Hey everyone, newbie to the forum looking for some advice on my P0171 issue.

The truck is a 2002 Ford Ranger with a 2.5L I4 5 speed manual. Like I mentioned the truck has a P0171 - running lean B1S1.

Backstory:
Bought the truck about a month or 2 ago with 214k miles (wasn't too concerned about miles cause these things run forever if they're properly maintained.) After driving the truck for a few days I got a P0171 code. Fuel trims were pretty high, ST about +40%, LT about +10% Did some research, did multiple smoke tests with a vape at home, found many vacuum leaks and repaired some (big one was on upper intake manifold gasket), replaced the upstream o2 sensor, replaced the MAF, filled the tank with some 91 and fuel system treatment, and the code went away for about a week and a half with regular daily driving (enough for me to even pass smog during that window.) Fuel trims dropped to specs (about +/- 10% ST and LT.) After that week and a half, the code came back, fuel trims shot up to the same numbers as when I first got the code, and I decided to vacuum test one more time and it seems I have a pretty good sized leak coming from towards the back of the engine by the firewall and my best guess (I'm not 100% sure, smoke seems to travel upward from rear of engine) is its one of the two vacuum hose that connects to the back of the intake manifold. I have looked everywhere online for what these hoses lead to but I have had no luck at all. I have tried to follow the hose with my hand but since its so close to the firewall and the engine is in the way, I can't seem to pinpoint where the hose is routed to.



I'm also confused why the truck would run damn near perfect for a week and a half after everything I did and all of a sudden give me the same code again? My thought is if it was in fact a vacuum leak, the code would have never left and the truck's fuel trims wouldn't have corrected themselves after my repairs. Any help or advice on the situation from what I've given would be appreciated. Thanks in advanced.

 

Welcome to the forum

2002 Ranger has the 2.3l Duratec I4 engine, not that it matters much, but it is a completely different engine than the 2.5l Lima, 2001 and earlier
It does have IMRC which can cause problems, 2004 and up didn't have that

Yes, Lean code means too much "unreported air" or low fuel pressure

Vacuum leak is more common, i.e. unreported air
I don't have a vacuum hose routing diagram for this engine, but larger vacuum ports were used for Power brake booster and PCV Valve in most engines
The 2.3l can have an oddball setup for power brake booster, it used ported vacuum AND regular vacuum

Most vacuum hoses do not need a clamp since negative pressure inside tends to pull the connections together, not push them apart, but.....................

In some applications there can be a sudden positive pressure that will push a hose off, this creates a vacuum leak that wasn't there before

And PCV Valves can get stuck open, the 2.3l PCV Valve is a pain to get at, drivers side middle and lower down, just above oil pan
Ford recommends changing this valve once a year, just FYI
PCV is a controlled vacuum leak, but if valve sticks open then its just a vacuum leak

 

After taking a second look around the engine bay today I noticed that one of the vacuum lines was disconnected. I'm assuming it is just as you said, the line must have blown off by some rare case of positive pressure. I reconnected the hose, threw a clamp on there for safe measure, and the truck is back to running as normal. Fuel trims are good too. Thanks for the response

 

Good work

Thanks for the update and THE FIX