Chapter
E:
Computerized Engine Control Diagnosis And Repair (Including OBD II)
91
indicate more fuel is being added. When the ECM sees
a rich (high voltage) signal from the HO2S, the fuel trim
program subtracts fuel to lean the mixture. The scan
tool will display a percentage on the negative (-) side of
0%. If short term fuel trim is necessary in one direction
(rich or lean correction) for a period of time, the ECM will
command a correction of long term fuel trim. When A/F
control is out of acceptable range for too long a time, a
DTC will set. Long term fuel trim represents correction
to fuel delivery over time. If the oxygen sensor voltage
is fluctuating, but is mainly below 450 mV, indicating a
lean A/F ratio, long term fuel trim will increase and the
ECM will command longer injector pulse width. If oxy-
gen sensor voltage is fluctuating, but remains mostly
above 450 mV, indicating a rich mixture, long term fuel
trim will decrease and the ECM will command shorter
injection pulse width to adjust fuel delivery.
Short term fuel trim is useful when confirming fuel con-
trol. Observe short term fuel trim on the scan tool while
adding propane through the intake system. The addi-
tional fuel will cause a rich mixture. If the fuel system is
in closed loop, short term fuel trim will move in a nega-
tive direction as the fuel trim program shortens fuel in-
jector pulse width in response to a higher HO2S voltage
signal. Driving the system lean by pulling a vacuum line
will cause short term fuel trim to increase injector pulse
width. The scan tool display will move in a positive di-
rection.
During diagnosis, be sure to look at both short and long
term fuel trim. A problem that has existed for some time
will cause long term fuel trim to record high or low. Once
the problem is repaired, long term fuel trim will not
change for a while, but short term fuel trim will begin im-
mediately to move in the opposite direction. A restrict-
ed fuel filter for instance will cause a lean mixture. Long
term fuel trim will eventually show a positive percentage
(more fuel) as the system compensates for the lean mix-
ture. Once the fuel filter is replaced, the A/F ratio is sud-
denly too rich. Comparing short and long term fuel trim
immediately after the filter is replaced will reveal oppo-
site readings: a negative percentage reading in short
term fuel trim because the ECM is attempting to return
the A/F ratio to normal by subtracting fuel, and a posi-
tive percentage reading in long term fuel trim because
the long term program still "remembers" the lean cor-
rection and is waiting to see what happens.
Misfire Monitor
Engine misfire monitoring uses the CKP signal as the
primary sensor. When a misfire occurs, whether due to
engine compression, ignition, or fuel, crankshaft speed
is affected. The ECM is programmed to notice the inter-
mittent change in CKP pulses, figure 5-6.
Camshaft position is used to identify which cylinder mis-
fired. Because outside factors such as electrical inter-
ference and rough roads can mimic a misfire, most ECM
programs keep track of how many times a cylinder mis-
fires in a given number of engine rotations. The ECM ac-
tivates the MIL when misfire reaches a predetermined
percentage of rpm.
Remember, misfire monitoring, like fuel trim monitoring,
is a two trip monitor. The MIL will glow steadily once a
misfire is detected. If misfiring becomes severe enough
to damage the catalytic converter, the MIL will blink con-
tinuously until the misfire becomes less severe.
Catalytic Converter Monitor
As mentioned earlier, the catalytic converter monitor
checks converter efficiency by comparing upstream
HO2S signals with the downstream HO2S signal. In nor-
mal operation the upstream HO2S signals will switch
frequently between 200 mV and 900 mV and the down-
stream HO2S signal will show very little fluctuation and
a voltage that tends to stay above the 450 mV
thresh-
old,
figure 5-7. As catalyst performance begins to de-
grade, less oxygen is used in the converter and so less
ends up in the exhaust, causing voltage fluctuations and
a lower voltage bias, figure 5-8. When the downstream
HO2S voltage signal begins to fluctuate within about
70% of the upstream HO2S signal on two consecutive
trips, the ECM will record freeze frame data, set a DTC
and actuate the MIL.
EVAP Monitor
A vehicle will fail the EVAP monitor if the ECM, using in-
formation from the fuel tank pressure sensor, sees vac-
uum decrease too quickly after the EVAP vent and EVAP
purge solenoids have been closed. Keep in mind that
simple problems like a loose, damaged, or missing gas
cap will cause this code to set. Be careful when making
quick repairs. For example, after replacing a damaged
gas cap on a vehicle brought in for a lit MIL, you may be
tempted to clear the DTC and return the car to the cus-
tomer after a short road test. However, the EVAP moni-
tor won't run if the engine is warm (above 86°F) or if the
fuel level is not between 1/4 and 3/4 full. If the EVAP sys-
tem has other problems and the EVAP monitor doesn't
run during the road test, the MIL will come on after you
return the vehicle to the customer.
Some technicians use a sensor simulator to simulate a
cold start so the monitor will run. A scan tool can be
used to check EVAP system integrity, even with a full
tank of gas. First, idle the engine. Then using a scan
tool, close the EVAP vent solenoid and open the EVAP
purge solenoid. Intake manifold vacuum will draw a vac-
uum in the EVAP system.
Содержание ASE-A8
Страница 2: ......