Hello I have a 1999 F150 with the 4.2L and it has been running very poorly. I found a few vaccum leaks at the pcv valve in the rubber 90 degree. I fixed that and it ran great for 3 days. Now its back to running poorly again and wouldnt go over 50 mph as I limped it home it kept slowing down and now it wont go over 10 mph and its bogging out bad. (no power) has anyone had the same thing? just looking for a place to start . Trouble codes are bank 1 sensor one lean and bank 2 sensor 1 lean. both O2's are new just did a tune up a month ago and had to put both exhaust manifolds on. one was cracked between cyl 4 and 5 and the other had a hole right at the flange by the head on cly 3.

 

How does it idle? Manual or Auto? When's the last time you changed the fuel filter?

 

It has a rough idle and I think those codes were from the vaccum leak. now it is throwing random missfire and missfires on cyl 4,5,6. Its been a while since the fuel filter was changed I say maybe 4to5 month ago. The exhaust is very very rich all the o2 sensors are reading around 700ish Mv but one is sitting at 4 Mv so I know that is a problem and its bank 2 sensor 1. Also I noticed a pretty good rattle coming from bank 2 pre cat. Maybe partialy clogged? And it is an automatic.

 

Clogged cats.

 

Sounds like it's time for a new exhaust.

 

Collapsed muffler baffles or a pinched pipe can do that too. I'd take it to an exhaust shop for a full inspection, most of them these days have an infrared gizmo to check for clogged cats.