I have a 2003 F150 FX4 with the 4.6L engine. It has 114000 miles on it. My service engine soon light started coming on when I turn the AC on. The codes are P0171 and P0174. I have learned these codes mean the truck is running lean and may have a vacuum leak somewhere. I wondered if it happening when the AC is on would lead me to a certain area to look or it's just putting the truck under a load and that's the only connection to the codes? Side note, the truck seems to be running fine other wise with or without the codes. It did surge slightly sitting in the drive way when I turned the AC on Max air. It went from 600 to 1000 a couple of times. The SES light didn't come on. I turned it off. The next time I turned it on a few minutes later SES came on without surges. I can drive the truck 15 or 20 miles without the SES light coming on but when I shut the truck off, then get back in a few minutes later and start it back up, is when the light will come on. Anyone have any insight? I like to educate myself a little before I take it somewhere so I will understand what a mechanic is telling me and let them know I know a little bit so don't try to mess me around. TIA for any info.

 

Start by fixing the intake vacuum leak. PCV hose and elbow around the back are the usual locations.

 

So I changed the pcv valve, hose and elbows. So far no SES light but the truck starts, acts like it's going to die, then idles high then levels off. Then idles fine. What would cause this? Wasn't doing that before even with the SES light on.