I recently bought an 86 XJ6 S3 that sat for roughly 12 years. The car runs good (40K miles) but has a few quirks. The latest issue, with the weather turning much colder, has been with the heater. When I have the fan control on the Auto position, the car will initially provide heat but will then go into high fan mode and blow cold air. The temp control is at the max setting but cold air continues to blow. After a while it will make a few noises (some from behind dash on passengers side, some behind dash on Driver's side) and heat will come on again. Changing the fan controls changes the speed but no effect on the heat. Outside temp is 30F. This seems to happen more when car is first started for 1st half hour...I haven't run it longer than that so I don't know if it goes away after 1/2 hour or not. When the temp control is changed, I can hear a whirring noise on the passenger's side which I assume is for the water valve. I get a clanging noise on the driver's side and then heat seems to flow for a bit until the next cold cycle starts a few minutes later. I initially thought something was sticking and preventing the heat from getting thru, but the pattern is too uniform for something in partial malfunction.
One other thing I noticed is that the water temp indicator doesn't go into the green area but stays 1/4 of the temp range or less..I'm not sure if it is a defective gauge or a thermostat issue. FWIW, the AC doesn't work...I'm guessing it is low/out of freon but I'm not worried about that until spring.
Any insights into this hot/cold temp issue? Thanks in advance for any and all suggestions!

 

It is easy for you to eliminate one possibilty and that is by checking the operation of the water valve. It is situated inside the engine compartment right behind the engine. Very visible. It functions by vacuum being fed to it that will cause it to open allowing the hot coolant to flow through the heater core. You can see the operating lever on it move to open and close the valve. Should mention that they operate very quietly. You should not be able to hear it at all.

Sometimes these valves get corroded and they stick. Somehow though I don't think this is your problem but it would help to check it out first before pulling anything else apart.

 

hi,
1) the clanking on the driver's side must be the Blower Case Flaps; There are two flaps that open and close to let fresh air in, or to recirculate air; One flap is visible if you get on the floor on your back with a flashlight; There are two blower cases, one in the driver's side, one in the passenger's; The flaps have foam stick-on seals that desintegrate and when the flaps close under vacuum pressure, you hear the clanking;

also these flaps are held and moved together by a series of Linkages which themselves are held together by little vinyl Pins which break, causing the linkages to fall out of sync or to get stuck trying to open or close the flaps, compounding the problem;

your blower case might have one or both problems; Repair is very involved, the entire blower case assembly must be removed, and for that you need the Jaguar Factory Service Manual to follow each step, otherwise you end up damaging more parts, vacuum lines, and wiring.

2) the whirring sound in the passenger side is the Servo doing its thing; It opens and closes the different floor or face Vents, and defrost Vents; It also works by way of many linkages. Don't mess with it; The fact that it is whirring is a good sign.

3) the Amplifier is one component that goes bad and then you don't get Heat or A/C, or you get Heat when you want A/C or viceversa. Only way to check is by substituing it with a known good one. John's Cars in Dallas sells a replacement improved Amplifier, www.johnscars.com you have to call them for it. Not cheap but a lot cheaper than the factory part.

4) the heater system in these cars does not run until the car has warmed up sufficiently; you need to get it warmed up first. It's like that, nothing we can do about it. In fact, if you start the car in 30 degree weather, set the Mode switch to Auto and the Temp switch to 85 degrees, the blowers/fans won't even run until the engine and coolant are hot. I installed Seat Heaters in my '84 to be able to drive when it is very cold and not be shaking from the cold.

5) check the coolant, these cars inhale an enormous amount of coolant, check the coolant level when the car is cold, never when hot.

6) to check the Heater Valve operation as suggested by Bob, warm up the engine, have someone sit inside the car while you watch the Heater Valve mounted on the center of the firewall behind the center of the engine; Have someone set the Mode switch to Auto, and then start moving the Temp switch from 65 to 86 and viceversa; you should see the linkage moving under the valve, opening and closing the valve, also watch for any leaking of coolant at the valve; If it leaks, replace it or tighten any leaking hose. Heater valves are sold on eBay cheap.

I hope this helps you.

 

here's for everybody, Merry Christmas!

 

First and foremost, thank everyone for their input! After reading these and many other posts, I think the issue could be in the AC Amplifier. I get heat initially, but then it switches to full fan cold air for a few minutes, then back to heat. Sometimes it settles down and provides heat like it should after 20 minutes or so, but its cold until it decides it will work properly. The heater valve is working properly, the pair of air doors seems to work properly (although they clang a bit), and the coolant level is good. Gotta be the AC Amplifier.

Jim