I have a 99 Silverado with a 4.8 that throws a P0303 #3 misfire code. I've checked the obvious, new plugs, swapped wires, and coils, compression check (150 psi), and checked for an intake leak. No valve train noise to speak of so I don't believe its a lifter or lobe. It runs rough at idle, and just a hint of roughness between 1700-2300 rpm (will set code here, but not at idle), but smooth everywhere else. Any ideas? injector? Some are saying fuel pump, but not sure I agree with that.

 

Try swapping #1 and #3 injectors and see if the misfire moves with the injector. The injectors on the early silverados are prone to plugging so if #3 injector is bad you may want to replace all 8.

 

Yeah, that was my next step.... I have a second set of injectors, so i'll just swap it out. I just want to get it running decent enough to sell.

 

Well after reading all the forums, and reading how the dreaded p03xx has plagued many, it was just an injector..... 220k on it, and it runs great now. Good enough I may want to keep it, and do a Denmah style build... too bad its a rclb.
Thanks turbo68bird for the heads up on the early Silverado injectors, I didn't know they had problems.