So I'm finally managing to put some time into my XK8 Coupe (1996 4.0 V8) which has taken second place to the X300 over the last few months. I bought as a project as there are a number of issues that need attending to. I've had the car for 3 months and for the most part it has sat on the drive with only the occasional outing. One thing that became apparent from day one was that there was no hot air coming out of the vents when the heating was turned on or up. Today I started her up and left to warm up then went back and went through the heater controls just to see if something had changed....nothing had until I put the rpm upto about 3k and then warm air started to circulate,once the revs were allowed to drop back down the warm air cooled off. The heater fans are strong and all the airflow controls work. Is this something anyone has experienced? I initially thought that maybe there was a sticky thermostat but the engine fans do cut in when necessary. I also vaguely remember a reference to a type of valve connected to heating system that might be relevant I'm not upto speed on the XK8 setup as yet so any advice would be greatly appreciated.Many thanks.

 

First question, have you upgraded the thermostat housing, water pump and thermostat. The early water pumps were prone to disintegrating and blocking the coolant pathways.

You could also try back flushing the system but personally, I would strip it down clean it all out and fit the 3rd generation parts.

 

Hi Jim,
Thanks for the response. The housing has been upgraded as it's not plastic,old receipts show the water pump was done in 2006 which was approx 30k miles ago. I have a new thermostat ready and waiting for me to put in. I shall do that and clean out at the same time and then see what happens.Thanks.

 

It's fairly common for the heater core to be plugged with sediment if used infrequently. Backwashing the core should take care of it.

 

In my experience, cold air from a heater until revved up, means low coolant in the system. I would burp/ bleed the system 1st.

 

Also, check the heater pump. There is a non trivial setup for the heater that includes an electrically controlled valve and a separate pump. The engine water pump is not expected to push water into the heater core as in most cars I know. There are several posts about this.

First, check the fuse to it.

If you can convince yourself that it is not working, you can start with the link in the post here for all the gory details: xkr-failed-heater-pump-resolved-151643/

This is good, too: heater-pump-removal-how-96841/

Best of luck, keep us posted.

 

Looks like I've got a list to work through which is fantastic,thank you. I shall prioritise probably with the easiest first (ie fuse) and work my way through. I really appreciate all your suggestions and shall let you know how it progresses.

 

I'm with fmerz on this one - once the revs go up, there's enough pressure from the coolant pump to push some hot water through the heater matrix. But - when ticking over, you need the "auxiliary heater pump" to get any heat into the cabin.
If you are practical, they are not difficult to repair for a few pounds worth of new brushes. Access and removal is not easy and the so-called "octopus hose" has couplings which can be difficult to disconnect. At the very least, you will need on of those cable operated hose clip removal tools.
Look at links above to get an idea of what is involved.
Obviously - check the fuse(s) first, but my 50p is on the pump.

 

Twack, same problem for me a few years back with the same year and model, in my case it was the water heater pump:

Part no MJA6710AA

Check on Ebay and you'll find one seller with the following helpful text:

"This item is the main source of hot air in the cabin. If you are unable to get hot air through the vents then it is very likely that this part is the fault"

I've owned mine for 12yrs and have done quite a bit with/to it so if you want to compare notes am happy to. Also know a few really good suppliers of used parts. For example I bought an alternator from a chap local to me, exact fit and with a 120 day warranty...the princely sum of £20. I've recently bought a used transmission from him, including TC for just £320...delivered!!

So get in touch am only just down the M5 and I've actually got one of the water heater pumps spare (the garage that fitted mine refurbed my old one for £25).

 

Thanks for that MarkyUK, good to have a local contact. Had a fair few of the XJ's and a couple of the 3.2 V8's but this is the first XK8 so it's going to be a bit of a learning curve, I'll be sure to take you up on the offer of note swapping as there's a bit of a list to get through...worth it in the end though.

