Seems like Superblue’s heater is just not getting as hot as it used to in past winters. She’s full of coolant and I replaced the heater valve last season (although the old one seemed to work O.K., just wanted to be sure). The engine always runs at the right temps. I had wondered if maybe switching to a “winter” thermostat from the other one might make a difference, but it is my understanding that whenever the thermo is fully open (at whichever temp it is supposed to) that has no effect on the temp of the coolant in the system past that point. 
Is the system clear of air bubbles?
A common issue on older cars is a plugged-up heater core: see if both sides of the supply pipes are hot: if so, then check to see if the heat is Ok on low, and if it cools off, on high, that’s an indication flow is diminished, which ofttimes is a plugged core.
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another issue would be the vacuum controlled flaps not closing/opening properly.
I have the same issue as you. One test that may confirm this, if I let the engine warm up with the fan/heater OFF, then turn on the fan/heater, I get nice warm air for about 5-10 seconds, but it then slowly changes to lukewarm.
My guess is a flap is not closing properly allowing outside air to mix.
That’s the typical symptom of a plugged heater core. Flow is way down, but when it’s not loaded it gets good 'n hot anyway, so when you first engage it it flows hot air. It can’t keep up, though, so gradually the output falls off. It’s unlikely to be caused by a flap gradually moving after it’s been set.
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Greg,
Have you verified that your heater valve, located on the firewall, is opening and closing properly? If it is the OEM version, like the one in these pictures, the valve should be open with no vacuum applied to the valve.
Then with vacuum applied the valve closes as shown in these picture.
I have had a few of these valves fail in our XJ6s and XJS. They can fail in the fully open position, the full closed position, or anywhere in between. A failure to open completely might explain your issue.
Paul
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Thanks Paul, yeah, I replaced that last month with the new A/C Delco plastic one, no change.
I’ll eventually get to the heater core, but since I rarely drive my XJS in the winter, and when I do it’s not that cold here in Seattle, I will probably put this off until another year.
Also, looks like a difficult/expensive part to find. $270 Eurospare is all I could see.
BTW, I found a great write-up by a member here, for the Mk3 unit (which I have in my 88).
http://www.goflyrc.com/projects/XJS/xjsheater.htm
Greg,
I parted out three Series III XJ6s with the Delanair MKII climate control systems and getting that major assembly out of the car is a major undertaking in order at access the heater core. I have removed three of them in my parts cars but fortunately I haven’t had to remove and replace one in our driving cars. It is not for the faint of heart.
Paul
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There’s not enough money in the entire… Alpha Quadrant, that would compel me to ever again R&R a heater core, in put-near any modern car!
looking at the Mk3 instructions, it doesn’t look that bad? Is it the Mk2 and before that is the headache?
I’ve done heater core on my late 90s Volvo 850, and although it was painful, it wasn’t any different than other tough jobs.
I;m about to tackle my radius arm bushings, so I may WELCOME some time with my heater core! 
Heater core on a Mk III is a cake walk compared to the Mk II.
The Mk III really isn’t all that bad as heater core replacements go
Cheers
DD
Remember, the mkII gets a lot easier when you cut the pipes.
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It does actually seem to blow not as warm on the higher speeds as it does on the lowest speed, and I wondered about that. But then, when a fan blows on a hot object doesn’t the faster the fan go, i.e. the higher the velocity of the air draft, the cooler the object gets? Or does it not work that way with a heater core? 
So, can a heater core be back-flushed to clear/clean it, and if so, what is the procedure (face-lift)?
Can that happen with the 4.0s? How could I tell, if so, and get rid of the bubbles, as there is no air bleed valve in the system, unlike w. the 5.3s. 
Just reverse the flow of water through the core, did you have the heater set to hot when you filled the radiator?
If the air is moving too fast then there isn’t enough time for the heat to transfer from the elements in the core to the air flowing through it to raise the temperature very much - you get a lot of lukewarm air. But the amount of heat extracted may be similar to when the fan is blowing slowly - a small amount of hot air that doesn’t really heat the whole cabin.
Here’s how I do it, briefly:
-Disconnect heater hoses
-Temporarily attach two new hoses, about two feet long.
-Via one of the new hoses fill heater core radiator cleaning product of your choice from the local auto parts store. Arrange the hoses so the the open ends are higher than the heater core so the cleaner stays in the core. Do nothing else for 12 hours.
-Buy plastic fittings so a garden hose can be attached to each of your new hoses. A couple dollars at the local home improvement store
-Attach garden hose and turn on the water slowly to push out the cleaning product and the gunk it has hopefully loosened. I flush several times in both directions. Direct one hose downwards so the ugly output doesn’t end up all over your engine bay
-I can’t tell you exactly how much water force to use. It’s a gut feeling thing based on input versus output. Gradually increase flow/pressure. Although I’ve never had it happen there’s always some worry (for me, at least) about bursting an elderly core if it is seriously clogged.
Cheers
DD
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Before starting disassembling the universe to get to the heater core, this is what I would do fist:
Disconnect the heater hoses at the firewall and connect two extra (long) hoses to the core
Fill up the heater core with white vinegar and leave it for 24 hours
Back flush with garden hose and see what comes out
It might clear it enough to make it work again.
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Well, if it happens, you will know immediately.
Don’t ask.
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give it a go on SuperBlue, and report back. Then maybe I’ll try it