2002 Ford Windstar P0171 and P0174 Diagnosis: Lean Mixtures, Vacuum Leak, MAF Sensor Issues
I've completed the TSB for intake gasket replacement, but it's not resolving my issue. I'm now completely stuck trying to diagnose this. When driving, the engine produces a whining sound similar to a turbo kicking in—this noise stops when the engine runs smoother, though there is still a slight misfire. I've already tested for vacuum leaks using carb cleaner, replaced the fuel filter, spark plugs, plug wires, and followed the TSB instructions for isolator bolts and O-rings. No improvement. When I disconnect the MAF sensor, there's no significant change in engine performance. Could the EGR valve or DPFE sensor be at fault? Both banks are showing lean conditions (P0171 and P0174), which makes me suspect a systemic issue rather than isolated components. I'm looking for reliable troubleshooting steps to identify the root cause—especially since both banks are running lean. Any insights on EGR, DPFE, or vacuum system diagnostics would be greatly appreciated.
The PCM won't highlight a major fault—it's up to you to find the actual issue. Since both engine banks are running lean, what could cause this? While intake gaskets were a common starting point, did you replace only the upper plenum gasket or also the lower intake gasket? O2 sensors are less likely to fail simultaneously unless there's an exhaust leak affecting both signals. Have you inspected your exhaust system for leaks? Next, refer to the vacuum diagram in the engine compartment. Check every vacuum line for disconnections or damage. Don't forget to pinch off the brake booster hose—leaks here can cause vacuum issues (though you'd typically notice a hard brake pedal). If all these are intact, focus on diagnosing the MAF sensor. Many 2002 Ford Windstar models have known MAF sensor inaccuracies that underreport airflow, leading to lean mixtures. A whining noise during acceleration is often a sign of a vacuum leak—locate it. It's almost always present and must be found and sealed.