Intermittent Spark
 

Recently did a tune up on my 98 Blazer, changed plugs, wires, cap, rotor, to repair random cylinder misfires that triggered DTC Codes. Now I am getting the P0304 code, along with idle issues, black exhaust, fuel smell, all distinctive misfire symptoms. Tracking down possible causes I did a fuel pressure test, holds fine just under 58PSI for 15 minutes and holding as I ceased the test. Thinking not fuel issue. Compression seems ok, didn't actually do a test, Blazer runs fine with the exception of misfire. Pulled the plug, checked for signs of malfunction, no cracks, chips, or signs other than black sooty plug. Again, consistent with misfire. Fired engine to check spark at the plug, bright bluish spark but intermittent. Fires fine then quits, fires again, quits, random no spark at the plug. Did research online and checked through various threads seeking info on the coil, or ICM, to limited success. Is it more than likely the coil is faulting here? Any help appreciated.

Your problem is amongst the listed above.
From what You have written it seems the distributor cap may not match the rotor (or any other things that prevent high voltage passing through). Do You still have the old ones?
As for the spark plugs, You may put back an old wire in between coil & distributor and one of the plugs and see if that changes anything...
If it's misfire of a one cylinder only, less likely it would be the coil. A damaged coil should DTCs for other cylinders as well. Not only that exact one.

Not 1" no, don't think so. Plug fired but not like that. Distributor not removed, checked timing with #1 @ TDC, right on the mark. 99.9% certain rotor is correct, will only go on in one position

Should also mention all replacement parts are of the AC Delco brand

But still You had random cylinders before and now it's a 4th (P0304) one only?

Yes, typically previously the misfires were random, the odd time get specific to #3. This turned up after replacing the components listed. A rotor would be a quick easy swap to verify. Did not keep old parts

The old cap and rotor showed signs of obvious wear, burned tips on cap with some arching too. Rotor likewise had tip fried looking and signs of arching and blackening

Got it.
So it seems to me that Your ignition module, coil and a wire between coil and a rotor cap are OK. Otherwise the misfire would occur on a random cylinder. There could be something within:

• passing the spark from rotor to a cap or
• a cable from cap to a spark plug or
• the plug itself

I think You may try swapping cables (and plugs) between #3<--->#6 and see if the misfire goes to another cylinder. That would give an answer to the last two options.

OK I changed the rotor out, ran bit better but still missing. Changed plug, again ran bit better but still missing. Went for short trip to gas station,1-1.5 miles and back, started throwing P0300 code, at least not cylinder specific. Put old rotor back in, still running better. Seemed to be surging so started checking EGR valve, disconnected power no change, pulled PCV hose off. Immediate change in idle, swinging up and down to point of stall and noticed the vacuum has a surge like sucking sound ?? Sounds like a vacuum should but surges in intensity, shouldn't that be an even sound? Better yet, would vacuum difficulty provide idle issues as well

If your vacuum is surging when your engine is, that would be ok. As the throttle plate opens vacuum decreases. This strikes me as more of an effect, not a cause. I would do a compression test at this point to rule out any engine internals.