Automotive Diagnostic and Programming Case Studies

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Automotive Diagnostic Case Studies


Scan Data is not always reliable

This Subaru had a P0101 MAF code and would barely maintain idle. The model year did not comply very well while using scan data on two different scan tools, so I had to take a different approach. I used a multimeter to monitor the MAF signal KOEO and KOER. The KOEO voltage was .2 volts, and the KOER voltage was around 1 volt. At this point, I believed the P0101 is the result of another problem, but what?

My next move was to disable the ignition and listen for a cylinder imbalance while cranking. And after doing so, the engine sounded like there was a mechanical problem. Instead of locating the bad cylinder using an amp clamp, I decided to check the Intake Pulls utilizing a pressure transducer. The waveform’s blue trace is the Intake Pulls, and the Red Trace is the Ignition Sync. The #2 cylinder has a cut at the bottom and shows a weak intake pull. The #3 cylinder shows the same on one of its two Intake pulls. We are dealing with mechanical failure from at least one cylinder. The engine’s inability to intake the correct amount of air would explain the rough idle and P0101 fault code.

automotive diagnostic case studies



P1450- Unable To Bleed Up Fuel Tank Vacuum

Using Freeze Frame to identify an active fault.

This Transit had a P1450 – Unable To Bleed Up Fuel Tank Vacuum. Although there are many possibilities, a stuck open purge valve is the most common. Vacuum testing of the purge valve shows that it is not stuck partially open. I did not suspect a restriction near the vent valve because refueling is not a problem, nor was the fuel tank pressure sensor showing any failure signs. Everything seems good, right? Maybe we should return the vehicle to the customer? 

This is where Freeze Frame Data comes into play. In picture #1, the snapshot data occurred during a deceleration event. During this event, the Purge Valve Command dropped to 0% while the Tank Pressure remained at -3.0 inH2o. With this information, we can now test drive the vehicle while monitoring the Purge Valve Command and Fuel Tank Pressure Sensor PID’s. Picture #2 is during the test drive, the blue trace is Purge Command, and the green trace is Tank Pressure. Notice that when the Purge Command falls to the 0% mark, the Tank Pressure is at -2.8 inH2o. This value is close enough to the -3.0 inH2o in Freeze Frame Data to say there is an active fault. We can now invest time into troubleshooting the vehicle.  We hope you enjoyed learning about our automotove diagnostic case studies today.

So what fixed it? The purge valve was bad. Sometimes parts don’t fail the like they usually do. Picture #3 was taken after the Purge Valve replacement. Notice the moment during the Purge Command drops to 0%. At this point, the Tank Pressure has already risen to -.75 inH2o…This vehicle is fixed! I believe it’s safe to say the original purge valve was not closing fast enough, causing the evap system to remain in a vacuum.

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