P0171 and P0174: fuel trims high at idle but improve at 2,500 rpm

Question

I am diagnosing a 2012 Ford Escape 3.0L with 148,000 miles. P0171 and P0174 return after roughly 15 minutes of driving. At hot idle, STFT is about +9% on bank 1 and +11% on bank 2; LTFT is +19% and +21%. Holding 2,500 rpm with no load brings the combined correction on each bank down to roughly +6%.

Fuel pressure stays near 55 psi and does not sag during a snap throttle. The MAF reports 3.7 g/s at hot idle and 14.8 g/s at 2,500 rpm. There is no current misfire. The RPM-dependent trim pattern makes me suspect unmetered air shared by both banks. Would you smoke-test the intake first, or is there another data check that should come before that?

AirStation · Accepted Answer

The fact that correction is much higher at idle than at 2,500 rpm is strong evidence for an air leak whose flow becomes a smaller percentage of total airflow as RPM rises. Because both banks move together, start with a shared path downstream of the MAF: the intake duct, brake-booster line, purge path and PCV plumbing.

A low-pressure smoke test is appropriate. Seal the intake as the service procedure specifies and inspect the underside of elbows, not just the visible top. You can also briefly isolate the PCV and purge branches while watching trims; a quick improvement identifies the branch but is not a substitute for finding the leak. I would not condemn the MAF or fuel pump from these numbers alone.