Hi I have a 96 Chevy S10 4.3 that has been throwing the code P0304. I have changed all plugs, wires, new distributor cap and rotor and a new coil. Unfortunately I still have the code. Although, it doesn't sound like it is missing. This past weekend I was doing some brake work - changed the master cylinder and the abs sensor. When I got it all back together I wasn't showing the misfire code anymore and I didn't unhook the battery. I drove it for a couple days and still no code. I didn't understand but I thought great! Unfortunately, I was under the hood yesterday and I noticed that when I removed the intake hose to have better clearance to the master cylinder I forgot to attach the fresh air hose to the back of the intake. This is the hose that is connected on the back that runs into the manifold. When I hooked it back up the code returned. Any ideas or could you point me in the right direction?

 

Probably an air leak in the intake manifold, or in the EGR or PCV system.