MODERN SPEED DENSITY ADAPTATIONS
With a few added refinements, the speed density system might perform more reliably on modern engines than mass airflow systems. As mentioned earlier, the MAP/Baro sensor speeds up the air/fuel computation by instantly responding to changes in MAP. To further refine the system, many speed density configurations separate the MAP and Baro functions. The Baro sensor might, for example, be built into the ECM circuit board.
On some applications, the MAP sensor might measure a broader pressure range than the barometric pressure sensor. A broad-range MAP sensor can be used to not only indicate MAP, but turbocharger boost pressure and turbocharger exhaust backpressure as well. And, on some EcoBoost applications, the IAT sensor might be combined with the MAP sensor. At KOEO, the ECM might monitor sensor calibration by comparing or rationalizing Baro, MAP and boost pressures.
When operating parameters are exceeded, a DTC will be stored for the odd-reading sensor. As smaller cylinder displacements and turbocharging become more common, look for speed density configurations that can detect various intake air system malfunctions.
With that said, we’re going to see more adaptive operating strategies that will tend to reduce performance complaints by substituting a default value in place of the faulty sensor. So the datastream you’re seeing on your scan tool might not indicate a performance complaint, but will result in a stored DTC.
At this point, most of us in this industry haven’t experienced enough of this new technology to define pattern failure characteristics or devise diagnostic methods that will solve the more elusive failures.
But remember that modern engine and drivetrain diagnostics are becoming very scan tool-based. For that reason, it’s always better to survey an engine management configuration by connecting a scan tool and looking for the various MAP and intake air temperature sensors.
The next step is to work from a wiring schematic to determine the relationship of the various combinations of barometric pressure and temperature sensors reporting to the ECM. Always leave the intrusive testing as a last step.
While this three-step procedure might appear to be added work, that, in itself, is the nature of dealing with the new generation of speed density systems found in 21st century engine technologies.
