My service engine soon light is on. I took it to the local shop and they said the codes are P0171 and P0174 - lean codes. They kept it for a couple of days and ran some tests. They did several smoke test to check for a vacuum leak, but could not find anything. My light is still on, but now my gauges are acting screwy. Whenever I start my truck, all of the gauges go all the way up and back down, then rest where they are supposed to. Ex, speedometer goes all the way to 100mph and back down to 0. Ex, the gas goes all the way to full tank and back down to empty and then to half tank or whatever is really in there. All the gauges are doing this. I guess my question is what is going on. Is my truck actually running lean, or are my gauges screwing up and sending false signals.

 

The most common cause for codes P0171 and P0174 are a rip/tear in the PCV hose elbow. Other causes can be other intake leaks.
As for the gauges jumping up when you turn the truck on, that's your battery going out and the ECM is recognizing the voltage drop.

 

I will change my battery and see if that works. I did replace my pcv valve, but not the little elbow. I will give that a try. Thanks for the info, I appreciate it

 

Ditto on the battery going out as that happened on mine , and my gauges were doing the same thing . If you have a Tuner on your truck , the low voltage can also cause it to malfunction/act strange .

 

did you get your problem fixed?
I tried all the above and it ended up being the gasket and screws on the intake that was leaking but could not be detected any other way.

 

I replaced my battery, which was bad, and my gauges are working as they should. The service engine light has not come back on yet. I am not sure if the code was cleared when I took the battery out and will come back on, or if the battery was the problem, causing a hick up and spitting out a false code. I will know more in a few days if there really is a reason it is running lean.

 

Did you look at the PCV hose elbows?

 

I also that problem with my 5.4 2001 F250 Super Duty. I did all I could on the outside off engine. Put it in the shop and found out it was a broken valve spring.
I hope yours is a simpler fix.
Just something to think about.

 

I replaced the battery a week ago and so far so good. The engine light has not come back on since, hope it won't.