I'm working on a 1998 Expedition with a 5.4. I have lean codes and intermittent rough idle. I've replace spark plugs and fuel filter, checked for vacuum leaks with a smoke machine, and the fuel pressure is within specs. I used my scanner to check cylinder balance test and it show miss-fires on every cylinder and a pattern of miss-fires shows up. after about 5 minutes of watching I can predict which cylinder is going to miss-fire next. Can anyone give me an idea of what could be causing this condition?

 

Coils? Run some Techron through the gas, you may have dirty injectors.

I know you used a smoke machine, but manually check the PCV elbow behind the intake. It rots and leaks.

 

I felt back there for that leak before and hose wasn't soft on bend like I've found before Just tried to pull elbow off and it ripped in half. I guess next time I need to take it off to inspect it but it a bear to get to on this one.

 

If it ripped in half, it was already bad. It's a lot easier to get at if you remove the doghouse - even easier if you also pull the throttle body and elbow to clean them.