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Recurring P1131 and P1151 Fault Codes on 2002 Roush Mustang Stage II - How to Fix Rough Idle and Rich Exhaust?

Model: Roush Mustang Fault Code: P1131 Posted: 2008-02-20 15:31

I own a 2002 Stage II Roush Mustang that has been showing recurring OBD-II fault codes: P0171, P0174, P1131, and P1151. After replacing both upstream HO2 sensors, the MAF sensor, and the fuel pressure regulator assembly along with the fuel filter, the issue persists. Fuel pressure readings fluctuate between 32–38 psi, which is within expected range but inconsistent. Spark plug gaps were checked at 0.54 inches, and compression tests on cylinders #5 and #6 showed around 120 psi — within normal limits. After clearing the codes, they do not return immediately, yet the vehicle continues to idle roughly with RPMs bouncing between 550–650, then jumping to 1000–1100 before settling back into a rough idle. This behavior has led two mechanics to suspect a sticking Air Inlet Control (IAC) valve. The IAC was replaced, all vacuum lines were double-checked and reconnected, but the P1131 and P1151 codes returned — with no improvement in idle quality. My question: Could these persistent lean/lean-rich misfires be caused by a faulty Throttle Position Sensor (TPS) or a failing Powertrain Control Module (PCM)? I'm an active-duty Navy member with limited time and budget, so I need practical, cost-effective solutions. Are there known common causes for this specific combination of codes on the 2002 Roush Mustang? Any real-world fixes or diagnostic steps that have worked for others? Thanks in advance for any insight — this issue is frustrating to troubleshoot without access to expensive diagnostics or specialty tools.

Related fault codes
P1131P1151
Comments (6)
Anonymous 2008-02-20 16:55

P1131 and P1151 are often linked to lean conditions, but they can also result from vacuum leaks, incorrect fuel pressure, or a failing DPFE sensor. For your 2002 Mustang, what is the manufacturer-specified fuel pressure range? Are there any visible vacuum leaks at the intake manifold or hoses? Could timing be off — especially given the rough idle and RPM fluctuations?

Anonymous 2009-03-23 00:40

I have a 1995 V6 Mustang with similar codes. The car bogs down severely during acceleration. My reader shows EGR not ready, CAT not ready, O2 not ready, and HO2 not ready. I suspect the EGR valve might be faulty — could that be causing the issue? Any advice on how to diagnose or fix it?

Anonymous 2009-03-23 16:37

Rich exhaust smell and rough idle point strongly toward a fuel delivery problem. Check both fuel pressure and flow. An EGR malfunction typically causes a rich condition, not lean — so if the engine is running rich (as indicated by CO2-like fumes), it's less likely to be an EGR issue.

Anonymous 2009-12-22 18:04

I’ve seen P1131 and P1151 codes on a 2002 Explorer alongside P171 and P174, with intermittent misfires. After replacing the fuel filter, EGR valve, and a dry-rotted hose connecting the PCV valve to the throttle body, the errors disappeared for about six months. Recently, they returned along with rough running — especially when temperature changes occur. My friend has a 2003 F150 with identical symptoms; he’s replaced EGR, PCV, and plugs but still experiences issues. A dealer recommended running one full tank of E85 to clean the system — though I’m unsure if it's safe for this engine model since it wasn’t designed for E85. When all four codes first appeared, I replaced: - EGR valve - Fuel filter - PCV valve - The hose from PCV to throttle body near EGR The symptoms improved temporarily. Now the codes are back — and the engine runs rough again. What could be causing this recurrence? Is there a deeper issue like intake gasket leaks or sensor calibration?

Anonymous 2009-12-22 19:01

For vehicles with persistent EGR or O2-related errors, cleaning or replacing the MAF sensor is often helpful. If you still see EGR faults, consider swapping out the DPFE (Differential Pressure Feedback) sensor with a dealer-grade unit. Additionally, rough idle can stem from a dirty throttle body — remove it and thoroughly clean both the throttle body and intake ports. This step has helped many users resolve similar symptoms.

Anonymous 2009-12-22 21:26

Intake gasket leaks are common in older Ford engines and often allow unfiltered air into the engine, causing vacuum loss and misfires. The upper plenum gasket is a frequent culprit, but lower intake gaskets can also let crankcase pressure enter the intake system. A simple test: Remove the dipstick tube, plug the PCV hose to prevent crankcase draw, cover the dipstick hole for 10–20 seconds, then release. If the engine 'sucks in' or RPM drops significantly — that indicates a bad lower gasket. For upper gaskets, a propane test is effective: apply small amounts of propane around the upper intake connection while the engine is cold. If idle speed fluctuates or drops, it suggests a leak. A smoke machine would be ideal for this, but not everyone has access. This type of vacuum intrusion can cause erratic idle and trigger P1131/P1151 codes — especially when combined with poor fuel mixture control. I recommend checking these gaskets as part of the troubleshooting process.