Good job. If the misfire persists, there are several other things we can check into. Also, your 1999 is a candidate for a GM TSB involving a binding condition with the valve guides under heavy load acceleration.

 

Thanks. If it were a part of the TSB, wouldn't it likely read a P0300/P03xx for multiple misfires under heavy load?
I just took it down a muddy dirt road, and while I could feel it miss at WOT, and it did set a P0303 with SES light on, I looked and the wire was rubbing up against the exhaust manifold. Good thing the wires are on the way.
I guess when it cools down, I can wrap it with electrical tape and use a zip tie to keep it out of the way for now. Had a similar issue on my 93 Cheyenne, which had the TBI 4.3. The wires ought to fix it.

 

Binding valve guides will usually cause the check engine light to flash and set a DTC in memory.


Sounds like you need new wires too

 

It was probably arcing on the #3 wire. I've got it taped and out of the way, I'll go hit the dirt road and see how it does.
Still going to run MMO through the PCV line to help clean the valves. Did wonders for the Alero. Different vehicle and engine, yes, but same idea. That cool MMO hitting the hot valves ought to help break up the carbon build up. I probably shouldn't do it at home. I did it with the Alero near home, and it put up one hell of a smoke screen.

 

Just for laughs, I went to Autozone and got a 24" wire and modified it to fit. No miss at idle or WOT now, nor has it set off the SES light or any codes my scanner could see. Pulls steady and smooth. Can't wait for the proper wires so I can be finished.