Heat starts out working then fails

(Douglas) #1

Here’s an XJS heating question for you all. Apologies if it’s been addressed before but I haven’t found much on this specific presentation of a heating issue.

I turn on my '89 and allow it to properly warm up. Once temp guage is showing its habitual spot (a bit below N and I know my car’s thermostats are goo so the engine really is warmed up) I turn on the heating system in manual mode and set the temp dial all the way to 85. I should add that my blower fans both work on all settings (low, norm, high) and defroster blows like it should when selected as well.

As soon as I turn the system on the vents adjust as they are supposed to and the majority of the airflow is from the footwell vents. The center console vent shuts off like it is supposed to as well. And then nice hot heated air blows out mostly from the footwells. So clearly the system knows I want heat and it delivers that heat without issue.

But then after a minute of good heat, it just disappears. The air keep blowing from the correct vents but it becomes ambient temperature or maybe even a little cold. If I turn the dial to full cool and wait for the vents to switch and then turn it right back, I can get another minute or so of heat until the same thing happens. This cycle can repeated pretty much indefinitely it seems.

I’ve read a lot about failed heater valves, etc. but I’m unsure that that is my issue considering that I can get heat, if only temporarily.

I’m wondering if it could be the ambient temperature thermometer or maybe a different thermometer somewhere in the system? Any thoughts on this?

I should add in case it’s relevant that my A/C system does work but poorly. It’s not charged fully and I haven’t had the time to find any leaks and/or sort out an R12 option or substitute. But seeing as we are moving into colder weather, I’m more interested in getting the heat working reliably before I address the cooling recharge (I
do know that getting it work fully properly is important too though not just for passenger comfort but also because of its role in cooling return fuel).

Any thoughts on what might cause this short term good heat that disappears but can be gotten back by adjusting the temp dial?


(equiprx) #2

My experience with the climate control is almost exclusively on my 94.
Your system could be different so don’t just jump on my bandwagon.
I was never able to get my system to do what I thought was correct.
While I was trying to troubleshoot it, there were some odd discussions.
It was said that the system in my car always runs with both AC and heat.
The actual output temp is regulated by blending heat & cool with the climate control ECU.
I never got the chance to replace the ECU, but there was talk of common ECU failure.
This is another place where the Jaguar PDU can help diagnose a problem.
If you can get access to a PDU take advantage of it.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #3

It sounds like your heater core is obstructed. When off, a small trickle of flow through it will get it nice and warm, so when you turn it on it feels like it should. But once there’s air flowing through it, the trickle of flow isn’t enough to keep it warm, so it cools off. I’d be checking for flow through the entire heater circuit, and fix any obstructions.

(John) #4

Have you replaced coolant recently? There might be some air locked in the heater.

(Douglas) #5

Interesting thought Steve–I’m not sure if I can connect a PDU to my '89? I was under the impression that the onboard diagnostic “plug and play” if you will came along with facelift. If that’s not the case though, then I’m glad to learn it and will search someone local who might have one I could use!

(Douglas) #6

Yes I have actually! (Due to a leaking seal by one of the thermostats) I recently had to add in quite a bit of coolant. It was the better part of a gallon I would say. Therefore an airlock seems like an interesting possibility to explore. Any tips for how to go about purging air in the heater core specifically?

As an additional note, I should add that my overflow tank over-topped and drained a little coolant to the ground behind the left front fender (as it’s supposed to I understand). Therefore I am quite confident (other than possible airlock(s)) the system is full of coolant.

(Douglas) #7

Thank you that makes a lot of sense. I like John’s idea of a possible airlock in the heater core and am going to have a go at looking for that. Any specific advice for resolving a heater core specific block? Thanks again.

(John) #8

I would start by disconnecting top heater hoose from heater valve and try to fill heater core with coolant through there.

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #9

Or, as recommended in the Book, just go ahead and install a “flushing tee” in that heater hose to facilitate properly filling the coolant circuit in the future.

