I did this a while ago, and I was never able to completely get rid of those false triggers (see the pic with the 1 little noise spike going above the trigger level)

I've got some ideas on how to get a nice, stable, clean signal but I haven't tried them yet. Most of the ideas include things already tried like more sheilding, a better 8 volt power supply, or maybe some kind of LC filter on the input. An inductor might work, or the correctly sized capacitor. The thing about the capacitor is that if you put one in there that is too small it won't be able to smooth out the noise. Too big and the square wave signal gets all rounded off.

2016 WRX STi Limited (current)
2001 A4 1.8tqms (donated)
1991 GVR4 #1933 (sold)
old.dsmregistry.com/detail.php?carid=218 (dismantled)