Did a little more today
From the circuit descritpion:
• U1b operates as a low-noise Sallen and Key, Butterworth low-pass-filter with a cutoff frequency of about 103Hz. R7 and R8 provide a gain of about 1.6 and allow the use of equal values for C3 and C4 but still producing a sharp Butterworth response. The rolloff rate is 12dB/octave. C3 and C4 can be reduced to 4.7nF to increase the cutoff frequency to 1KHz to hear respiratory or mechanical (automobile engine) sounds.
I have a sepehoscope from harbor freight I can just place it on the running injector and hear it "snap" on problem!
I dont like wearing it around moving parts and 100K volts!!
Some of these injectors are hard to get to! Infact some engine dis-assembly is required to get to them, and thats just to check them for mechanical movement. Doing this with a prob using sound or light short-cuts this whole process, for the shade tree DIY'er!
I did some experment today with a (portable phone) earphone,I hooked the ear phone to my meter and a red LED , when I tapped the earphone the LED blinked dimmly and got a reading of about 29.0 MV, AC, used a min-max meter. Tap was more like a slap!
Thanks for your thoughts!