[Johnathan]

Ok, I'm still hunting this issue down (and I'm tired of throwing money at it). Turns out the #4 spark plug was fouled, and the plug wire had some corrosion on it at the distributor cap. So new cap, wires, and coil (also tested bad) all went in... Same problem.

I did buy a cheap leak down tester and compared cylinders I was getting about 3% on #4 (unless I was performing it wrong). I could hear a faint leak from the tail pipe, and a slightly louder, but faint leak through oil fill. Does this sound normal?

I'm at the point where I'm going to pull of the HE and see how the gasket and exhaust value look..

Would a damaged valve/valve seat cause a plug to foul?

Any other ideas?

The car runs great otherwise, and I have a HPDE coming up, could I do any further damage this slight backfire at idle?