Chart S5: Engine Misses - Mercury MerCruiser Service Manual

Step
Check for the following:
• Low compression
12
• Deposits on the intake valves
Was a problem found?
Review all of the procedures in this table. If all procedures have been completed and no problem found, inspect the
following:
13
• CDS G3 scan tool data
• All of the electrical connections within a suspect circuit or system

Chart S5: Engine Misses

Definition: Steady pulsation or jerking that follows engine speed, usually more pronounced as engine load increases. The
exhaust has a steady spitting sound at idle, low speed, or on hard acceleration. Fuel starvation can cause the engine to miss.
IMPORTANT: For additional information on diagnosing engine misfires, refer to the PCM 112 Service Manual.
Step
1 Was the Visual/Physical Check completed?
2 Did the onboard diagnostic (OBD) system check complete?
• Check for contaminated fuel.
• Check fuel filters and the water‑separating fuel filter.
3
• Check for poor fuel quality and improper octane rating.
Was a problem found?
Check for proper fuel pressure while the condition exists.
4
Was a problem found?
Check the fuel injectors.
5
Was a problem found?
Check for adequate spark at all of the spark plugs.
6
Was adequate spark present?
Remove the spark plugs and check them for moisture, cracks,
wear, improper gap, burned electrodes, or heavy deposits.
NOTE: If the spark plugs are fouled with gas or oil, the cause of the
7
fouling must be determined before replacing the spark plugs.
Was a problem found?
Check for electromagnetic interference (EMI). A missing condition
can be caused by EMI on the reference circuit. EMI can usually be
detected by monitoring engine RPM with a scan tool or
8
tachometer. A sudden increase in RPM with little change in actual
engine RPM indicates EMI is present.
Was a problem found?
Check for the following:
• Low compression
• Sticking or leaking valves
• Bent push rods
• Worn rocker arms
9
• Broken valve springs
• Worn camshaft
• Improper valve timing or valve train problem
• Restricted exhaust system
Was a problem found?
Check the intake and exhaust manifolds for casting flash.
10
Was a problem found?
90-8M0099748 eng DECEMBER 2015
Action
Action
Yes
For repair, refer to Section 3 ‑
Engine Mechanical.
Yes
Go to step 2.
Go to step 3.
Use known good fuel.
Replace the fuel filters.
Go to Chart T3.
Repair or replace the faulty
injector.
Go to step 8.
Replace the spark plugs.
Locate and correct the EMI
source.
Retest the system.
For repair, refer to Section 3
‑ Engine Mechanical.
Repair or replace.
Retest the system.
© 2016 Mercury Marine
Troubleshooting
No
Go to step 13.
No
Go to the Visual/
Physical Check.
Connect the CDS G3
service tool and scan
for faults.
Go to step 4.
Go to step 5.
Go to step 6.
Go to Chart T4.
Go to step 8.
Go to step 9.
Go to step 10.
Go to step 11.
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