Chapter
E
: Computerized Engine Control Diagnosis And Repair (Including OBD II)
92
Now close the EVAP purge solenoid to trap vacuum in
the system. Observe the fuel tank pressure sensor read-
ing on the scan tool. If the system is leaking, voltage will
climb toward as pressure increases. As always, test and
repair procedures must be followed exactly. Some test
procedures, the IM 240 for example, specify testing
EVAP system integrity with pressure instead of vacuum.
Freeze Frame Data
A Freeze Frame is a miniature 'snapshot" (one frame of
data) that is automatically stored in the ECM/TCM mem-
ory when an emissions-related DTC is first stored. If a
DTC for fuel control or engine misfire is stored at a later
time, the newest data are stored and the earlier data is
lost. All parameter ID (PID) values listed under "Scan
Tool Data" are stored in freeze frame data. The
ECM/TCM stores only one single freeze frame record.
Scan Tool Data
Figure 5-9 shows the different types of information that
can be displayed on the OBD II scan tool.
Checking Open-Loop and Closed-Loop
Operation
Electronic engine control systems have two basic
operating modes: open-loop and closed-loop. In open-
loop, the PCM ignores the heated oxygen sensor
(HO2S) and controls fuel metering based on engine
speed, load, and temperature signals and its own pro-
gram. In closed-loop, the PCM responds to the rich and
lean signals of the HO2S as well as signals from other
sensors. The PCM then controls fuel delivery to main-
tain the air-fuel ratio in relation to overall engine require-
ments. In most conditions, except high-power opera-
tion, the ratio is 14.7:1.
A cold engine starts in open-loop and goes to closed-
loop as soon as the HO2S is warm enough to send an
accurate voltage signal to the PCM. Some systems also
have a timer circuit that keeps the engine in open-loop
for several minutes after any start-up regardless of
HO2S and engine temperatures. In addition, some sys-
tems return to open-loop at idle and during full-throttle
acceleration. For accurate diagnosis, it is important to
know the open-loop and closed-loop characteristics of
the particular engine being serviced. Along with check-
ing trouble codes, verifying open-loop and closed-loop
operation is a primary point in diagnosing most drive-
ability problems.
Use a scan tool to determine if an engine is operating in
open-loop or closed-loop. Most systems transmit a dig-
ital parameter that displays what strategy the PCM is
operating in. When a loop control parameter is not avail-
able, simply monitor HO2S feedback signal voltage. If
HO2S voltage fluctuates rapidly above and below the
mid-point of its operating range (generally, 450 milli-
volts), it is sending a reliable feedback signal and the
system should be in closed-loop. If HO2S voltage does
not move from the mid-point, the PCM considers the
signal unreliable and maintains open-loop control.
Driveability Symptom Tests
Many engine control system problems produce clear
and recognizable driveability symptoms but will not set
a code.
Typical driver complaints or symptoms include:
• Rough or high idle
• Surging or hesitation
• Stalls at idle or during deceleration
• Pinging, or detonation
• Poor fuel mileage
Test procedures or troubleshooting charts for these and
other common symptoms are usually available in the
factory Service Manuals. However, in order to use these
tests, the symptom must occur or be present at the time
of testing. Symptomatic tests are not designed for use
on intermittent problems.
When using a symptomatic test, be sure to follow the
one for the specific vehicle year, model, and engine. The
tests are often different for cold and for warm engines.
Fig. 5-5
. Fuel trim corrections may be displayed on the scan tool as
percentage of correction.
Fig. 5-6.
The ECM is programed to notice the sudden change in CKP
sensor pulses.
Содержание ASE-A8
Страница 2: ......