Hah! The problem returned again. I pulled the injectors again and found that #2 was shooting a straight jet of fuel.
I have another spare injector, so it's gone in now also and the car is once again running well.
It's possible there were just two intermittently bad injectors, it's also possible I got turned around during the earlier diagnosis and swapped out the wrong injector.
But in saying all that, none of them have the correct spray pattern, #3 and #4 are "OK" but still have an inconsistent fan. The spray pattern improves at WOT but at low loads it's really bad.
That last comment is something I think if you are reading this considering whether your K-Jet injectors have problems, don't just open the air flap right up, lift it just a bit so the injectors are only just flowing, that's where the inconsistencies really make themselves obvious.
One thing I notice is that as I mentioned before, the -015 injectors have a higher opening pressure than the -007, and this results in the 015 injectors staying shut when the air flap is ever so slightly lifted, while the -007 injectors dribble fuel. I don't think this is a big deal, because even at idle in neutral the air flap is lifted much further, but it may explain why Volvo switched to the higher opening pressures.
