Actron CP9135 OBD II AutoScanner - Manual (Page 32)

B-3 About OBD II
it uses fuel injectors (either in a throttle body or at each intake port). Fuel
injectors are far more precise than carburetor jets, and create a much finer
fuel “mist” for better combustion and increased efficiency. In addition, most
fuel injection systems have ways of measuring exactly how much air is
entering the engine, and can calculate the proper air/fuel ratio using lookup
tables. Computers no longer have to “estimate” how much air the engine is
using.
In
many modern systems, the computer also uses information provided by
sensors to give it an idea of how well it is doing its job, and how to do it better.
Sensors can tell the computer how warm the engine is, how rich or lean the
fuel mixture is, and whether accessories (like the air conditioner) are running.
This feedback information allows the computer to “fine tune” the air/fuel
mixture, keeping the engine operating at its peak.
What the computer needs to know
• Engine operating condition. Sensors used are: coolant temperature,
throttle position, manifold pressure (vacuum), air flow and RPM.
• Air intake. Sensors used are: mass air flow, manifold absolute pressure,
manifold air temperature and RPM.
• Air/fuel mixture status. Sensors used are: oxygen sensor(s).
Open and closed loop modes
Open or closed loop operation refers to the way the computer is deciding how
much fuel to add to the air entering the engine. During cold start and other low
temperature situations, the computer operates in open loop mode. This means
that it is relying on a set of internal calculations and data tables to decide how
much fuel to add to the incoming air. It uses sensors such as the coolant
temperature s ensor (CTS), the throttle position s ensor (TPS), and the manifold
absolute pressure sensor (MAP) to determine optimum mixtures. The important
difference here is that it does not check to see if the mixtures are correct, leaving
the computer adjustment loop open.
In closed loop mode, the computer still decides how much fuel to add by using
the sensors listed above, and by looking up the appropriate numbers on a data
table. However, it now checks itself to determine whether the fuel mixture is
correct. It is able to check itself by using the information provided by the oxygen
sensor(s) (O2S) in the exhaust manifold. The O2S will tell the computer if the
engine is running rich or lean, and the computer can take steps to correct the
situation. In this way, the computer closes the adjustment loop by checking itself
and making necessary corrections. It should be noted that the O2S must be at a
very high operating temperature (650°F) before they begin sending information
to the computer. This is why open loop mode is necessary—to give the O2 sensors
time to warm up to operating temperature.
As long as the engine and O2 and Coolant Temperature Sensors are at operating
temperature, the computer can operate in the closed loop mode. Closed loop
mode constantly corrects to obtain an air/fuel mixture at the ideal 14.7:1. But in
stop and go cycles, the O2 sensor may in fact cool down enough that the computer
will need to rely on a set of internal parameters and go into open loop mode again.
This may happen during extended periods of idling. Many newer vehicles now
use heated O2 (HO2S) sensors to prevent this condition.
In many vehicles, the computer controls other systems related to open and closed
loop modes, including idle speed, electronic spark control, exhaust gas
recirculation, and transmission torque converter clutches. In open loop mode,
some of these systems will be adjusted to speed the warming of the engine and
get the computer into closed loop mode as quickly as possible.