Finished the flushing of the coolant system 3 times today then replaced the heater core. Refilled with VW G13 coolant, burped the system, ran it up to operating temperature and voilà, consistent heat once again.
Nissen is a quality brand. They’re designed that way to produce optimum heat. I think it’ll be a long time before you have to deal with clogging again.
I just had the same symptoms and it was the heater core. 2014 Jetta SE 1.8 TSI with 54K miles. I hope that isn't it for your sake.
Quick update: 12/29/18 Ended up taking apart the heater box and fixing the vents and their pesky foam, and also replacing the old heater core which looked like it was original factory.
Bought a OEM vw core. Not taking a chance on jobber. Seeing I didd the labor myself the job only costed me 160$
Spend the extra couple of bucks and get a Valeo heater core. It is a pain in the butt job but not actually hard.
I found a Febi unit it at a local euro parts dealer that somehow always has the most obscure mk2 parts in stock when I need them. Ill give the Febi a try but if this one fails I'll have to bypass it like a hack.
My buddy bought a Meyle for his MK4 and it unfortunately leaked within a few months and he had to redo the job.
I've had an aftermarket (Mahle/Behr) heater cores explode on me after a year.
They replaced the core in my 2003 Ranger when I bought it. Internet said it was a common problem, and that it was electrolysis. The soloution was to ground the heater core, which I did. Five years later it let go again.
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