Quick summary: My P0304 code was caused by an intake manifold gasket leak on the #4 cylinder. When I reinstalled the intake manifold after doing the PCV, I ran the lower bolts in pretty far before sliding the manifold onto them, because they are tricky to get to. In the process of installing the manifold, I caught the gasket at the bottom of # 4 and pushed it down to where it was leaking. Lesson learned: leave plenty of slack on the lower bolts so that the manifold is not in contact with the gasket when you first put it onto the bolts.

Longer history: I had posted that I suddenly developed a P0304 code after doing the motor mounts. My experience has been that, if you suddenly have a new problem after you fix something, it is most likely caused by what you just did. So, after swapping coils did not make the problem move to a different cylinder, I spent a lot of time trying to track down a damaged wire to the #4 coil,which I assumed was caused by raising the engine. Finally, checking the #4 coil with an HEI spark tester showed that the coil was firing, and I was chasing shadows looking for a damaged wire. So, I swapped fuel injectors and still had the problem on #4. I already knew that the compression was OK, so that left an air leak. Using a spray can of carb cleaner and the thin straw that comes with it, it was clear that spraying carb cleaner around the #4 port made the idle speed change. So, the manifold had to come off. Once I pulled back the manifold, the problem was obvious. The gasket was badly damaged.

New gasket and care in reinstalling solved the problem. On the plus side, with new motor mounts and no air leak, the idle is really smooth. I still don’t know why I was able to drive 50 miles with no check engine light, after the PCV service and before the motor mount replacement. I suspect that the computer can only detect an air leak at idle and the engine just didn’t happen to be idling when the computer ran its checks.