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2001 Ford Expedition 5.4L P0171 Code After Intake Manifold Replacement - Fix and Diagnosis

Model: 2001 Ford Expedition Fault Code: P0171 Posted: 2015-11-27 08:10

I own a 2001 Ford Expedition Eddie Bauer with a 5.4L V8 engine, currently at 209,000 miles. I've maintained the vehicle since new and it has never had major issues. Recently, I discovered a coolant leak in the front right-hand corner, which turned out to be due to a failed intake manifold. The intake was replaced with a Dorman unit that includes built-in 'O' rings—these are designed to prevent gasket movement during installation. After completing the replacement and installing new spark plugs, plug boots, and injector O-rings, the truck runs smoothly but exhibits a higher idle (approximately 150 RPM above baseline). My fuel economy has dropped by about 2 MPG. Two days after the work was completed, I received a diagnostic code: P0171 — Left Bank Lean. The engine runs strong with no rough idling or misfires. However, the persistent high idle and reduced mileage are concerning. I’ve already checked all vacuum lines, fuel injector connections, and used carb cleaner to test for leaks. No issues were found. I also considered a malfunctioning MAF sensor, though I’m uncertain if that would explain the long cranking time during startup. After replacing the PCV valve and vacuum hoses (including the VAP system hose), there was no improvement. The idle became slightly rough when stopped at traffic lights after driving about 4 miles. I then changed the fuel filter, which resulted in a normal idle and disappearance of the check engine light for approximately five days. However, today the light is back on with codes P0171 and P0174. The idle still stumbles occasionally at stops but is much smoother than before. My current theory is either: - A fuel delivery issue caused by a Lucas fuel treatment I added to the tank, or - A faulty Idle Air Control (IAC) valve—especially since I used brake cleaner inside the throttle body during maintenance, which may have damaged the valve. Ford advises against cleaning internal throttle body components. I’m now considering that the root cause might be a vacuum leak, even if it’s not obvious under normal inspection.

Related fault codes

P0171

Comments (6)

Anonymous 2015-11-27 08:34

P0171 indicates a system-wide lean condition. Combined with high idle, this strongly suggests a vacuum leak. Try the 'spray test'—use a small bottle with a spray nozzle to apply water or carb cleaner along suspected areas such as intake gaskets, injectors, and throttle body seals. Listen for any noticeable change in RPM.

Anonymous 2015-11-27 09:10

Perform the spray test using a pressurized spray bottle—apply water or carb cleaner to potential leak points like intake manifold gaskets, fuel injectors, throttle body, and vacuum hoses. Use a strong stream if needed. A sudden drop in RPM after spraying indicates a vacuum leak.

Anonymous 2015-12-03 19:27

I’ve driven the vehicle for three days and covered 86 miles without resolving the issue—still showing P0171 and P0174 codes. The idle remains elevated, though it fluctuates between normal and up to 750 RPM. I've checked all electrical connections, loosened fuel injector rails to ensure proper seating, and ran a cold engine test with carb cleaner. No leaks were detected. I sprayed the intake, injectors, and all visible vacuum lines. I suspect either the MAF sensor or an internal throttle body issue, especially given the long cranking time during startup.

Anonymous 2015-12-24 12:24

After failing to detect any leaks with a smoke test (using propane or carb cleaner), I discovered that the Dorman intake manifold has a rear elbow for the PCV hose. When I removed this elbow, I found it had an O-ring socket but no O-ring was installed. After installing a $0.49 O-ring, the idle stabilized completely and both P0171 and P0174 codes disappeared. It’s surprising that official O-ring kits for this intake do not list this specific component—this is likely the root cause of the lean condition.

Anonymous 2015-12-24 12:51

It's unlikely that Lucas fuel treatment caused the issue. Start by checking fuel pressure—did changing the fuel filter help? Cold weather can expose weak fuel pumps to performance issues. Are you using an oiled air filter? A dirty or unfiltered intake may contribute to lean conditions. Consider cleaning the MAF sensor—it can be done safely with a throttle body cleaner (not carb cleaner). Brake cleaner is similar in composition, but Ford warns against internal throttle body cleaning. Carb cleaner should be avoided as it can damage the throttle body coating. Merry Christmas.

Anonymous 2016-01-13 21:55

FOUND IT! The Dorman intake manifold includes a rear elbow for the PCV hose. During my smoke test, no leaks were detected using propane or carb cleaner—until I inspected this elbow. It had an O-ring mounting point but was missing the O-ring entirely. After installing a $0.49 replacement O-ring, idle returned to normal and both P0171 and P0174 codes cleared. This small omission in the installation kit is not widely documented—many users may overlook this detail.