Calculated Engine Load
California Air Resource Board (CARB) defines this as airflow divided by peak air flow. Peak air flow may be adjusted for altitude via barometric means if available.
For diesel engines, the number is current output torque divided by peak output torque at current rpm. We have noted that vacuum leaks can have dramatic effects on this value.
Modern engine performance depends heavily on the accuracy of the sensor monitoring coolant temperature. This sensor is often independent of the measurement reported on the dash.
It should read close to ambient temperature at startup, and run to between 180°F-220°F after warmup. Injector pulse width is heavily dependent on this sensor, causing engines to run very rich at startup (remember the manual choke?). Fuel enrichment is usually tapered off around 150°F.
Fuel trim refers to adjustments being made dynamically to the base fuel table to get the proper ratio of fuel to air. Short term fuel trim refers to adjustments being made in response to temporary conditions.
Long term fuel trim is used to compensate for issues that seem to be present over a much longer period. Fuel trims are expressed in percentages; positive values indicate lean (add fuel) and negative values indicate rich (subtract fuel). Fuel trim banks refer to the cylinder banks in a V style engine.
Cylinder #1 is always in bank 1. Fuel trim is generally calculated by using a wide set of data values, including front O2 sensors, intake air temperature/pressure (or the more elegent air mass sensor), engine (coolant) temp, anti-knock sensors, engine load, throttle position (and change in throttle position), and even battery voltage can effect fuel trim. Long term fuel trim generally should not exceed +- 10%.
Manifold Air (or Absolute) Pressure (MAP) sensor measures intake vacuum, used to calculate engine load and timing advance. It may also contribute to fuel trim.
O2 sensors monitor exhaust gases for the presence of oxygen. Heated sensors are commonly referred to as HO2S, which allows sensors to begin giving valid data much earlier in the warm up cycle.
Most sensors output a voltage between 0 and 1.0 volts (usually .10 to .90), 0v - .45v for lean conditions, and .55v - 1.00v for rich conditions. Upstream sensors are primarily used to adjust engine air and fuel flow, while downstream (of the catalytic converter) sensors are used to monitor the effect of the catalytic converter.
Sensors do not usually run at constant voltages, rather oscillate rapidly (at least once every few seconds) in a nice sine wave. The frequency is referred to as switching frequency, or as switches.
The rear sensor should display a much slower switching frequency than the front sensor, the difference indicating the catalyst capability. These sensors are invaluable in the diagnostic effort, and can indicate conditions ranging from fuel delivery issues, vacuum leaks, to engine oil overfill conditions
Air intake temperature is used to correct fuel quantity (pulse width) since air temperature effects air density. (colder air is much more dense) Approximately 70°F is generally used for 0 correction, temperatures above will result in less fuel to compensate for the lower air density, and conversely temperatures below 70° will result in steadily richer (by volume) mixtures. Bad readings will effect ECM attempts to keep the air to fuel ratio proportional.
Deliberate IAT Sensor Distortion
OBD2 systems use the IAT, MAF, and HO2S sensors to regulate the fuel to air mixture. To keep the exhaust as clean as possible, the OBD2 systems will attempt to run the system fairly lean.
Performance enthusiasts would prefer a richer mixture, at the expense of fuel economy and clean emissions. A fairly simple way to get a richer mixture is to replace the IAT sensor with one that claims the air is cold and dense when it really isn't.
Such devices are inexpensive, fairly easy to install, and comprise the majority of the 'performance chip' upgrades advertised on certain popular auction sites
The increase power claims are mitigated by the actual outside temperature, the amount of inefficiency in the stock fuel map (most maps make the ratio richer when heavy throttle is present anyway), and the amount of compensation that will be done when the O2 sensors see the results of reduced oxygen in the exhaust.
The proper correction is custom tuning as we do at Team ZR-1 Corvette Racer's LLC