Its buried in the harness pretty good.

Maybe check resistance between the ECU and the sensors. Might tell you if there is a partial break somewhere.

Do you still have a megasquirt? Can you take a log of it happening? Might be able to catch a sync loss.

I'm sure you dug through M-S about this (if not, you should) it seems to be a relatively common issue with various culprits. Have y'all checked the egr valve?

Don't just focus on the timing sensors, it very well may not even be related to that system at all.

I'm not versed enough with the Megaquirt to see if a log there can help seek out the issue. The only reason I knew there was an issue was changing back to the stock ECU.

So, it sounds like analyzing some data logs to see if anything comes through with enough resolution is worth a shot?

What you would do would be to log for 15 minutes and see if the sync loss counter went up. If it does you need to take composite logs to try and catch the sync loss. Then you can look at the cam and crank logs and see if something is amiss.

Cool. Guess I need to get some plates on there then.

I would do this, and if it does not show you anything I'd get a scope and start checking sensors and outputs until you find the issue. With a scope you should be able to diagnose the issue in a few hours. If the cam/crank is clean according to the MS log, check it with a scope and then scope the injectors and coils. If it's actually a misfire from loosing sync in some way it will drop an ignition event or injector firing, which is very easy to see on a scope. You can also scope every other sensor to look for any odd behavior.

So, the P0300 did not come back after driving it around and I thought I had finished the drive cycle so I went to emissions and while waiting in line got P0106 instead. I'll investigate this later for the manifold pressure sensor range. I'm still not convinced that the random missfire is solved, either.

Any updates on if the P0300 returned?

It will likely remain unsolved unless someone buys the car from me and figures it out. It will stay for sale until the spring at which point I will fully part-out the car and afterwords, drag the chassis out of the garage with my bare hands and toss it down the driveway.

Does the code not trip if you keep the RPMs say under 4k? I had this exact same thing happen to me last year when it was time for smog check. The only way I was able to pass was to drive around staying under 4k RPMs until all my readiness were ready.

I'm super surprised that the car turned ready with no rpms over 4k.

Even if that bypasses the p0300 to get tagged, I still have the issue, and other CELs have emerged that also need attention. So, I might try this but likely unbolting the whole this is easier at this point.

Just an update on this, it now throws a P2006, which is a VTCS system error, and then the P0300, which I believe is a lean misfire. I've found one thread where someone discusses the P2006 here:

https://forum.miata.net/vb/showthread.php?t=485541

With, of course, no resolution. I've visually inspected the system externally, it stays cold until over 3000 rpm when cold and then stays open above 140 degrees.

At this point it may need a lower intake manifold clean, or it could be something random like an EGR valve needs cleaning. I'm not hugely familiar with the system.

The car is still for sale however, so I thought I'd update this thread. Car still drives very well however.

Curly, can you scope the cam and crank sensors?

Yeah, that is what is so interesting. And "wel"l being relative to the stock ECU of course.

And....the car is sold.