Problems after switching from 4x to 58x trigger wheel – unstable idle and light misfires

[tobsec]

Hello everyone!

Background:
My engine is a Chevy small block V8 which originally has 4x crank and 1x cam, running i a boat, converted to rusEFI. See also this thread.
I wanted to get rid of the distributor and do things properly with 8 individual coils, fired sequentially. For that, I bought a new trigger wheel from EFIconnection to convert from the original 4x setup to a 58x (or 60-2, depending on naming). Setting it up and getting the engine running wasn’t an issue – just some minor adjustments and of course dialing in the trigger angle advance using a timing light.

For reference, I’ve uploaded both tunes to rusEFI Online:
- previous stable tune
- new adapted tune

Problem:
Since switching to the new trigger wheel, I immediately noticed the idle is no longer as stable as before. Previously, I had a near-perfect idle using ETB closed-loop idle air control combined with closed-loop ignition timing control. Now, the idle oscillated, and I had to re-tune the timing control loop – reducing the integral term from 0.1 to 0.05 and adding a derivative term of 0.002. That gave the best result so far, but it’s still way worse than before.
Additionally, when driving at low RPM (e.g. around 1000-1200) with constant throttle (note: it's a boat), I feel small misfires. You can’t really hear them due to the underwater exhaust, but you can clearly feel intermittent deceleration in the boat’s movement every few seconds. Top speed seems unaffected, but I only did a brief test drive. I haven’t used the boat since then because I don’t fully trust the ignition control anymore.

Additional notes:
- I'm using two VW 4-cyl coil packs from a 2.0L engine (VW #: 06A 905 097). I confirmed a nominal dwell time of 1.5ms with one aftermarket manufacturer and bench-tested the voltage curve carefully for the voltage multiplier.
- To rule out coil-related issues, I reverted to the original single coil and distributor setup while keeping the new trigger wheel --> no changes.
- Logs of the test drive showed no trigger errors (-> can upload if someone is interested?)
- I did extensive bench testing using an Arduino to simulated trigger signals (matched to what I scoped on the running engine). I verified ignition outputs without any observations. I even randomly introduced single missing tooths which were correctly detected and reflected in the counter.

Now I'm stuck, looking for any ideas what can cause this behavior. Theoretically, ignition timing should be much more precise with the 58x wheel compared to the 4x. But in practice, the engine runs worse. I'm thinking to revert back to the 4x wheel if I cannot solve this issue, I'm searching for a solution since almost one year now and I'm a bit frustrated, I’m even considering buying a good logic analyzer to record ignition/injection events while driving to see if any events are missing or misaligned.

Any ideas or suggestions are highly appreciated!

[AndreyB]

high rate data log to see how exactly it looses sync - is it extra events (noise) or is it gap ratios outside of expected ranges?

what hardware? all capacitors legit values?

logic analyzer yes, the final step

[tobsec]

Thanks for the reply.

Just to clarify (might have gotten lost in my original post):
No trigger errors were counted during the test drive - the counter remained at 0 for more than 30 minutes of runtime, with RPM peaks of over 4000.

Regarding HW: I'm using my Proteus-based "Eidothea" (see this thread). I bench-tested the trigger signal path – rise/fall times and signal quality looks good, even at simulated signals up to 8000 RPM. Caps and filtering are identical to Proteus.

To me, sync and trigger signal integrity appear solid, but I’m seeing the worse idle and light misfire behavior since switching to the 58x wheel. It feels like one (or more) cylinders occasionally don't fire, or fire at the wrong angle. I haven’t driven it since autumn last year, as I don’t want to risk engine damage due to incorrect ignition timing.

Any further suggestions?
Btw: Has anyone done a direct comparision on the same engine between different trigger wheels (e.g., 4x vs. 58x)? Could there be an issue with 58x in general? If I didn’t have the direct comparison with how the engine ran before, I might have assumed these small misfires are just part of running a non-OEM ECU or tune.