Ill try to keep the background story short and to the point. I used to be a ford tech, so this kind of work is second nature for me.

I bought a 97 F150 with a 4.2 with 180,000 miles with a blown head gasket on cylinder 3. Had misfire codes for cylinder 2, 3, and 5. Replaced both head gaskets, intake gaskets, injector o rings, pintle caps, plugs, wires, coil. Runs good now expect for a skip at idle, bad enough to eventually set a P0305 code.

Swapped 4 and 5 injector, plug and wire. Code remains cylinder 5. Replaced coil with no change. Aside from the skip at idle the truck runs great. Towed a car 100 miles with it today. No issue off the line or cruising. Only at idle.

Checked all injectors with a stethoscope and they are all firing. Unplugged and capped EGR line just to see, no change. Compression is app 120 on all 6. Plug from cylinder 5 (has 200 miles on it just like the rest) looks the same as every other plug in the engine.

If you pull #5 plug wire while running there is a noticeable change in the idle, its gets worse. Acts like the cylinder is low on compression almost but its not.

I am lost and out of ideas.

 

Have you done a cyl. leakdown , air hold test ?No chance of a vac leak ? Almost sounds like a slightly burnt / warped valve .

 

One last thought , did you ck piston travel ? They all came full TDC ? Not a slightly warped rod ? I have seen that after a hydrolock . Just a slight tweek , lowering piston travel .

 

Have not done a leak down test yet. No vac leaks. That was my first thought as well. Did not check piston travel as the head gasket was not leaking on this bank. Funny thing is cylinder 4 has a small crack between the valve seats yet that cylinder gives no trouble.

 

Oh and I chec valves with carb cleaner while heads were off. No leaks past valves.

 

I find brake fluid works better than carb cleaner . If there is a leak , it will find it . I would do a leakdown test and running compression test next . I was thinking GM firing order when I mentioned piston travel . Lots of heads have small " cracks " between the valve seats . As long as they do not extend too far , no worries . We use to peen them , back when , if we found them .