On this model Ford, the BARO PID is inferred by the ECM from data gathered from the Mass Airflow sensor (MAF). This PID is updated under part throttle and wide open throttle conditions and then stored in Keep Alive Memory (KAM). Ford uses a "hot wire" type MAF sensor. The wire is maintained at a constant temperature above ambient (as detected by the "cold" wire). Airflow across the hot wire cools it, and the current required to keep it hot is proportional to the airflow itself.

MORE THAN ONE CULPRIT

Take a look at the fuel trims: Short Term Fuel Trim (STFT) on both banks were shifting normally at plus/minus 5 percent either side of zero. But the Long Term Fuel Trim (LTFT) had learned a correction for an overly rich condition that was being reported back by the oxygen sensors. This could also point to the MAF. Because a contaminated MAF sensor will overestimate airflow at idle, the ECM is providing more fuel than really needed. The oxygen sensors report this rich condition and the ECM corrects for it by adjusting LTFT. The trim Figures are not quite high enough to trigger a code, but it's an impending problem nonetheless.

Figure 2 This Ford MAF sensor illustrates the location of hot wires. Look here to actually see hot wire contamination.

Personally, I had all the information I needed to condemn the sensor and add it to my recommendations to my customer. I knew that, even though it wasn't bad enough yet to set a code, it was in this customer's near future, and I didn't want that to happen on his trip.

BUT IS THAT THE CAUSE?

I still had some investigating to do. Because the STFT readings were normal, I didn't think the MAF was the cause of the running complaints. Reviewing the current data, I noticed that the Delta Pressure Feedback (DPFE) sensor was reading 1.74 volts at idle.

This sensor is part of the Exhaust Gas Recirculation (EGR) system, and it measures the flow of exhaust gases across a fixed orifice in the EGR feed tube and reports those findings to the ECM. The difference in pressure between the upstream hose and the downstream hose is the "delta" pressure.

Faulty readings from this sensor are not uncommon, and many Ford models had a recall on these sensors. But I didn't think this was a faulty reading. With the key on and engine off, the DPFE sensor was reading 0.87 volts, which is OK. Hmmm, could the EGR be stuck open? It would explain the rough idle.