Experiencing a check engine light (CEL) along with a P0174 code can be concerning for any car owner. In the case of a modified Ford GT, this issue led to an interesting diagnostic journey, revealing a potential problem related to aftermarket long tube headers and O2 sensor placement, specifically affecting bank 2. The P0174 code signals a lean condition in engine bank 2, which is the driver’s side. When the Powertrain Control Module (PCM) detects this lean condition, its failsafe response is to richen the fuel mixture. While running too rich isn’t ideal, a lean condition can be significantly more damaging to an engine, making the PCM’s reaction a crucial safety measure. In this particular case, despite the P0174 code and the system running rich as a result, the engine performance remained seemingly normal, albeit with the eventual issue of spark plug fouling.
This Ford GT was not stock; it included performance modifications such as a pulley and tune, an Accufab throttle body, and FRPP/Borla long tube headers and muffler. These modifications are important to note as they play a crucial role in understanding the diagnostic process and the ultimate cause of the P0174 code.
The diagnostic process involved a series of systematic checks to pinpoint the cause of the lean condition. The technician undertook the following steps:
- Vacuum Leak Check: A smoke generator was used to meticulously verify and rule out any vacuum leaks in the system.
- Fuel Injector Flow Verification: The fuel injectors were tested to ensure they were delivering the correct amount of fuel.
- O2 and MAF Sensor Operation: The functionality of both Oxygen (O2) sensors and the Mass Air Flow (MAF) sensor was confirmed to be within specifications.
- Compression and Leak Down Tests: Engine health was assessed through compression and leak down tests to identify any potential cylinder issues.
- PCM to O2 Sensor Circuit Testing: The electrical circuits from the PCM to the Bank 2 O2 sensor 1 were examined for any faults.
- Exhaust Valve Spring Visual Inspection: A visual inspection of the exhaust valve springs was performed to check for any mechanical anomalies.
At this stage, a critical distinction between Original Equipment Manufacturer (OEM) exhaust manifolds and aftermarket long tube headers became apparent and crucial to the diagnosis.
OEM exhaust manifolds are designed such that the upstream O2 sensor (sensor 1) monitors the combined exhaust gases from all four cylinders in that bank. This provides an average reading of the air-fuel mixture for the entire bank.
However, long tube headers often alter this configuration. In this Ford GT with long tube headers, the O2 sensor 1 in bank 2 was positioned to sample exhaust gases from only cylinder 7.
This significant change in O2 sensor placement means that the PCM was only receiving air-fuel mixture information from cylinder 7 in bank 2. Consequently, any lean or rich conditions occurring in cylinders 5, 6, and 8 of bank 2 would go undetected by the O2 sensor and the PCM.
The question then became: what could be specifically wrong with cylinder 7 to trigger a false lean reading?
To investigate further, the technician ingeniously swapped and rewired the O2 sensors so that the sensor intended for cylinder 7 was now sampling from cylinder 8. Remarkably, after this modification, the O2 sensor voltage and the air-fuel mixture readings returned to normal. This real-time demonstration was observed and confirmed using a laptop, providing solid evidence that the issue was localized to the original O2 sensor position or its immediate surroundings.
To pinpoint the exact cause, a smoke test was performed on the exhaust system itself. Rubber gloves were placed over the exhaust outlet tips, and smoke was introduced into the exhaust system. This test revealed that smoke was leaking from a slip-fit header connection located very close to the original O2 sensor 1 position for cylinder 7.
The technician concluded that the exhaust leak at the slip-fit connection was drawing in external air, directly affecting the O2 sensor reading in the cylinder 7 tube. This extra air falsely indicated a lean condition to the O2 sensor, which in turn triggered the P0174 code. Crucially, the O2 sensor in the cylinder 8 tube, now in the position previously used for cylinder 7, was located further downstream from the leak and was not affected, hence providing accurate readings.
Currently, the rewired O2 sensor setup may serve as a viable workaround. Simultaneously, efforts are underway to determine if Borla, the manufacturer of the headers, will provide a replacement bank 2 header under warranty, addressing the slip-fit connection issue.
This diagnostic case highlights a rather unusual problem, especially for those running aftermarket long tube headers. A key takeaway for anyone with headers is to understand the implications of O2 sensor placement. Don’t assume the PCM will reliably alert you to lean conditions across all cylinders when using long tube headers, as the sensor may only be monitoring a single cylinder’s exhaust output. This case serves as a valuable lesson in understanding the nuances of modified exhaust systems and their impact on engine diagnostics.
