So I've been re-reading all of this, totally forgetting I was the last person to reply to this. It might be a bit of a long post...please go with me on this.
Since the day I posted that comment, I have done LOADS to the car but we're only about 50-60% of where I'd like to be with it. We get hot air from the car (sometimes) and there have been days were the far-right vent (car is RHD) is almost as warm as the far-left (According to OBD11 reading the temperature sensors) but they have been few and far between.
I've since done a full de-grease and flush of the car. We ran 800ml of Forté bio-degreaser through it and after around 300 miles with that in the coolant, the vents did begin to warm up.
Before: After:
Not a massive increase but the only change in the testing was the use of the degreaser. So it was dislodging something!
We then dismantled the matrix from the heater box and checked it out - when we filled it with hot water while investigating it, the thing heats up evenly, meaning it can't be blocked.
I will say, my situation is different - I had the silica gel back in the coolant bottle but it wasn't ruptured.
There also wasn't any sand in the system. All I can assume was causing it was the breakdown of the glycerine in the G13 coolant...collecting in the core.
Or, so we thought...
My Dad and I have been working extensively on this, ever since - we've really got the bit between ourselves on this and have basically deduced that THIS is the problem:
Perhaps not the best picture, but hear me out. I can only comment on models with Denso heaters in them but given the problem affects both, I think it's safe to assume the same applies.
The hot coolant flows into the bottom of the unit then works it's way along the bottom, up and then out the top. I can't confirm exactly when my heating broke but we're both pretty certain that it happened after July of 2019, when the water pump in my car was replaced along with the timing belt.
The reason for this is that since flushing it out, I have noticed that I get considerably
more heat from the car after I release the air pressure (and any accompanying bubbles) from the coolant system when the car has been running for a while. This has been the pattern since we drained it, took it out, replaced and re-filled it - albeit only with water, for now. I don't want to put coolant back in until we're certain the problem is resolved adequately.
Dad's a mechanical engineer and is of the same opinion as I am - because the coolant flows in and out of the same side of the core, if an air bubble gets trapped in there, due to the angle it's sitting at, any water + air which gets trapped in the matrix following the draining of the system will arrive at a state of equilibrium - with the water trapping it inside. Since the water will always find the path of least resistance, the air is likely going to collect on the far end of the matrix and stop it warming up to the correct temperature.
This would explain why it still gets warm but immediately cools down with enough air blown through it.
VW have designed a bad system here, plain and simple. I highly doubt that we'd be seeing the same thing if the inlet/outlet pipes on the matrix were on opposing sides; one bottom left and the other top right. That would force the water, when it's being filled, to work it's way all the way along, then up, forcing any air out so that it can find it's way up to the bleeding tubes, on the top of the engine block.
I am convinced that this is the reason why so many people never get their heating working again after they've had this problem - the system is physically incapable of evacuating the air which naturally works itself in here. Having an engineer for a Dad means we're never short of...imaginative...ways of trying to resolve this. In some ways, it's fun but the entire time we've been working on this, I've always ended up being right and I'm currently grappling with him over the use of a vacuum pump to empty the system entirely and then re-fill it (as the Haynes manual recommends) to see if the system behaves any differently.
I share this only to bring my experience here. I appreciate there is a lot of insight into this issue with lots of different, varying experiences, but I can't help thinking that so many of these cores have been thrown out unnecessarily. When I tell you that mine warmed up evenly after we ran hot tap water through it, I mean it - there is no blockage in mine anymore. And it
does work when it's in the car, we just can't make it do so consistently.
Yet.
I mean it when I say that every time I've had a solution to this, I've ended up being right. This is only worth commenting on because computers are my thing - not mechanics. That's Dad's area. But I am definitely of the mind that VW's "self-bleeding" system is perfectly fine for bleeding the engine block & radiators but this thing, stuck in the bowels of the dashboard, is absolutely the black sheep - the unfortunate, "gimpy" one.
Curious to know what other people think. This is absolutely still a work in progress. We had it working, perfectly - we had de-greased it, drained & flushed it and re-filled it. The fun started when we drained it to flush it again (because the engine was still full of de-greaser).
It has to be an air bubble. There is nothing else wrong with my car - miraculously. She is in tip-top condition and it really surprises me, given how many miles she's done.