Ford Expedition Problem...
 

Well, I'm not sure where to turn from where I am now, so I'm throwing it out there for anyone who might have some experience.

I have a 1998 Ford Expedition 5.4litre 4x4.

I get a flashing code P0304 Cylander #4 Misfire. When this happens, it runs rough. Then it goes away. It'll bump in and out of rough feeling ride, shaking, then perfect, shaking etc. It seems at idle, it's fine. Coasting, it's fine. Under acceleration it does the in and out lightly rough run, my keys jingle. Heavy acceleration, it shakes hard, and will sometimes cause the CEL to start flashing.

So, thus far I swapped #2 and #4 coil packs(ug... about as easy as an SVX), to see if the problem followed the coil packs. No go, remained at cylinder #4. So I pulled the spark plug, and it was fowled pretty bad. I thought, "Lucky!" and replaced it. No fix... So my thought of why it looked so bad, with build-up on the tip, was because of whatever is causing the issue.

Also, when it is misfiring, it shakes really hard. However, if park it, and rev it, no problems. It's only under load. Also, it's not high rpms really either. Dominantly at 1500-2000rpm in 4th gear, hitting the gas to increase air/fuel, just before the point where it would upshift to 3rd, so it's dumping a lot of fuel in there, and accelerating. If it upshifts to 3rd, jumps to 2500-3000rpm, the problem of the shake seemingly goes away, or is significantly less noticeable.

Suggested to me by my buddy at Advance Auto Parts was either EGR or IAC. I thought, that doesn't sound right. His explanation was that the cylinder #4 may be more worn, if there's a vacuum leak, that would cause the problem. I'm going to call around first thing tomorrow to see if anyone can do a smoke test for me to try and trace down vacuum leaks.

Also, is there any chance the transmission might have something to do with the problem?(unlikely, but still...) When I go around tight bends from a stop, i'll feel something from the front driver side wheel, almost like binding, but not. 4wd is defidently not engaged. But something does feel off.

Well, throwing it out there to see what comes back. Apparently it's happened before to folks on the internet, but no one has posted the solution. Or at least not one that I've found applicable to my situation, yet. Appriciate all the help you guys are. Thanks.

An injector gone stupid perhaps? Maybe the injector for that cylinder isn't delivering enough fuel under higher load?