Truck has a rough idle with lean bank 1 and 2, codes P0171 and P0174

LOGANMECHANIC

MEMBER
2003 FORD F-150
5.4L
V8
2WD
AUTOMATIC
226,000 MILES

First thing is a rough idle and sometimes shakes until truck finally shuts off. I have changed the ignition coils, regularly checked spark plug gaps and replaced, checked for vacuum leaks, but to no avail. I have even checked the resistance for the ignition coils, but they read good after replacing. The only thing I know for sure is that the cylinder 1 continues to have a misfire code. Any advice on what to check next would be very helpful thanks.

1 Reply

SCGRANTURISMO

MECHANIC
4,897 POSTS

Hello,

I have a Mustang GT with a 4.6L engine which is identical to yours the only difference is your engine has a longer stroke and from experience I would almost undoubtedly say that this is an vacuum or intake leak. It is extremely hard to check the intake manifold gaskets in the engine's valley and that is where I had a leak and I was pulling my hair out for weeks trying to find it. Please double check the intake manifold gasket for leaks by spraying starting fluid or similar by the intake manifold gaskets, inside and outside and listen for the engine to pick up. If it does than you have found your vacuum leak. If not, please get back to us and we can go from there.

Thanks,
Alex
2CarPros