Our 2008 expedition (148k) had the heater hose bust over #3, leading to a tune up; replaced all plugs.
Under load, at about 3500 rpm, it will now violently misfire (at first, I thought a piece of spark plug was tearing up a piston).
Auto Enginuity shows codes for a #3 misfire. Pulled the plug; looks fine. Replaced the COP/boot and it still does it.
Any ideas?

 

what plug and cop did you replace it with? Check that the connection did not somehow get wet or dirty during the plug and cop swap. If it seems good, swap the cop with a known good one. If it is still there I would then swap injectors. Still there? Compression test.

 

I had bought a new coil.
I swapped injectors 2 and 3 and it's still giving a misfire on cylinder 3. I also got a noid light to make sure that the FCM was giving a good signal... it looks good.
I don't think I mentioned before; this only happens at higher RPMS (2500+). It loses power and there's a loud popping noise. I was thinking maybe fuel was getting in the exhaust manifold and causing the popping, but out of curiosity, I left the #3 injector unplugged after I used the noid light and ran it down the street still popping: what else could be causing that?
I got a compression gauge, but it looks like I need an adapter to fit it in the spark plug hole. Ordering that tonight.

 

Compression test was good. Any ideas?

 

Took the valve covers off. The timing chain on the driver side is loose... probably a failed tensioner or guide. The timing cover comes off this afternoon.

 

Ok. After changing the timing components (both tensioners had the gaskets blown out... guides and chain looked okay, but I changed them anyway), it still had the problem. Took it to the muffler shop for a backpressure test, and the passenger side cat is bad.
At least now I know...

 

What codes were you getting? I have a similar problem (see my post) I get a Too Rich Bank 2 P0175 code and I get a random popping sound and loss of power. Just wondering if my cat is bad too?

 

All I was seeing was a cylinder #3 misfire. At one point, I got a #8 injector malfunction and a bank 2 O2 lean; but I think it was just a loose connector. The local muffler shop did the backpressure check for free; they just pull the O2 sensors and check if there's a pressure drop... you can probably rig something up similarly (plumbing fittings, tubing and a 0-20 psi pressure gauge) if you can't find someone to do it for free.