Encountering a check engine light and suspecting a vapor pressure sensor issue? Recently, I tackled a tricky problem where a faulty vapor pressure sensor was causing a recurring “low purge flow” code. While initial tests with a vacuum pump seemed normal, a deeper dive using a car scanner revealed the real culprit. Here’s how I diagnosed and fixed it, highlighting the importance of accurate readings and using your car scanner effectively.
The initial diagnosis involved using a vacuum pump to test the vapor pressure sensor. On the surface, the sensor appeared to be functioning within its range, and the voltage readings on the scanner responded to vacuum changes. However, the key detail was the unit of measurement. Vacuum pumps often read in inches of mercury (in/Hg), while the sensor operates and reports in millimeters of mercury (mm/Hg). This discrepancy masked the sensor’s inaccuracy.
With everything reassembled in the car, I used a car scanner to perform a function test and scope the sensor’s voltage. This revealed a sluggish and erratic voltage drop. Instead of a smooth decline, the voltage would jump down in jerky increments. Furthermore, the fuel tank was visibly starting to collapse inwards, pulling away from its straps. This was because the engine control module (ECM), sensing the inaccurate readings, was continuously commanding vacuum in an attempt to lower the pressure further.
The solution was replacing the vapor pressure sensor. After replacement, the difference was immediately noticeable when observing the sensor readings via the car scanner. The voltage signal now dropped smoothly and rapidly. Using a stopwatch, I timed the voltage drop – it reached 1.5 volts within 25 seconds with the new sensor. In stark contrast, the old sensor took a sluggish 2 minutes to reach only 1.9 volts, clearly indicating its malfunction and triggering the “low purge flow” code.
This experience underscores the crucial role of a car scanner in diagnosing sensor issues accurately. While basic tests can provide initial clues, a scanner allows for real-time monitoring of sensor behavior within the vehicle’s system. It also highlights the importance of understanding sensor specifications and measurement units for accurate interpretation of diagnostic data. For future troubleshooting, I’ve learned about Dwyer’s mm/Hg gauges, which would have been invaluable in this diagnosis and something I plan to add to my toolkit. Hopefully, this detailed diagnostic experience helps others facing similar vapor pressure sensor challenges.
