Ok, Now what the hell went wrong. All I did was replace the stock exhaust with new pipe 2". and installed a Flowmaster 40 series SIDO muffler and two turndowns before the rearend now all of a sudden my check engine light came on with a P0171 and P0174 code. Both are for System Too Lean. Did anyone else run into this. Any idea let me know.

 

Forgot to tell you that my truck is a 98' F-150 4.6 manual tranny. the rest of the truck is stock except the 265 tires.

 

Check your o2 sensors did you remove or alter any when you changed your piping?

 

no I relaced the piping just behind the o2 sensors so I didn't have to mess with them. Did that on purpose.

 

I've got a dumb question, but did you unhook your battery to reset the computer after you did that?

 

yup, I unhooked it and waited about 5 min to rehook it up

 

What type of gas are you using? Around by me they removed the mtbe and replaced it with ethanol at some gas stations that will play havoc with your 02 sensors also

 

look in two places...I had exactly this problem....4.2 liter 6

1st place...under the fuel injection manifold...in the exact center on the bottom side... there is an elbow vacuum hose about 3/8 in diameter...the elbow decays over time...that's where mine failed...it is a bitch to find it and even more of a pain to replace unless you have small hands.

2nd place...at the back of the lefthand (passenger side) valve cover is your PCV valve...coming off that valve are some vaccuum hoses...make sure they are not decayed/cracked/collapsed.