I've been having this problem for some time now, code P0300 on my 1996 LT1. No other codes are being thrown. The spark plugs, O2 sensor, and MAF sensor are good. The car starts hard and backfires, especially under any load, when it hasn't been started for a day or so. I have a decent video of it if that'd help. It also occasionally puffs white smoke from the exhaust when it starts, although I'm not down on power so I doubt its a head gasket.
Any ideas what could be causing the misfiring? Fuel delivery, opti?

 

Check your fuel pressure. Might need a fuel filter or possibly a fuel pump. Once the fuel pressure is confirmed, you can move on from there. The opti is the go to for LT1s, but it is an 18 year old car at this point, so any number of things need to be checked as well.

 

Alright, I'll give that a shot this weekend. Any ideas on an easy opti test? I've heard of the 20MPH 5th gear full throttle test, I'll give that a go if its useful.

 

I would go on shbox.com to see what kind tests you can perform. Great website. Has helped me out a bunch on my SS.

 

Pull a couple plugs and see what that tells you.

 

Bump with an update, I replaced the fuel filter and tested the fuel pressure, it jumps to 40ish when I first turn the key but drops to under 10 after sitting for a second. Fuel pump?

 

You can try changing the pressure regulator at the rail, but I almost guarantee it's the pump.

 

Pump.

 

Super late update, pump fixed it. No codes, running well, only problem now is occasional puff of lingering white smoke on startup. The only thing I could think of is a head gasket issue, but there are no other signs of this as far as I can see. The engine feels strong, nothing looks milky

 

I might be wrong but that sounds like a bad valve guid seal if it only does it when you first crank it up. If the seals are bad it will let oil down in your piston thus when you crank it up it burns the oil off and that would be you smoke you see. Hope that helped