The warmer and not-so-snowy weather has been a treat and allowed for additional troubleshooting of this misfire issue I've documented here.
Discussions w/ Joe Perez and DIYAutotune left me w/ the symptom of a poor CKP signal causing a loss of spark at above 4.5k. If the signal was bad from the stock CAS, then it needed to be filtered. I'm not into making circuits, so I arranged a Dual VR v2.1 from JBPerf (Jean Belanger). It arrived last week, so this past weekend, I reviewed the latest (2015-10-12) MS3X V3.57 hardware manual as Jean's instructions detailed needing a pull-up resistor for the input, which sounded similar to what I had read in the MS3X setup documentation (https://www.diyautotune.com/tech_art...azda_miata.htm
I determined that my DIYAutotune MS3X did not have the internal 1k pull-up at R57 on the main board, so I installed one early December since Jean's instructions also called for it.
I reinstalled the MS3X w/o adjusting the VR pots and logged this:
Since I was losing sync above 5k, I was fairly certain I wasn't meeting the voltage threshold required to create a CKP pulse to fire the coils. Adjusted Main and Expander pots to get 2.5V w/ 5V applied (at top of R54 and ZC test point, respectively), I got the following (log was huge, so just a screenshot):
Nothing crazy, just shows pulls to 7000 rpm…but after months of not being able to do this and not being able to figure out why, it feels like a small win!
This allowed me to continue tuning the VE Table (as you can see, I had conservatively WAY too much fuel above 5.5k) and start exploring closed-loop idle tuning and AE for throttle tip-in.
I appreciate the help along the way in troubleshooting this issue. The solution forced a much deeper understanding of the MS3 and how it triggers the Miata’s coils via the CAS, but in the end, the idea was to better understand engine management…so another win!
Key words: MS3, MS3X, high-rpm misfire, no tach signal, no rpm signal, no sync loss, R57, pull-up resistor, VR circuit, crank input, CKP