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2001 Honda Prelude P0172 P0170 Running Rich - Black Exhaust & Fuel Trim Issues

Model: 2001 Honda Prelude Fault Code: P0170 Posted: 2007-04-25 11:48

My 2001 Honda Prelude-SH has been running rich for an unknown period of time, indicated by persistent black exhaust fumes and unburnt hydrocarbons. The check engine light is showing two fault codes: P0172 (System Too Rich) and P0170 (Fuel Trim Malfunction). I currently use a piggyback ECU controller (APEXi VAFC-II), but despite adjustments, the car continues to emit black smoke from the exhaust. This suggests an ongoing fuel delivery issue or sensor misalignment. The engine plugs are noticeably wet, indicating significant cylinder washdown due to excessive fuel mixture.

Related fault codes
P0170P0172
Comments (8)
Anonymous 2007-04-25 12:17

Check and replace the fuel pressure regulator as it may be causing excessive fuel flow leading to a rich condition.

Anonymous 2007-04-25 12:27

I have an aftermarket fuel pressure regulator (AEM) installed, but I'm not sure if it's contributing to the issue.

Anonymous 2007-04-26 07:31

Try reinstalling the original factory fuel pressure regulator and see if the symptoms resolve.

Anonymous 2007-04-26 11:00

The AEM fuel rail has different mounting dimensions, so it won't fit back into the stock setup.

Anonymous 2007-04-26 11:23

Let me rephrase: return to stock components and carefully adjust fuel pressure and volume until the vehicle runs within factory specifications and the check engine light turns off. Unless another system has failed, this is likely a fuel-related issue. Wet spark plugs and heavy cylinder washdown are strong indicators of a rich mixture.

Anonymous 2007-04-26 11:56

Alright, I'll try swapping back to the stock fuel pressure regulator. Thanks for the suggestion!

Anonymous 2007-04-26 13:26

I reviewed the AEM fuel pressure regulator (Product ID: 309) — it's labeled 'High Volume & Adjustable'. According to instructions, you can reduce fuel pressure. If adjustment isn't effective, revert to the stock regulator. The original factory unit is generally recommended for long-term reliability. Aftermarket regulators are typically used in performance applications like turbocharged or supercharged engines with high boost levels. For a standard 2001 Prelude, stock components offer better consistency and longevity.

Anonymous 2007-04-26 15:31

With the piggyback ECU controller installed, even if it's a modified setup, persistent rich running suggests the fuel delivery is still above stock levels. This is essentially the same issue as older mechanics facing problems with oversized carburetors on stock engines — cause and effect matter regardless of vehicle model or era.