EVAP Code P0452 - simple fix, right?

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.

Now that I had grabbed what data I could, I decided to grab a graphing DVOM and have a look at the wires right at the FTP sensor in hopes to get an idea as to what was going on. It is a two-channel meter, so I monitored the 5v ref, signal wire and ground all at the same time and it revealed exactly what the scan tool was showing. When the signal wire dropped to 0v at 3000 rpm, the ground and 5v ref stayed perfect. Or so they seemed. Not really knowing where to go next, I decided to just double check the circuit integrity of the signal wire from the ECM to the FTP sensor. I unplugged the FTP sensor and the ECM, supplied battery voltage on the signal wire and with a 780mA test light on the other end and it seems to carry current just fine (Figure 7).

At this point, I really just don't know. With a shop parking lot full of other work, it was getting hard to concentrate. It is a terrible feeling as a mechanic and shop owner, as many of us know. Part of me is thinking the ECM is gone wonky but the fact that it is so predictable and it only does it at 3000 rpm is still weighing in my mind. I decided to play some "swaptronics" and substitute a known good (used) FTP sensor (Figure 8). A quick trip to see my friends at the local salvage yard and we were back at the shop. Used sensor is now installed and low and behold it does the same exact thing! Needing to think for a bit I decided to gather a bit more data on a test drive and it revealed that driving it made no difference and it still drops out at 3000 rpm. Engine torque, bumps, forward, reverse, hot or cold, nothing made a difference.

Not beaten yet!

Feeling like I am at the end of my rope and out of ideas for the moment, I decided to use my "phone a friend” life line. I called my good friend and mobile tech Keith DeFazio from New Level Auto Diagnostics in Staten Island, NY. We discussed what tests had been performed and what the possible causes were, and tried several things such as unplugging DPFE, TPS and retesting. 

Low and behold, unplugging either the DPFE or the TPS sensors caused the problem to disappear! WHY!? Was something happening on the 5v reference? It was time to have a closer look, using the scope feature on my DVOM. With the scope hooked back up, there it was — staring me in the face. At 3000 rpm, the 5v reference would go into a high frequency hysteria, for lack of a better term (Figure 9). Seeing this only brought about more questions though. The main one being, where is all this noise coming from and why is it effecting the signal wire and making it drop right to zero volts? 

While I was still contemplating ideas in my head knowing I was one step closer, I needed to know if the signal was being "sent" by the ECM or "received." The easiest way to do that was to just cut the wire near the ECM and scope both ends of it and I discovered it was coming from the FTP sensor to the ECM. I still had no idea why though. However, it did lead me away from thinking it was the ECM at the time. All I knew was something was changing at 3000 rpms and was effecting it. Of course, at this point I have tried unplugging the coils, the alternator and various other components, just to try and get an idea where this noise is coming from and was not making any progress. I decided to study scan data some more to try to gather some clues as to what could be changing. I thought to myself, is the alternator duty cycle changing, is there a device or output turning on at the same time, ANYTHING! Still nothing. 

Deciding to call it a night after a busy day at the shop and working on this in between other cars, I figured it is best to go home and sleep on it hoping it comes to me in a dream. Laugh if you want, but I am certain many of you have been there before! I woke up in a panic at 3 a.m. and rushed back down to the shop to fix this mind-bender. On my way out the door, it dawned on me I never saw a Power Steering Pressure (PSP) switch input on the scan tool. Could that be changing? I clearly remember thinking to myself, "that is a 5v switch, right?" I walked back in the shop, fired the scan tool back up and unplugged the PSP switch. I GOT IT!!! The problem was gone! But why!? 

At this point, part of me did not care, but the rest of me wanted to know what was going on. I grabbed the scope, back probed the connector and there it was (Figure 10). The PSP switch at 3000 rpm was opening and closing like a mad man. I also discovered I was wrong; it is not a 5v switch — it is indeed a 12v switch. Good thing I was tired and did not give it a second thought. Had I known it ran off a 12 circuit would I have gone back in and checked it? Hard to say at this point. As they say, hind sight is always 20/20.

A new PSP switch was fitted the following morning. The EVAP code was repaired, drive cycle was completed and the symptoms were corrected. As a mechanic who grew up in this field and learns from self-study and by getting my butt kicked from time to time, this one humbled me and taught me a few things. The first thing it taught me was to use my scope feature from the get go even if it seems unnecessary. You never know what it might show you. It also reminds us sometimes we do need to walk away, clear our heads and not give up! I also learned the value of having a great friend in the industry who shares the same passion for repairing vehicles as I have and is always willing to bounce ideas around.

I tend to chuckle every time I have a vehicle come in now with an EVAP code tripping the MIL because you just never know. It may just need a new power steering pressure switch! Who would have thought.