Got a 99 with the 4.3. Started throwing a P0300 out of no where while crusing on the highway. Changed the plugs and wires on it. Checked the cap and rotor. Seemed to be carbon on cap tips so i cleaned it up as well as the rotor and you can still feel a slight misfire. I checked my static fuel pressure and it was good and didn't drop what so ever, so I've elimated my fuel regulator. Misfire only occurs at idle and once you increase RMPS misfire goes away. Not really sure what else could be going on. Maybe a vacuum leak? (checked over the lines shortly and they seemed ok.) or maybe just time for a new cap and rotor?

 

see if this helps you out

https://blazerforum.com/forum/diagno...4-5-6-a-35153/