Hello forum I'm new here so if I post in the wrong place or something I apologize I have a 2007 f150 2valve 4.6 at a 120,000 miles the number 3 cyl went out 0 compression I used a endo scope down the spark plug hole and found in one spot it looked like the piston had come away from the cyl wall I replaced the eng with a used motor with 60,000 on it the new motor has been working great for 2years no problems the other day I was on the freeway and noticed it was missing and had no power I got it home and scanned it I had multiple cyl misfire code p300 p303p305p308 I figured the coil packs had gone out I replaced all the coil packs and spark plugs cleared the codes started it and it still ran crappy I just did a compression test and number 3 has 0 compression the rest are right around 130 has anyone run into any problems like this its crazy both eng would loose number 3 both at 120,000 miles thanks

 

Have you looked at your valvetrain? Could also be that the #3 spark plug threads are damaged and is leaking compression.

 

I've checked the plugs and threads there fine

 

Check the injector for that cylinder. The o-ring could have gone bad, allowing air to escape.

If it's not that, it's time for engine disassembly to check the head gasket or the piston.

 

Did you move any components over like the intake or injectors. Bad injector orings would not cause compression failure . But a leaking injector can hydro lock cyl and ruin it or blow head gasket . A failure on the same cly on two motors is very odd. No 3 is hard to get to but so is no 4 for testing and plug change . Did you scope it and do a wet comp test . Are these the same heads or something like that ? I would think you got a complete new used motor .with even new fuel rails .This would be a completely odd happening since there have been no reports of a weak 4.6 cyl manuf defect but what do I know ...

 

the injector could have ruined the piston ring on that cylinder. very possible

 

Thank you everyone for your help I went to do a leak down test on it and removed the valve cover so I would know when I was at TDC found a broken exhaust valve spring got all the parts out except the keepers hope there in the oil pan I have the heads off my old engine that are in good shape does anybody know if I have to pull the front cover to change the head? If I can don't without pulling the cover it will save a ton of time