(gcoder1) #10

After spending years trying everything else to cure the same problem as yours, I found that Kirby was correct in the heater core being obstructed. I back flushed mine and now my heat works great. It seems that the same debris that plugs up the radiator also plugs up the heater core. My recommendation would be to back flush the heater core first. Be sure that you purge any tap water from the heater core before refilling with coolant.

(Carl Hutchins, Jr. ) #11

Cynical view : And now the clean heater core leaks.

Better view. A generic one. A sensor is telling the unit that the air in the cabin is warm enough and shuts off the hot water.

Thermister comes to mind…


(Robert King) #12

A bit of advice- remove the overflow bottle, remove the pad underneath, and rinse the area thoroughly. Used antifreeze is corrosive and will rot away the bottom of the fender. Ask how I know…

(Carl Hutchins, Jr. ) #13

You suppose that is because it is now mere H2O, bereft of glycol and inhibitors???

Apology, I just could not resist…


(Robert King) #14

Actually, my understanding is that it is the metals from used coolant. But I have to believe that the felt pad holds even plain water , notice how many rot at that same spot.

(Douglas) #15

Reviving this thread awhile later!! Could you advise on the procedure/steps you followed to backflush the heater core? Where/what should I disconnect and so forth?


(Douglas) #16

Got the “flushing tee” to put in–where exactly is best spot for it?

Sorry again for reviving and my older thread!

(Kirbert - author of the Book, former owner of an '83 XJ-S H.E.) #17

It needs to be as high as possible. If you have an OEM heater valve dead center of the firewall, in the line just to the starboard side of it works well.

(Douglas) #18

Ok perfect thanks. The valve is OEM and mounted on the firewall. Appears to be working fine (I’m basing that off the little bit of heat I can get any time as described above)

(gcoder1) #19


If you look at the hoses on the heater core you will notice they are difficult to remove at the core itself. On the drivers side follow the hose to the spot where it can be disconnected at the inner fender. That is the spot to disconnect on the left side. Same idea on the right side but that is more assessable. Toward the back of the coolant rail heading straight up is the fitting where you disconnect on the right side.

You will need a funnel with a small enough end to fit in the hose, an air compressor, a bottle of radiator flush, some distilled water and new antifreeze.

Disconnect hoses per above. I put another hose over the right hand side so I could redirect the waste into a pail.

Flush from left to right. You can use a hose to run water through the heater core or just blow out the coolant with air. Once the core is emptied clamp of the right side hose with a clamp or flat vice grips so the fluid stays inside when you add the flush. Use the funnel to add flush into the heater core. Let it soak for a while. Each flush is slightly different so read the instructions for soak time. After that, rinse out the flush with the distilled water. Blow out with compressed air again, then reattach hose on the right side, refill the heater core from the left side hose. Then reattach the left side hose. Fill the balance following standard procedure to be sure the air is out.

You may want to completely drain and refill the cooling system at the same time as any debris in the system will quickly plug it back up.

Good luck,


PS- do not use full force on the compressed air or risk damaging the core. I just put my finger around the nozzle/hose to keep the air directed but allow some blow back for flow with out pressure.

(Carl Hutchins, Jr. ) #20

A bit of goating and gloating. Apologies for the latter!!

I needed to do a couple errands today. And some cabin fever relief. As the weather is not too bad, I chose the Jaguar.

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  4. Four tasks planned for the shop. Flunked 2 of 4. .
    A. Replace the blade in my recip saw. Slippery. Grub screw and blade refused to align. Deferred, tomorrow is another day.
    B. Fix the “my bucket has a hole in it”. Beyond all… Tossed.
    C. Wipe and put tools away. Partial victory.
    D. R&R interior light door switch. Deferred. I suspect more than the switch is involved…

Telephone counsel with son. Canvas ripped on one of his military tents that shelter some of hs toys in the Sierra foot hills. Strong thread, curved needle and an awl. Where to find 'em. Horse place. Tack and saddle shop. Same tools used to work leather

Off to make lunch decision…