I'm getting the unhelpfully P0171 code on my 1997 Aerostar and am having problems finding the root cause. Its only throwing the code when the engine is heat soaked when restarted but not every time that condition occurs and when the code is throw the engine is running fine has good power.

Problems I'm experiencing (never when the code is thrown):
- Sometimes the van has no power/acceleration I'll go to accelerate and take like 5 minutes to get to 60. Engine sounds fine but doesn't sound like its trying.
- When accelerating up hills under load vent/ac will switch to defrost and then cut out entirely until the hill is complete.

State of the vehicle:
- Low total miles, ~129K
- Fantastic MPG ~25
- No problem idling
- Van has sat for long periods of time
- Packrats have been a problem
- I have smelled rotten eggs from the exhaust serval times, but not all the time (exhaust system looks to be original)

Things I've done to try to fine/fix the problem:
- Replaced fuel filter (frequency of code being throw dropped after replacing the fuel filter, think it may have been original)
- Ran fuel injector cleaner through the fuel system
- Checked the PCV (fine)
- Checked for vacuum leaks with carb cleaner everywhere I could access including under the dog house

Ideas I currently have:
- Cats maybe clogged? Since I've smelled the rotten eggs seems like maybe that could cause lack of power problem. Don't know of a way to test it.
- I'd think there is some sort of vacuum leak because of the hill problem, but maybe in part of the system that isn't used at idle?

So that said any idea's on what I should check next?

 

The p0171 code means the engine is running too lean. The most common cause is a leak in the vacuum line, or intake manifold gaskets.

The AC going to defrost on the uphills is a definite indication of a vacuum leak. Take a close look at all the tiny plastic hoses running around the blower housing at the passenger side of the engine compartment. Follow each line to make sure none of them are cracked. A vacuum leak big enough to cause the engine to run poorly will also make it idle poorly.

I think your engine has EGR, which, when applied too aggressively, will weaken engine performance. But a vacuum leak can prevent it from working properly, so this doesn't seem consistent.

It's also possible that the oxygen sensor is failing. That happened on mine. The replacement is only $25-$30, so it's something cheap to try.

 

if the climate system goes to defrost during acceleration, the plastic vacuum tubes are cracked. You can splice the cracked sections with vacuum hose.

if the engine is a 4 liter, most likely you need to upgrade to the revised intake manifold gasket, which means dismantling the top of the engine, a total pain but it must be done.