japie:
Interrupts that open injector, wait injection time and close it. (during that time the loop is stalled but who cares)
As well as being ugly, I'm not at all convinced that you will get it to work. What would IMO be far better would be to use the timer interrupt to start the injection and at that time configure a timer to trigger another interrupt when the injection needs to end. This way the real time part of your code runs with minimal latency and does not lock the processor up for the duration of the injection, and the background processing in loop() can run and do whatever it needs to. Just ensure that you don't do anything that disables interrupts.