sorry to hear your woes,
I resisted posting previously, particularly when you advised you were going to change the battery, and then the fault disappeared.
I had a starter battery replaced under warranty, would not hold full charge for a reasonable time.
Initially VW checked and returned the van to me with no fault.
Subsequently the van failed to start and the battery was flat.
VW assist advised the battery needed replacement. VW dealer carried out the required VW factory check (twice) eventually the battery met the “fail” criteria set by VW !
The starter batter was replaced (yah)
you may not want to know this next bit, but it’s worth checking to prevent potential longer term issues if not rectified.
I also had an intermittent cranking issue, there was no rhyme or reason as to why & when it would do it.
after the battery was replaced the problem seem to abate.
symptom initially presented as cranking for 10 seconds, not starting, turn ignition off, next crank the engine would fire up!
typically this would not present for a few weeks , possibly months At a time.
VW dealer initially said they could not find a problem & returned the van to me.
the problem persisted for some time (very intermittently)
when the problem presented, the starter battery would also drain because of the additional cranking. I kept the battery health 100% via a 240v smart charger as required (occasionally)
eventually I experienced one instance of rough running after starting. The van ran perfectly after this , no loss of power no warning lights etc.
I reported this to VW dealer, who said to keep an eye on it and bring the van in if it did it again
Shortly after (within a week) The van then failed to start. three or four attempts to crank 10 seconds each time.
VW assist attended and experienced the issue, the van was taken by flat bed to the dealer.
the VW dealer acknowledged the non starting,
they got it running but it was rough running they asked to keep the van to diagnose the problem.
diagnoses :
potential faulty HP fuel pump ? (i Raised this with the master tech when I reported the previous issue)
it’s a known issue the HP fuel pump (Bosch) the internal parts break up dropping minute swarf into the fuel system, eventually blocking injectors etc.
the Master tech had to get a bit of kit from VW:
effectively the equipment is an inline filtration system for the fuel, it traps any swarf in the system, (trapped on a inline filter) whereby it can be visually identified Confirming a HP fuel pump failure.
after the test, there was no swarf identified !
the Master tech asked if he could keep my Van for another day to repeat the test, particularly as he’d seen the rough running after not starting (Naturally I agreed) presumably the fuel filter was doing its job.
after the second test a minute amount of swarf was visually identified in the test kit filter.
The presence of swarf identified the fuel pump was breaking up.
the Bad news is the entire fuel system needs, fuel lines, filter, injectors, HP fuel pump, lift pump etc needed to be changed and the fuel tank needs to be removed from the van and flushed out. (fuel tank needs to be practically empty to enable it to be removed)
The reason the whole fuel system needed changing is that the unused fuel (from the high pressure side / common rail ) is returned to the fuel tank which means that swarf is deposited through the whole fuel system.
I sincerely hope this in not your problem as it is an horrendous cost (mine was under warranty).
I wish you luck in identifying and sorting your problem