SERVICE MANUAL NUMBER 24
FUEL INJECTION DESCRIPTIONS AND SYSTEM OPERATION
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 de-
sign 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 indicating
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 approxi-
mately 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 ob-
served, 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
90-861327
OCTOBER 1999
Page 5D-11
| Categories | Mercury MerCruiser Manuals |
|---|---|
| Tags | Mercury MerCruiser 305 CID, Mercury MerCruiser 350 CID |
| Model Year | 1998, 1999, 2000, 2001 |
| Download File |
|
| Document File Type | |
| Copyright | Attribution Non-commercial |
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