I started out my diagnosis on a 2003 Ford Escape with an automatic 3.0 VIN 1 24 Valve V-6 just like I would any other vehicle that I bring in the bay. First, the customer interview to gather the information and the complaint — with just shy of 200K on the odometer, the MIL light is on — then gather some codes and data, come up with a plan, repair and verify and then move onto the next one. Little did I know my plans were about to be derailed!
|Want more ? Enjoy a free subscription to Motor Age magazine to get the latest news in service repair. Click here to start you subscription today.|
ENTER CODE : ART30 AT CHECKOUT
The customer had no complaints other than a glowing MIL light. A quick scan revealed a P0452 "EVAP System Pressure Low" both pending and current (Figure 1). Nothing new for us in the rust belt to see EVAP codes on a day to day basis, that's for sure! While sitting in the driver's seat, I decided to have a quick look at the Fuel Tank Pressure (FTP) data PID and quickly discovered it was at 2.6v KOER, which is normal for a Ford EVAP pressure sensor sitting at atmospheric pressure. Wanting to see if the FTP sensor would respond to vacuum, I opened the vapor management valve (purge solenoid) with the scan tool and saw the voltage drop to around 2.19v (Figure 2). That is about what I would expect to see considering the canister vent was still open. OK, so what is going on here? I don't recall at the time what possessed me to increase the rpms, but I did. Perhaps it was in an effort to gain a bit more vacuum on the decel while watching the FTP data PID with the vapor management valve still open. Whatever the thought was, it turned out well because it revealed the problem! At EXACTLY 3000 rpms, the FTP data PID would drop straight to zero volts (Figure 3)! OK, you have my attention, I thought.
I now knew what the ECM was looking at and why it flagged the code. After all, it met the code setting criteria. It was time to get to know my enemy. I pulled up a wiring diagram to see what I could gather from it and verify what I already knew (Figure 4). It is a standard three-wire 5v pressure transducer that was easily accessible under the driver side rear seat. A ground wire, signal wire and a 5v reference. It also shared the same 5v reference as the Differential Pressure Feedback EGR (DPFE) and the Throttle Position Sensor (TPS). Thinking to myself that if the sensor shares the 5v reference and it was losing it, and it happened to be at or before that splice S105 that I saw in the diagram, certainly we could see that in scan data because it should affect the other sensors sharing the same 5v reference, right?
Time to test a theory
After a moment of recreating the conditions and monitoring the TPS and DPFE PIDs, they appeared to be unaffected. I knew it could not be losing the ground side because if that were the case, the voltage should go high. I verified this by simply unplugging the sensor and indeed it did go high (Figure 5). While I still had the sensor unplugged, I recreated the conditions of the fault and the signal stayed nice and steady. However, after plugging it back in the condition was still present (Figure 6). That being said, we did not correct the condition by unplugging it or messing with the harness. You know as well as I do how frustrating that can be.