9/10 times in my experience this sort of random massive over-fuelling is due to a sticky injector (getting stuck in the open position). I think by the fact that you have found that when you remove it and pump petrol through it things work for a short while is a big clue.

You could try running some injector cleaner through and it's probably worth running the injector test in diagnostic mode. Realistically though, the only way you can identify/cure the problem is by substituting your injector for a known good one.

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