Non-Scan Diagnosis Of Driveability Concerns (With No Codes Set) - Mercury MerCruiser MCM 454 Mag MPI Service Manual

SERVICE MANUAL NUMBER 23
CLEARING CODES USING SCAN TOOL (SCAN)
1. Connect scan tool.
2. Start engine.
3. Select clear codes function.
4. Clear codes.
5. Turn key OFF.
6. Turn key ON and read codes. If codes are still present, (there is a real fault in system)
NOTE: When clearing codes without the use of a scan tool, the battery must be fully
charged. The ability to clear codes is directly dependent on the battery being fully charged
and able to start the engine with adequate cranking rpm.

Non-Scan Diagnosis of Driveability Concerns (With No Codes Set)

If a driveability concern still exists after following the diagnostic circuit check and reviewing
"Troubleshooting," an out-of-range sensor may be suspected. Because of the unique
design of the EFI system, fail-safes have been incorporated into the ECM to replace a
sensed value with a default value in the case of a sensor malfunction or sensor wiring
concern. By allowing this to occur, limited engine performance is restored until the vessel
is repaired. A basic understanding of sensor operation is necessary in order to diagnose an
out-of-range sensor.
If the sensor is within its working or acceptable parameters, as shown, the ECM does not
detect a problem. If the sensor should happen to fall out of this "window," a code will be
stored. A known default value will replace the sensed value to restore engine performance.
If the sensor is out of range, but still within the operating window of the ECM, the problem
will go undetected by the ECM and may result in trouble later.
A good example of this would be if the coolant sensor was reading incorrectly and indicat-
ing to the ECM that coolant temperature was at 20 F, but actual coolant temperature was
175 F. This would cause the ECM to deliver more fuel than was actually needed and result
in an overly rich, rough running condition. This condition would not have caused a code to
set as the ECM interprets this as within its range.
To identify a sensor which is out of range, unplug it while running the engine. After
approximately two minutes, the diagnostic code for that sensor will set, a code, and replace
the sensed value with a default value. If at that point a noticeable performance increase is
observed, the non-scan code chart for that particular sensor should be followed to correct
the problem.
NOTE: Be sure to clear each code after disconnecting and reconnecting each sensor. Fail-
ure to do so may result in a misdiagnosis of the problem.
Index
90-861326--1 MARCH 1999
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check following note and repeat Step 1.
MULTI-PORT FUEL INJECTION DESCRIPTIONS AND SYSTEM OPERATION
Page 5B-11