It seems like this is a recurring problem with these cars. This is the first cold weather we had this year. I had posted this problem a year or two ago. My 1999 XJ8 now produces no hot air despite putting the temp control on Hi. I have tried leaving on recirculating and tried every vent setting and using only the defroster. I was told last time they looked at this problem there is an air mixture box with a door that blends cold air and hot air and that over time these fail. They checked the heater core and control module and said they were functioning well. Last winter I had a little warm air, but it is worse now. How many have had similar problems and does anyone have an answer to improve heating? Thanks.

 

If you had a little warm air last winter, this is what I'd do.

1. Check coolant level.
2. Run engine up to temp.
3. Set heat to max temperature setting with recirc on. I'd have the fan speed set in the middle and to have the air mode door set to come out of the dash to blow on your face.
4. Make sure that both heater core hoses at the firewall are getting hot.
5. If they are not getting hot, you need to check the auxillary electric water pump for operation. You could have a bad electric waterpump.
6. Check the electric coolant valve just downstream from the aforementioned electric waterpump for operation.
7. The #1 most likely cause is that of a clogged heater core.
Hope this helps

 

Option 7. above most probably, as Addicted2Boost said. Start with that. It's a yearly ritual on my '01. Have to flush / backflush the heater core every start of winter. I get my cabin heat back each time :-) Cheers

 

I would add that #5 is about equal to #7 in probability. Look to archives here or on Jag-lovers for procedure to change the motor brushes. Neither of these issuers are hard to diagnose. They are just a little messy in that you need to open up some pipes and see coolant flow!.

 

Not that I expect everyone to think like I do, but this is the exact reason why I sporadically during the spring and summer time I turn the heater on full blast and drive it like a bat out of hell!! Just for a few minutes though is all I think I need.

 

Blue_XJ8, not to hijack this thread, but now we are on the subject, and as it seems you got lots of experience, how do you do your flushing?
I will need to address this too.

 

Hi Ericjansen. I will be happy to (try to) describe how I do it, but don't have pics. I believe there are other threads on the forum that will have some pics along with the steps, just in case my explanations are vague. OK - so - bottom line is that you need to locate the two hoses that connect to the firewall (on drivers' side - for left hand drivers cars) - the heater core being on the inside (in/under dash somewhere). One hose connects to the heater valve (that little unit just below the top coolant expansion tank, to the right), the other to the auxiliary pump (half tucked away under the brake booster 'housing'). So my steps are the following: disconnect top coolant expansion tank electric connector, then remove tank holding bolts to let the tank 'loose' (for ease of access). Gently move the tank aside, disconnect the heather valve electric connector, locate the hose that links to firewall, loosen (and push back) the hose 'tightener' and remove the hose from the valve [NOTE: coolant will spill out]. Next disconnect the electrical connector to the auxiliary pump. Remove the pump holding nuts to release the pump and gently pull out towards you (not much, but access not easy so every 1/4 inch counts!). Locate the hose that links to the firewall (I believe the one shooting upwards - hence not the one that connects straight to the heater hose matrix / non-return valve). loosen (and push back) the hose 'tightener' and remove the hose from the pump. [NOTE: coolant will spill out]. You now have both hoses (in/out) to the heater core 'free'. I use a garden hose 'male' coupling connector (that fits the size of the heater hoses) that also has a shut-off on it to control flow/pressure. Turn on garden house, shut off the coupling connector. Fit the coupling connector into one of the two heater hoses and VERY GENTLY turn the shut off valve to let some water pressure go through. I generally go a few times on and off, very little pressure, until I see some water starting to come out the other hose. Only when water is starting to pour out will I slightly increase the water pressure - BUT NEVER go to full pressure. The heater core is not designed for that level of pressure. I then remove the coupling connector and place it in the other heater hose, and perform same step (backflush). I go back and forth the hoses a few times (flush / backflush). Make sure water is pouring out towards the ground - not up towards the engine. Reconnect the heater hoses / secure back in place all unbolted units / reconnect electrical connectors and - naturally - re-fill coolant via the top tank until the floater is covered. Start the engine, get heater on HI, keep an eye on the 'low coolant' warning as you rev' a little to get the coolant to circulate. I generally visually inspect the coolant level in the tank before it gets too hot to open (warnings sign or not). As long coolant level is good, let engine get to sufficient running temperature to start feel cabin heat ! I hope this was helpful. Cheers

 

Thanks, Blue-XJ8.

I saw several pictures over the years of ppl. flushing from the front side (assisted with air), but also figured out that you can flush the core only by working closer to the firewall.

I only wonder how you handle the non-return valve (7), where do you disconnect exactly?
I presume you do it on the non-return valve?

Thanks again,

Eric

**NOTE: Message revised and attachment deleted to prevent later misunderstanding**

 

Hi Eric - the diagram can be confusing / misleading, for sure! And when I looked at your question / diagram, you had me very concerned for a second there, thinking 'WTF have I done!' LOL - So the hoses that get used to flush do not have a non-return valve in the way (note: I did once mistakenly pulled wrong hose and tried to get pressure in... yeah, splashed all over - the valve ultimately blocked the way !!). See below diagram where 4 and 7 are the hoses to the firewall/heater matrix. I added colors to the hoses used to flush. From memory, the 'blue' stream is from firewall to heater valve, the 'red' stream from pump to firewall/heather matrix. Hope this helps. Cheers!

 

Now this will help for sure.

My diagram was from a much older post, saved "for later" on my laptop.
I revised my previous post to prevent misunderstanding.

Thanks for all help for me, and hopefully also for the OP.

Eric