Pardon me for thinking out loud here, but maybe someone reading this will have an idea.

With a brand new distributor from DUI, new wires and plugs, I can be reasonably certain that spark is not an issue.

With new injectors, proper fuel pressure, no fuel rail blockages, and good wiring, I can also be certain that there are no fuel delivery issues.

Having performed leak down and compression tests, torn the engine apart, had the heads inspected, and changed the camshaft, it's safe to say that the engine is mechanically sound.

Having gone through the TPI harness and ohmmed out each connection along with putting the so-called "noid" light on the injector plugs and swapping injector wiring banks from side to side, I'm sure that the wiring is not an issue.

Since all sensors have been verified with multi-meters and/or my scanner, it's probably within reason to assume it's not a sensor issue.

With an out-of-the-box factory 1990 Z28 EPROM still having the same misfiring problem as my custom PROMs, I can rule out my custom PROMs as the culprit along with my new ECM.

Have I missed anything?

What about vacuum leaks? If there's some persistant leak that I've failed to seal in the last four intake manifold removal/replacements? I can't see the mechanism for a misfire from a vacuum leak- especially on a MAP car. Am I wrong?

 

Did you - or anyone who owned the car before you change the timing chain or pull the distributor shaft?

I went through the same thing you did once and here is what happened. This may sound odd - I had this happen to me on 2 occations.

I jumped the gas at a light and the car ran like crap (missing on 1 or 2 cylinders). I did everything over that weekend - checked and re-checked. Oddly enough I found the problem - the timing chain jumped exactly 180 degrees.

 

When I pulled the cam, I inspected the timing chain as well as the gears and found nothing unusual. The timing dots were still in alignment and the chain was not loose by any means. Since I have less than 20,000 miles on this engine, I'm not surprised. The lifters have also been replaced.

Again, the distributor is brand spaking new. When I installed it, there was no change in the misfire problem from my old distributor (which, by the way, I had recently rebuilt).

 

Well it sounds liek you have all the sensors and externals covered is it possibly a valve or valve train issue?

 

Wow, it's been a while since I posted this...

Here's the latest...

I've discovered that there's apparently an inline 1.5 ohm resistor on my primary 12V ignition feed to the coil. Since I have a 1971, the original distributor was a points type. When I converted to HEI, I never by passed that resistor. Somehow I got it in my head that the resistor was an external one and only now did someone mention that it's internal! Anyway, this Saturday I'm going to fix it.

Funny thing is, I have had an HEI for the last eight years or so and if this resistor wire is truly the problem, why is it only acting up now?