a chance to backflush the core or test anything else, so this will be a good mental exercise/battle plan for me as well.

If it heats very well on HI but poorly in other situations, it's possible I guess that the interior temp sensor "In-Car Aspirator" is bad.

I think I've read some reports of people rebuilding the pump with new brushes, etc. which may do the trick. While the x308 heating control isn't quite like the traditional heater core with a blend gate (not sure of correct term), it's not too complicated. The scavenging line from engine coolant hot side leads to the valve which is switched open/closed by the ac module. from there it goes through the pump, thru heater core, thru non-return valve, and back thru said control valve to engine coolant cold side. There's good info in JTIS under Electrical->Heating and Ventilation->Description and Operation.

You should be able to feel the pump running if you put your hands on it (it is outside the false bulkhead housing the brake system bits). This doesn't necessarily mean it's working perfectly, but it's a fail/pass situation to test other things. If you suspect the non-return valve is clogged, I'd assume you could replace it with a basic coupler and test the system again, just try and avoid running the car with the heating system off so that coolant doesnt backflow in the case where it wouldn't with a valve in there. There's another non-return valve on the other side of the water valve, maybe worth testing as well. You could also test the control valve, either by manually feeding it a signal to open, or again bypassing it with couplers and testing.

So in short, backflush, bypass inner nonreturn valve, bypass outer non return valve, bypass control valve. Isolate each possible cause and eliminate it. If none of those work, I'd look at a new pump. If that doesn't help, I don't know what else to look at. Lots of this I got from a handy diagnostic chart, also in JTIS in the same heating ventilation section.

 

Check heater control valve, coolant circulation pump and clogged matrix.

Here is some info.

I have had to flush heater cores, even cars not in this VIN range.

bob

 

If your heater works well when set to High but not when auto it is possible that the ambient air temperature sensor is defective.
Press the selector on the control panel check what the reading is you are getting for the outside temperature.
My heat would only run on high and was all over the board on auto. The ambient air sensor was telling the ACCM that the outside temp was 138 degrees.
No heat but air conditioning was great.

 

Based on this:

But otherwise, in city driving, i almost always feel this cool breeze from the left and right vents when it's on auto...and if I take it off auto and off AC, and to have it just blow on the front, it sometimes starts to get cool when the car's not moving. It's all over the place, but not hot, ever. Engine's warming right up, so that part is ok.

My money is on the heater pump brushes. Without the pump there's not enough warm water in the heater matrix. When revs are above 2000 water will start flowing a bit by itself which explains why you feel some warmth when driving at higher speeds and only cold air in city driving.

Removal of the pump in the XJ8 is not too hard, most tricky part is to loosen the hose clamps. No need to drain coolant, just top up after the job. Get a reconditioned unit or even better spend a few dollar on brushes that you will need to sand down to the correct size.

You'll have a nice and warm car after this job!

Noud

 

Agreed to the above, it's the exact same symptoms I had with mine, and it was the pump. I was lucky enough to score a used one online, and since it was just hitting the colder winter days when mine was out, I was glad to replace it with something known working, and that frees me to work rebuilding the brushes (which were VERY worn out) at my leisure. Replacing the pump took maybe 5-10 minutes total, just the maneuvering the hoses and pieces to get clearance. Had heat within minutes afterwards.