 

Certainly seems to be the consensus,thank you also,I shall be checking the fuses tomorrow and post as it unfurls.Thanks again.

 

Well before the rain came down I managed to check out the fuse in the engine bay and no surprises there, it was fine. On the fuse issue just so I have it right. Fuse 3 in the fusebox in the engine bay on the passenger side (RHD) is the one we're looking at,yes? I checked all of them as a matter of course but I mention it as it's not labelled in my handbook for fuse locations/purpose etc.Did this change for the later models.? I've attached a picture and the security sounder is on two locations and front fog lamps have a separate fuse location also. When the rain abates I shall purge the system of air and then decide the best course of action to tackle the heater pump. I have read over the numerous threads and the 'How to' and it certainly looks like a challenge. If anyone is facing this I highly recommend reading through the links in 'fmertz' s reply (#6)


1996 XK8 Vehicle Care page

 

Are we talking about the security sounder or the heater pump ?
Heater pump is fuse 15 - 10 amp.


Edit - If you haven't already done so, download the 1997 wiring guide from Gus' excellent site www.jagrepair.com
You will thank him every day for at least the next year once you have done so !!

 

Wow,that is a fantastic resource,thank you. I now know the relay involved so can check that out. I was talking about the fuse for the heater pump. I picked up on various threads saying it shared a common fuse that also did the security sounder and foglights. I've read so much over the last couple of days I may well have got my threads crossed,as it were. On my diagram (Fuse box-Engine bay-left hand side-passenger side) Fuse 3 has various circuits but not listed as heater pump, and Fuse 15 is for the Air conditioning coolant pump. I've checked all the fuses anyway and they're all good and now I can check the relay, it would just be could for future reference to know exactly which fuse is responsible for the heater pump.

 

Jaguar LOVES to make your life difficult.
The Air Conditioning Coolant Pump **IS** the auxiliary heater pump.

 

You are a shining light in my ever dimming world....ok,thank you first and foremost. So using the info from 'Gus' I know where the pump relay is and now know from your good self the proper identification for where the fuse for the pump is.....and according to the attached table Fuse 3 shares with A/C pump relay coils...so there is some kind of logic there. I think.

 

twack


Yes - there IS logic but not as we know it !!


The wiring diagrams are fantastic compared with many of the other cars I have owned and tried to work on (think Mercedes for starters !!).
Once you have got your head around the "square", "round" and "octagonal" symbols, and how to get from the power distribution pages to the actual bit of non-functioning kit, it all makes sense. It's more like Sudoku than Electronics - very logical and straightforward in a perverse sort of way !! It even tells you where to find components, pin configuration, colour and type of plug - it is all there - just not easy to find at first.
You are obviously pretty practical, so much of this will be patronising, but :


Wiring diagrams

 

Again in your debt, I have bookmarked the site and shall hopefully have the chance to work my way through it and put together a folder of all the relevant sections. Many moons ago I used to repair 10-pin bowling machines and to start with anything concerning PCB'S was a case of swapping out the board and sending it away for repair, no schematics available and trying to diagnose on a component level was nigh on impossible. Still over the years as you built up a network of contacts the seemingly impossible became possible as someone had done it before and was happy to share.
That is why I am here, to be humbled by those like yourself and others who are happy to share their own experience and knowledge, I only hope that I can return the favour at some point.

 

This applies to all of us, and is exactly what this forum is all about!

 

Multi-tasking this morning. Thought I would cross a couple more items of the list. 'Burped','Belched' and thoroughly winded the car. Made no difference to cabin heat situation. After that I swapped out the relay for the pump, just in case, made no difference. The only point of interest I can raise and I don't know if this is good,bad or indifferent, the engine ran for approx half an hour and it took the best part of that time for the fans to kick in.It was sat on tick-over with just a few occasions of prolonged higher revs. Is that about right?
The temperature sits smack in the middle of the gauge. Not sure if any of that is relevant.