Just bought an 88 vert with no compression in rear rotor. When I took off the plugs in the process of trying the ATF trick, I noticed a trailing plug showed no signs of having been fired. Once I started the engine back up, I noticed the redline alert didn't work. How could a bad coil, driven over time, effect compression - what happened? I suppose it's academic, my engine probably needs replacing/rebuilding, right?

 

I don't believe a non firing trailing plug would be the cause of compression failure. If the trailing plug(s) did not fire, there would be a definite loss of performance, but no reason for compression loss.
My '88 TII which I purchased new, had a problem NGK and Mazda engineers were trying to conquer. That being no spark on the trailing plugs. I put 28,000 miles on the car waiting for the '89 TII to appear @ dealerships. Mazda bought back my '88, but never a loss of compression.

 

your car should really run fine without a trailing plug. you'll be down some ponies but that's about it. It's there mostly for emissions.

 

Thanks for the input. Apparently, my loss of compression is in no way related to the trailing plug or what caused the trailing coil to go bad or become inoperable.

If it looks like a duck, swims like a duck and sounds like a duck, it's a duck. My engine's shot!

 

Don't feel bad. Just had one put in my '89TII.