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1997 Mercury Cougar P0430, P0171, P0174 OBD-II Codes: Engine Stalls on Start, Fuel Trim Issues

Model: OSD-II Fault Code: P0171 Posted: 2007-01-18 13:05

I own a 1997 Mercury Cougar and am experiencing persistent OBD-II diagnostic trouble codes: P0430 (catalyst system efficiency below threshold), P0171 (system too lean), and P0174 (system too rich). Fuel trim readings show: - Cold start: Short-term fuel trim at +35%, long-term at 7% - Warm engine: Short-term fuel trim fluctuates between +5% and -5%, long-term at 7% Symptoms include: - Engine stalls immediately after starting - On cold days, I must press the accelerator to increase RPM until the engine warms up - In very cold conditions, I need to start the engine three times before it runs smoothly - Once running, the vehicle operates normally—no issues during driving, stopping, or acceleration I have already cleaned both the Mass Air Flow (MAF) sensor and the Idle Air Control (IAC) solenoid. I do not believe there is a significant intake manifold leak because I don’t experience post-start problems such as rough idling or stalling after warm-up. What could be causing this? Can someone explain the root cause? Additional details: - MAF reading: 1.2 lb/min at 900 RPM - Long-term fuel trim is consistently around 7%, which seems high for a lean condition - After disconnecting the MAF, short-term fuel trim dropped to -20% and long-term to +10%; when reconnected, it returned to +10% and +20% - IAC was removed and tested—air blown through with blocked bleed hole; no change in RPM or fuel trim I am hesitant to replace the MAF sensor ($150) without confirmation. Is there a more cost-effective diagnosis first? What should I check next?

Related fault codes
P0171P0174P0430
Comments (8)
Anonymous 2007-01-18 16:54

Vacuum leaks remain a potential cause, as do low fuel pressure issues—both could contribute to the symptoms described.

Anonymous 2007-01-20 08:38

Thanks for the reply. Would low fuel pressure cause acceleration problems? No one has answered this yet. I’ve reset the codes several times. This morning, I now see P0171 and P0300. I’m confused—why did P0300 appear after cleaning the MAF? Some forum posts suggest that if a car starts without the MAF connected, it won’t stall, and stalls when reconnected—this indicates a faulty MAF. However, I don’t want to replace it yet as it costs $150. My current fuel trim readings are long-term 43%, short-term 13%—both seem unusually high. What is the correct air volume specification for this vehicle? Where should I begin diagnosing now?

Anonymous 2007-01-20 11:33

I still recommend checking for vacuum leaks and verifying fuel pressure before replacing the MAF sensor. How old are your ignition wires and spark plugs?

Anonymous 2007-01-21 23:23

The vehicle has 90,000 miles on it. I may replace the spark plugs tomorrow. I’ve inspected all hoses and fittings for air leaks—no visible issues. How can I properly test for intake manifold leaks? This older model includes a cold start injector or choke system—does this car have one, or is there an equivalent electronic controller?

Anonymous 2007-01-22 10:41

Reference: http://www.alldata.com

Anonymous 2007-01-24 12:45

I won’t replace spark plugs until I identify the root cause. Code P0340 has not returned—why was it set initially? I’ve checked for air leaks and found none. If fuel pressure is low, I would expect acceleration issues—but I don't experience them. When I disconnected the MAF sensor, short-term trim read -20% and long-term at +10%; when reconnected, it returned to +10% and +20%. This suggests a lean condition with the MAF connected and rich when disconnected—does this indicate a faulty MAF? However, readings are within 0.5–0.6 lb/min, which appears normal. I removed the IAC unit and tested it by blowing air through—it passed through with the bleed hole blocked. When disconnected, RPM and fuel trim remain unchanged—this seems normal? My main issue is engine stalling at startup.

Anonymous 2007-01-24 12:45

Apologies—the code P0300 was mistakenly mentioned earlier; it should not have appeared.

Anonymous 2007-01-24 13:57

P0300 indicates a multi-cylinder misfire. If your spark plugs and ignition wires are original, consider replacing them with new ones from a Ford dealer—this is often the root cause in older vehicles like this one. The MAF may not be faulty; fuel pressure issues can sometimes manifest without noticeable power loss.