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2001 Ford F150 V6 P0171, P0174, P0303 Fault Diagnosis: Vacuum Leak and Coil Pack Issues

Model: 2001 F150 V6 Fault Code: P0171 Posted: 2009-01-30 20:04

My 2001 Ford F150 V6 is experiencing severe engine shuddering at lower RPMs, especially in gears 3 and 4 with a manual transmission. The vehicle has triggered OBD-II diagnostic codes P0171 (System Too Lean), P0174 (System Too Lean Bank 2), and P0303 (Random/Multiple Cylinder Misfire). I've already performed a tune-up, cleaned the throttle body and air temperature sensor, replaced the fuel filter, and checked for vacuum leaks. During inspection, I found that a connector on the passenger-side valve cover—where three hoses meet before connecting to the valve cover—was loose. After reattaching it securely, the symptoms persisted: engine shuddering remains, and at highway speeds, the check engine light flashes but no noticeable shaking occurs. The truck idles smoothly, which suggests that the issue isn't directly related to idle misfires. However, after recently handling the ignition coil pack (which I noticed had a crack and one bolt tab broken), the shuddering worsened in all gears when demand is placed on the engine. This makes me suspect a misfire originating from cylinder #3. I've verified that all vacuum hoses are intact and show no audible leaks during operation. The 90-degree PCV elbow under the throttle body was previously replaced with a straight hose and clamp, so I believe it's not the root cause. Despite these efforts, the P0171/P0174 codes remain unresolved—indicating an underlying vacuum leak or sensor issue that I haven't yet identified. What could be causing this? Is there a possibility of a small vacuum leak in a hard-to-detect location? Could a dirty Mass Air Flow (MAF) sensor be contributing to the lean mixture readings? Any guidance on cleaning or testing the MAF would be greatly appreciated.

Related fault codes

P0171P0174P0303

Comments (6)

Anonymous 2009-01-31 14:10

The P0171 and P0174 codes are often caused by a failing PCV system with vacuum leaks. Check all hoses, especially the 90-degree rubber elbow located on the rear of the throttle housing (not under the intake). A soft or dried-out elbow can create a subtle leak that goes unnoticed. If you remove the EGR valve vacuum line and drive the truck, and the shuddering stops, it likely indicates clogged EGR passages. Let me know if you'd like step-by-step instructions for cleaning or replacing the EGR system.

Anonymous 2009-01-31 14:21

I've seen this issue multiple times—especially on 2001 F150 V6 models. The 90-degree PCV hose behind the intake can dry rot over time, leading directly to P0171 and P0174 codes. For reference: https://www.obd-codes.com/p0303. This is a common cause that should be ruled out before moving on to other diagnostics.

Anonymous 2009-01-31 14:51

Thanks for the input—I’ve already inspected all vacuum lines and confirmed they maintain good vacuum pressure with no audible leaks. The original 90-degree PCV elbow under the throttle body was replaced with a straight hose and clamp, so that’s not the issue either. While troubleshooting, I noticed my ignition coil pack has a visible crack and one bolt tab is broken off. Given this, I suspect cylinder #3 may be misfiring. However, after recently working on the coil pack, the shuddering worsened across all gears when engine demand increases. This makes me think the misfire issue might now be more pronounced. But why are P0171 and P0174 still present? There must be a vacuum leak somewhere—just not detectable under normal conditions.

Anonymous 2009-01-31 15:11

Since the shuddering intensified after handling the coil pack, it's highly likely that the misfire is originating from cylinder #3. If the truck idles smoothly, any vacuum leak would be minimal—possibly undetectable by ear. A dirty Mass Air Flow (MAF) sensor could also cause lean mixture readings and trigger P0171/P0174. Have you tried cleaning it? To clean properly, locate the MAF sensor near the air intake, typically on the driver’s side of the throttle body. After cleaning, disconnect both battery terminals for about 20 seconds to clear the PCM memory (or use a scan tool to reset the KAM if available).

Anonymous 2009-01-31 15:12

Cylinder #3 is located as the third coil from the passenger side of the engine. The 90-degree rubber elbow isn’t under the throttle body—it’s on the rear of the throttle housing, near the center of the passenger-side valve cover. Follow the plastic hose along the back of the throttle housing to locate it. This elbow is a common source of vacuum leaks and often cannot be detected by sound alone—especially when the engine is running smoothly.

Anonymous 2009-02-01 17:57

PROBLEM SOLVED: I discovered a crack in the ignition coil connector for cylinder #3. After replacing that coil, the shuddering stopped immediately and the P0303 code cleared. I also performed a vacuum treatment: I removed the PCV valve and allowed about one cup of Sea Foam to be drawn into the engine via suction. When the engine began to run roughly, I turned it off and sprayed the MAF sensor with electrical cleaning solution (quick dry), then reinstalled everything. After restarting, the truck coughed and sputtered, spitting black smoke from the exhaust. I let it idle for a few minutes, then gently revved it until the exhaust ran clean. I cleared all diagnostic codes and drove the vehicle for approximately 15 minutes. Now there are no error codes and no shuddering. Thanks to everyone for the helpful suggestions—this was a great troubleshooting session!