[blix99]

Thank you so much for such a comprehensive reply. You come to more or less the same conclusions as me, all though I'm much less qualified : ) I generally make everything up as i go along!!.

So, here goes:

Based on the diagram, I don't see any reason why that dashed hole in the upper radiator hose pathway (as it passes thru the ET) would need to be there. I think the purpose of that hole would be to burp air from the upper end of the radiator/engine connection, but there would be no need for it since the small hose that runs from the junction at (5) to the ET would serve the same purpose. So maybe that's why it is not present, as it'd be redundant.

This diagram is actually from a slightly later manual (1986) so maybe they changed the filling hole from the smaller by-pass section in the ET (as mine has) to the larger one which feeds the rad? However, this van is almost 40 years old so can't see that the current system has had it overheating for all these years

This is actually a pretty nice design as shown in the diagram, with what appears to be better self-bleeding capability than the Volvo version has, since in this scheme any air trapped in the high point of either the engine or rad has a path to escape to the expansion tank. That's unfortunately not true for the Volvo installation, which is why we were talking about our special burping procedure that you won't have to deal with. I think we can be confident in this design that it should not have air in the system, since after running for any length of time it should have purged it on its own.

As above, I'm sure this whole system has worked fine for years : )

When the engine is running, do you see coolant flowing out of that hose that runs from (5) to the expansion tank? A lot or a little? And how large is that hose? Is it just a little (like 5mm) air purge hose, or a hose more like the size of a heater hose?

The lower hose of 5 is pretty big but just runs straight though to the top of the rad. The smaller hose that tees of it has an external diameter of approx 30mm (just over an inch) I've just felt the joint where this tees off and there does seem to be something harder molded within, so probably the 4.5mm or similar restrictor is in place. As mentioned, this is a slightly newer diagram so 4.5mm might not be accurate. However, the smaller hose is near the top of the expansion tank and I guess is only really for excess coolant under pressure so nothing really comes out of it that i can see. I've also blown down this hose to make sure it's clear and it seemed OK.

Congrats on getting it back running and doing the whole TB/WP job, that's pretty significant work and you made it sound easy. If you got that done without much sweat then I think you'll figure out this cooling issue soon. It can't be that complicated.

Ha ha. I fumble along. I wouldn't say it was easy but it's just one 'small' job followed by another. By small I mean 'uncomplicated' eg: Removing the crank pulley was just a matter of working out how to lock the crank and undo a large bolt : ) Then next small job is working out how to get said pulley off. Then on to the next thing and on and on ...
I think this is why it's bothering me so much; it's not that complicated but I can't seem to fix it by doing the things that should seem to fix it : (

Let me sum up where I believe you are saying things stand at this point, so that we are all sure to be on the same page with our thinking:
- Water pump is new and engine timing is set correctly (both cam and IP) - yes
- Thermostat is a new 87C from a good quality manufacturer, installed correctly (block-off disc facing inwards to the engine)- yes- also re-tried the old tstat back in and tested both in a pan of hot water
- The engine doesn't leak or appear to lose any coolant (right?). It gets hot when running, at least according to the gauge, but never over-pressurizes the system, blows coolant out of the pressure cap, has coolant mysteriously disappear and need to be topped off, makes air bubbles in the coolant or sooty/oily deposits, or any other indications of a leaking head gasket? - no. no leaks and coolant stays clear with level constant
- As best we can tell, the cooling system is full, with no air pockets, and coolant seems to be circulating correctly (heater gets hot on both supply and return sides when the heater valve is open? Hot air blows from dashboard?)- yes
- The only symptoms we see are that the temperature gauge reading is very high when running after a short time, yet hot coolant does not seem to be flowing through the radiator.- yes

If all of the above is true, then as far as I can come up with right now, there are only two basic possibilities for what is going on:

Variant 1: The engine IS getting excessively hot as the gauge indicates, due to the fact that for some reason coolant is not flowing correctly through the system and is not being cooled in the radiator. The obvious first suspect for this situation would be a thermostat that is not opening, or is installed backwards, or a failed water pump, but we seem to have pretty much ruled all of those out.- yes

The other possible reason for it, though, is that coolant is somehow finding a large volume bypass route around the radiator. I'm looking at the diagram and at your notes, and seeing that mention of a 4.5mm restrictor in the hose that runs from the upper rad passage into the expansion tank as an important possible piece of this puzzle. If that restrictor were not present, and if the hose is relatively large, then the result could be that hot coolant pumped out of the top of the engine mainly just recirculates through the expansion tank, thus taking volume away from what should pass thru the radiator core and achieving no cooling effect for the engine. How big are those orifice holes in the pass-through pipe for the lower rad hose? Big enough to move much coolant? Could that restrictor exist at the connection point to the ET, since it's not in the hose on your van? Is it definitely missing?

And in a similar thought process, is there anything else in the cooling system, like an aftermarket additional heater circuit or something, that could allow a large volume of coolant intended for flow to the rad to instead just loop around straight back to the WP inlet without being cooled at all?

As far as I can tell, everything in this system is original and correct. I also think the '4.5mm' hose is doing what it should; which is just allowing any excess air or coolant to be purged into the top of the expansion tank under pressure and not for anything else.

Note that in this scenario, if there were any possibility for coolant to bypass the radiator, that possibility would be amplified if the radiator itself contained any restriction to flow. Remind us once more: you said you did take the rad out during a previous repair step and confirm that it didn't appear to have any buildup, restriction, flowed easily, etc, correct?

I have checked the rad when out of the vehicle with hot water from a kettle and the flow / heat seemed OK. However, I think I'm going to check it again in situ. I have to drain the cleaner from the system anyway so will fill the rad directly from the top with hot water and see if the feed hose to the tstat gets hot. I'll also see how quickly this happens.

If we think that required 4.5mm restrictor may be missing, then one test (now that the cooling system seems to be purged of air) would be to gently crimp that bypass hose shut. Like with locking pliers with hose sections or tape over the jaws. That would halt flow through that circuit and determine whether it is affecting cooling capability.

As mentioned, nothing seems to escape through this hose but it will eliminate it as a possibility so will give it a try.

Variant 2: The engine IS NOT getting excessively hot as the gauge indicates, and the overheat indication is due only to an electrical fault resulting in a skewed high reading. Based on what we have learned so far, this is the possibility I would want to pursue first.

I had considered this and might have already mentioned it earlier. This would indeed be ironic. However, I had intended to change the WP and timing belt and check timing, so this time wasn't wasted. Aslo the tstat was a good thing to change / check. It's the rest of the time that's been a PITA!!
Getting an IR thermometer is definitely the next obvious step to save my sanity. It's not unreasonable to think the gauge might just be reading high.
As you say, the only thing alerting me to the possibility of overheating is the gauge : )


edit: I've just borrowed a BBQ thermometer that has a range -50 deg C to +330 deg C so that will at least tell me the temp of the coolant in the expasion tank.

I will certainly report back.

Thanks very much again.