Definitions
The world of auto repair is filled with an ever growing amount of acronyms and terminology.
Here is an attempt at defining some of it. This is a growing list and will
be updated as time goes by.
Coolant Temperature
This sensor measures the temperature of the antifreeze in your engine. It can be
measured in Celcius or Farenheight. Coolant sensors have been known to shift in
resistance causing incorrect readings. They have also been known to go open or shorted
causing a wide change in reported temp. Coolant temperature is a key input into
the base fuel schedule and can have an input into whether or not a control module
runs certain tests or functions.
Fuel Trim
This is a correction to the base fuel schedule of the fuel injection system. It
comes in a couple varieties and on most cars is expressed as a percentage. A negative
fuel trim takes fuel away and makes the fuel mixture leaner. A positive correction
does the opposite and adds fuel, making the mixture richer.
ShortTerm Fuel Trim aka STFT corrects the fuel delivery schedule using the swings
of the Oxygen sensor. When the sensr swings towards rich, STFT will correct
by going in the direction of Negative. When the sensor swings towards lean, STFT
will correct by going in th direction of Positive.
Long Term Fuel Trim aka LTFT corrects the fuel delivery schedule by trying to keep
STFT as close to zero as possible. STFT that drifts away from zero usually
result in the eventual change in LTFT.
Knowing how these trims and the fuel schedule work can be key to diagnosing lot's
of different drivability problems.
Manifold Absolute Pressure
aka MAP, This sensor is used to determine the Pressure inside an intake manifold.
This is one of the key sensors used in a Speed density type fuel injection system.
It can be measured in inches of Hg and in Kpa. Kpa is a little more accurate because
of the wider range and seems to show better when being graphed. Escan uses MAP to
determine the vacuum of the engine and displays it as a calculated pid. One feature
unique to the Escan scanner. Timing, Fuel injection control, and Trans shifting
are just some of the items that use the MAP value.
Mass Airflow Sensor
A vehicles Mass Airflow Sensor aka MAF is used to measure the amount of air that
enters an engine. This is a key sensor. The PCM needs to know how much air went
in in order to determine how much fuel is needed for optimum combustion. Not all
vehicles use airflow sensors. The ones that don't usually use some type of speed
density system which calculates how much air the car thinks it took in. The MAF
variety is a more accurate way of determining the mount of air. One problem with
airflow sensors is that they can easily become contaminated (especially by poor
air filters and oil type performance air filters). Cleaning them can be hit or miss
and replacement is the industry standard repair. Escans Fuel Trim Analysis and Volumetric
Efficiency Test make dignosing faulty MAF sensors easy. A lower than expected airflow
reading and trims that are negative at idle but switch towards positive off idle
are both indicators of faulty MAF sensors.
Oxygen Sensor
O2 sensors don't actually report Oxygen content they report the actual fuel mixture
or Lamda. There are many types but this definition is for the older basic O2 sensor.
When an O2 sensor reports a richer mixture it's voltage raises above it's switch
point. When it reports a leaner mixture it's voltage lowers below it's switch point.
The PCM uses this info to toggle the mixture rich and lean keeping it close to Stoich
or what is considered the desired mixture. Usually to speed up mixture control these
sensors are heated by an internal element. Poor heater operation results in
sluggish sensors. These sensors can become contaminated or damaged and should be
checked for response quality often to ensure good mixture control.
Throttle Position Sensor
This sensor aka TPS is used to measure how far the throttle is commanded open. It
is a key sensor and it is used to determine lots of things like, Transmission shift
points, Idle control, Timing, Fuel delivery, and more. A TPS sensor can be subject
to signal dropout or glitches that may be too quick for a scantool. When testing
it may be helpful to turn off the rest of the pids in order to speed up the data
rate of the sensor signal as displayed on the scanner.
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