I have a couple codes being thrown and I am curious if they are related to each other and be causing my truck to run lean.

(P0171 and P0174) Bank 1 and 2 running lean
(P0401) EGR flow insufficient detected
(P1506) IAC overspeed error (see P0507)

My truck seems to have always run lean and the IAC code is somewhat new. I recently have had horrendous gas mileage. 1/4 tank within 2 days. Sometimes 2 days.

Truck is a 1997 f150 4.6l v8. Hitting about 208k miles

 

I can go through a 1/4 tank in 3 hours. I'd say your doing great.

Get yourself a new IAC. Unless the gasket is bad, that's not the problem.

The 401 is either DPFE or most likley and since your a 4six, your EGR ports are plugged inside the plenum (TB Elbow). Yea, that's only a 4six problem. Those 4sixes SHOULD have a 5four EGR system. Then the problem would never happen.

The 171/174 nah, no relation unless the IAC gasket is bad or forgotten in some cases...I doubt it. Those codes are kind of like a bad gas mileage thing. There's too many variables that rely on good common sense, exceptional maintenance and troubleshooting. But for just the lean codes, -you start at the air box seal, MAF o-ring, IAT grommet, ALL tubing connections (specially at the TB, I've seen many leak at the bottom).

To get your mileage back, not that it was ever great to begin with as far as maintenance. Run a proper air filter and clean the MAF. Of course you wouldn't have to clean the MAF if your maintenance regimen is good. You have to go through exhaust connects, update sensors. I guess one the most critical sensors that are neglected that directly affect fuel are the O2 sensors. Most just replace the 2 up front. I never understood that until I found many posts stating the downstreams sensors have nothing to do with fuel. That's one of larger misconceptions I heard of on the internet. One persons BS went big that's for sure. When your going for a well running truck/engine, you start at the rear O2's AND the ones up front...not just the front lol. They are most definitely NOT there to make sure the converters are compliant...that is a BIG lie. One of the biggest lol. Slow switching down streams force your AF's in limbo...they get really bad with age. Every 100,000 miles, the O2's SHOULD be changed. Response time is everything and you could say it's one of the larger things that may be contributing to one vehicles bad economy.

Anyway, that's a little bit for yuh.