27k is a bit much. It misses half the injector pulses. I picked up a 47k potentiometer at Radio Shack today and will put it on there.
One other thought we could do is a voltage comparator.
Link. This is a very basic schematic that is generally what I was thinking of. Just the bottom half of the circuit is needed so it outputs when the voltage drop from vcc(battery voltage) is greater than 2.5V. The 10k pot in that schematic could be replaced with a pair of resistors once the value is known to make it trigger on a 2.5V drop or whatever is needed. So that circuit would add an op amp and 2 resistors to the overall design.
That type of circuit should work with just about any injector system and be very tolerant of just about anything that I can think of, as well as give a pretty clean output to the mpguino with just a standard resistor/zener input.