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1997 Mercury Cougar 4.6L V8 P0171 & P0174 Code: Diagnosis and Fixes After Intake Manifold Replacement

Model: 1997 Mercury Cougar Fault Code: P0171 Posted: 2008-08-06 21:53

I own a 1997 Mercury Cougar with the 4.6L V8 engine, currently at approximately 134,000 miles. I've been experiencing persistent OBD2 diagnostic trouble codes P0171 (System Too Lean Bank 1) and P0174 (System Too Lean Bank 2). Despite the engine running smoothly—achieving over 24 MPG on highways at speeds above 70 mph and idling steadily around 500 RPM with no loping—I still see the Check Engine Light (CEL) activate after driving about 100 miles, especially when I stop and restart the vehicle. The light reappears during acceleration. I recently replaced both the intake manifold (plastic upper unit), spark plugs, and ignition wires. However, the P0171/P0174 codes persist. One forum post suggested that if the engine runs well, the issue may not be a sensor-related fault—this aligns with my experience. I have no noticeable vacuum leaks during idle or operation. I’ve tried using Sea Foam in the fuel and cleaned the MAF sensor with carb cleaner. I also installed a K&K air filter that uses an oil mist system, which raises concerns about potential contamination of the MAF. The CEL has gone off temporarily after these actions, but returns under normal driving conditions—particularly when returning from rest stops on interstates. I’m seeking reliable advice on diagnosing this lean condition in a 1997 Mercury Cougar with a 4.6L V8. Is it possible the lower intake manifold or vacuum hoses are leaking? Could fuel pressure or MAF sensor contamination be contributing to these codes? Thanks for any insights.

Related fault codes
P0171P0174
Comments (7)
Anonymous 2008-08-07 09:50

Check fuel pressure and inspect the MAF sensor for dirt or debris—both can contribute to lean conditions like P0171/P0174 in a 1997 Mercury Cougar.

Anonymous 2008-08-07 10:05

I've added Sea Foam to my fuel several times and currently have a K&K air filter with oil mist. I plan to clean the MAF sensor thoroughly—any guidance on proper cleaning methods would be appreciated.

Anonymous 2008-08-07 12:05

Many customers report that their engines 'run fine' with P0171/P0174, but upon inspection, actual performance issues are often present. Did you replace the lower intake manifold? Were any vacuum hoses disconnected or damaged during the work?

Anonymous 2008-08-07 16:05

My son replaced the upper plastic intake manifold—known for reliability issues in Ford vehicles—and he's a Detroit Diesel technician, so I trust his work. The car doesn’t lop or misfire, which rules out major vacuum leaks. After replacing the fuel filter and cleaning the MAF with carb cleaner, the CEL turned off temporarily. It remained off during short drives. The pattern appears to trigger when returning from interstates—especially at welcome centers—when I restart after stopping. I haven’t driven long distances again yet. Thanks for all the input.

Anonymous 2008-08-09 17:55

It's worth double-checking the lower intake manifold for vacuum leaks—these are often hard to detect without replacement or pressure testing, especially in older Mercury Cougars with plastic intakes.

Anonymous 2008-08-09 19:06

Oil mist from air filters can contaminate the MAF sensor and cause lean misreads. This is a known issue that may trigger P0171/P0174 codes in older V8 engines like the 1997 Mercury Cougar.

Anonymous 2008-08-09 20:28

I've completed several short drives since cleaning the MAF and replacing the fuel filter. So far, the CEL has stayed off and engine performance is normal. This suggests a temporary improvement—though long-term stability remains to be seen. Thanks for all the helpful suggestions.