I agree - only change I made to get the car up and running (really well, actually) was the ECU. I'll swap the coils just in case, didn't know that a bad coil could back-feed and damage the ECU? Good to know!

I'm somewhat worried about this (hydrolock), there was a *decent* (maybe a cup, cup and half?) amount of coolant in there - rods *may* be on my list of purchases, just to be safe. Your tip about cleaning the crowns and measuring length is noted - thanks!

 

Potential for computer damage, via coil failure, is really just a product of how an ignition coil functions. Long story short, a short or partial short in the primary winding is what kills the ECU drivers. Total current flow of a straight piece of wire with zero effective resistance vs. the windings of the coil primary side is significant. If there is a short within the primary, current levels can be higher than expected, damaging the driver. Manufacturers and their engineers have finally gotten wise to this, I went to a FCA training event a few weeks ago and basically every driver on their new ECUs have current limiters which will disable the drivers if excess current flow is detected. I seriously doubt the miata ECU has current limiting drivers, but you never know. IMO if there is a concern of a damaged driver in the ECU, the driven component must be thoroughly inspected. Unless you have a lap-scope, there really isn't a good way to do this. Really I would only worry about the coil which was not being switched properly. If the other one worked fine, I wouldn't say it's required to be changed. Of course, coils aren't crazy expensive either.

This is the enjoyable aspect of my day-job, so I enjoy nerding out about it.



Good luck with your exploratory surgery on the motor.

 

Slight Update:

Pulled the engine/trans. Disassembled the engine down to the bare block (notice the nasty, brownish looking oil - MAJOR coolant dilution):



Looked over the whole thing in detail - some minor scoring on the cylinders, pistons/crank/bearings looked fine. No rods bent (a concern was hydro-lock from the coolant in the combustion chambers.) No cracks (visible or felt with fingernails - very 'high tech' analysis.) No worn-in ridge/ring at the top of the cylinders either.

In the process discovered this - beside the oil squirter for Cylinder #4:



Some of you know what this is - caught me MAJORLY off guard - those ragged edges looked so bad - I spent the night convinced I had grenaded my block. Turns out NOT to be a hole / casting defect, rather the oil return/drain passage for the head. PHEW.



Haven't gotten the any parts to a machinist for measurement - nothing within 3hrs of my location - will do this soon. The overall condition of the block/bore/crank/bearings seem promising - possibly just need a good cleaning, honing, rings/bearings/seal job?

I did take a look at my valve lash (thanks Greg Peters) - nearly HALF of my lifters don't fully close due to poor adjustment. It's possible this is where my compression test issues (look back a half-dozen posts) seem to have originated. Still doesn't explain my primary engine issues.