1997 F-150 4.6L 53K miles

I have been getting P0171 and P0174 codes for a few months. I replaced wires, plugs, MAF, filters, PCV and coils. I finally took it to the dealer and they told me that cylinders 4 and 8 are dead. One has zero compression and the other has 3 lbs.

Two questions occur to me:

1) What might have caused this? (both apparently went bad at the same time)
2) What are my options now?

I appreciate any input.

 

No compression is bad and could be caused by a few things.

1. Broken pistion rings
2. Valves stuck open or shut
3. Cam lobe wiped out

You can remove a cam cover and check that the valves, springs and valves are operating correctly.
Just crank the engine, don't run it with the cam cover off.

You can remove the sparkplug and squirt a few drops of engine oil into the cylinder. If that restores compression briefly then it's the piston rings.

You can buy an air fitting that screws into a sparkplug hole.
Get the dead cylinder to TDC and blow air in.

Then listen at the intake, exhaust or oil filler and that should point to where the trouble is.

Good luck.