Vehicle Surging with no CEL | Ghost Fault

4/4/2018

EFI closed loop diagram (Drdyno)

In my time as a technician I've encountered some strange issues, from defective new parts to ECM software bugs. Most of the time theres something obvious telling you there is a problem, like a check-engine light or noises. But on rare occasion, you get a customer that is very aware, and a car that only malfunctions ever so slightly in a brief operating window. This brings us to my very first Tech-Tip, how to capture ghost fault data.

In the picture above you can get an idea of how modern engines operate, there are many sensors providing feedback to the ECM(Engine Control Module) to maintain optimum performance. Most sensors themselves are also monitored to ensure accuracy, they are required to respond a certain way within a predetermined time frame. I'm sure you can guess what happens if a sensor responds too slowly or incorrectly, you get a fault code and a check-engine light. So what about those issues you feel but never get any red flags from the ECM?

Capturing and Analyzing Data

Erratic A/F sensor signal

One of the most important tools I use everyday is an OBD scanner. The most accurate way to analyzing data is by logging in graph view, otherwise you would never see the large variations in sensor output that are so close together. Look at the RED line in this picture, that is the A/F sensor signal and you can see it's going a bit bonkers next to the vertical FLAG line. Now look at the BLUE line, that is the REAR O2 sensor and it seems to be reacting to the same influence upsetting the A/F sensor. At this point it's safe to say something is causing a erratic lean condition, so now you should be testing your fuel pressure (ours was 52psi, good). Next I tested the alcohol content of the fuel, and it was exactly 10%, good. Knowing our fuel system is in good shape, I went back and monitored manifold vacuum, -9.6psi at idle, -10.2 at 2500rpm, and -12.0 on decell-all good here. Now I need to check a few other graphs...

MAF sensor and throttle inputs

Here we have the BLACK line showing the MAF sensor and the GREEN line showing the THROTTLE. The MAF signal looks very smooth and follows the pattern of the throttle, showing the increased airflow then returning to idle flow. At this point I did a smoke test on the intake manifold and the exhaust manifold, revealing no leaks. I did another drive cycle monitoring the VVT, EGR, and EVAP, and these systems seemed to be affected by the surging but still very much operational. I called the dealer help-line and informed them of all of this data, and they were confused as well.

Repair Attempts

1-A/F sensor-no change
2-EGR valve-no change
3-Evap purge solenoid-no change
4-PCV valve-no change
5-Accelerator pedal-no change

At this point I just decided to grab a loaner car and start swapping sensors, since talking to tech-line and ordering parts was taking ages. I started with the MAF sensor, swapped in the part and went out for a drive. I didn't feel anything going around the block, so I went around again and nothing. I went around the city a few times and never felt the surging again, so I logged the data again and behold...the A/F sensor signal is clean!

Good A/F sensor signal

So what can we learn from this so we catch it faster next time?

First I would say trust your instincts. I knew there was something affecting the A/F sensor and the car could have been fixed a lot sooner if I swapped in a known good MAF sensor first. I guess the MAF sensor can be faulty even if the signal looks good, witch is strange. I did everything possible, reached out to my peers, then started hypothesizing. Kinda sounds like life right? Take a look at the photos below showing the difference between the production part and the new part. When I fist saw the difference I assumed it was some type of update to improve flow, but I was wrong.

New MAF Sensor - clear flow path

Faulty MAF Sensor- flow obstruction

AR