I can get my heater to blow air, but it's only cold air? I am fairly new to these cars, where do I start to figure out why my heater won't kick out hot air, instead of just cold air (heat)? 1998 XK8 Convertible.

 

The heat from the engine coolant comes to the heater core through a semi complicated arrangement involving a computer controller water valve, an auxiliary pump and a dedicated coolant bypass loop. The pump is a DC motor that is running at all times the system is on. The most common problem is that the DC motor graphite brushes wear out and the motor stops, and no heat comes to the core.

Best is to go to jagrepair.com and check the electrical diagram. There is readily accessible fuse for this pump. When brushes are worn out, there is an open circuit ("infinite" resistance to the ground). If the motor is the problem, the bad news is that it is a challenge to reach/replace, but the good news is that the motor can be repaired (it is expen$ive from Jaguar) at the cost of new brushes and basic soldering.

There are other scenarios as well. The water valve can also be a problem but it is rare. FWIW, it defaults open, so simply disconnecting it can help diagnostics.

FWIW, I had good outcome out of a simple heater core cleanup. This is a normal conventional cleanup involving disconnecting the heater core hoses off the firewall by the brake booster and piping in clear tubing hooked to a garden hose. And yes, try and use radiator cleaner for the prescribed 10 min "while you are there".

Best of luck, keep us posted.

 

Does the light for Traction control work, or the front fog tell tale or heated seats lights come on?

If not then locate fuse 5 in the engine bay fuse box which controls those lights and also the heater pump - great combination eh?

I spent a lot of time trying to get the tell tale lights working including getting a replacement bank of switches but that made no difference. Once I rechecked fuse 5 and replaced it it bingo lights and heat!

We can hope it's that simple.

 

heater pump probably kicked the bucket. if it’s broken buy a generic aux pump and an EV1 pigtail

https://www.ebay.com/itm/For-VW-Heat...-127632-2357-0

https://www.ebay.com/itm/Bosch-Cobra...-127632-2357-0

 

Had the same thing happen to me. Yup - it was the pump. Yeh - it's a royal pain to get to. I kicked myself for not replacing it when I did all the hoses (sigh). Do yourself a favor: use this as an opportunity to replace all your rear hoses and the valley hoses as well. Getting the manifold and throttle elbow out of the way will give you more room to work. Replace everything you can back there when you go in. Get the pain over with now and avoid having to go back in over & over as various malfunctions may occur. Replacing your O2 sensors will be easier as well (hard to get to the connectors), but that may fall into the "if it ain't broken..." category. Spend a few bucks and buy the hose release set of rings - makes releasing the valley hoses a snap, and will also be used to release your fuel line. I went so far as to send all my injectors out for overhaul, and replace every seal and gasket I could get my hands on.

It sounds like a lot, but it's all just routine mechanics.

 

Replace the thermostat (careful it goes in the right direction...it can fit both ways) and flush the radiator. See if the heat is back. Spend time and/or money if easy doesn't work.

 

I can add a little to this. The water valve on the firewall,in front of the driver position, is, as stated, default to open. It is spring loaded open and allows hot coolant to flow to the heater matrix, plenum, box, or whatever you call it except when you turn on the air conditioning. With the A/C on the valve is electrically powered closed to keep the hot coolant from the plenum. I learned this because about 10 years ago or more my valve would not always close when I turned on the A/C so I got hot air out of the vents. A light 'calibrated blow" from a hammer and long punch solved the problem and it actually hasn't happened again for many years.
This valve being stuck closed would be unlikely because the internal spring would have to break.