Mercury MerCruiser MCM 502 Mag MPI User Manual

Name: Mercury MerCruiser MCM 502 Mag MPI
Brand: Mercury
Rating: 4 (1 reviews)

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STARTING SYSTEM SERVICE MANUAL NUMBER 23

Page 4A-2 90-861326--1 MARCH 1999

Starting System Components (Typical for All Starters)

72930

a-Ignition Switch

b-20 Amp Fuse

c-Starter Slave Solenoid

d-Circuit Breaker

e-Starter Motor

f-Wire Junction

g-Neutral Safety Switch

h-90 Amp. Fuse

i-Engine Ground (–)

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Index

Important Information 18
- Table of Contents8
- Introduction9
- How to Use this Manual9
  - Page Numbering9
- How to Read a Parts Manual10
- Directional References11
- Engine Rotation11
- Engine Serial Number Locations12
- Propeller Information13
- Water Testing New Engines13
- Boat and Engine Performance14
Important Information 118
- Lubricants / Sealants / Adhesives19
- Maintenance Schedules20
  - Maintenance Intervals20
  - Scheduled Maintenance that Can be Performed by Owner/Operator20
  - Scheduled Maintenance that Should be Done by Dealer21
  - Scheduled Maintenance that Should be Done by Dealer22
  - Scheduled Maintenance that Can be Performed by an Owner/Operator23
  - Scheduled Maintenance that Should be Done by Dealer23
- Firing Order28
- Fluid Capacities28
  - Engines28
  - Transmissions29
  - Sterndrive Units29
- 20-Hour Break-In Period29
- After Break-In Period30
- End of First Season Checkup30
- Specifications31
  - Fuel Recommendations31
  - Test for Alcohol Content in Gasoline32
- Power Steering Fluid32
- Transmission Fluid32
- Coolant for Closed Cooling System33
- Crankcase Oil33
  - Overfilled Crankcase34
  - Checking Engine Oil Level/Filling34
- Changing Oil and Filter35
- Changing Water Separating Fuel Filter36
  - MCM (Sterndrive) Models36
  - MIE (Inboard) Models37
- Power Steering System38
  - Checking Fluid Level38
  - Filling and Bleeding39
- Closed Cooling System40
  - Checking Coolant Level40
  - Flushing System MCM (Sterndrive)41
  - Flushing System MIE (Inboard) - Except 454 Mag MPI Horizon43
  - Flushing System MIE (Inboard) 454 Mag MPI Horizon45
- Transmission Fluid49
  - Checking Fluid Level49
- Lubrication50
  - Throttle Cable50
  - Shift Cable51
  - Shift Cable and Transmission Linkage . 1B-3451
  - Engine Coupler / U-Joint Shaft Splines52
  - Drive Shaft Extension Models53
  - Starter Motor54
- Cleaning Flame Arrestor55
  - Plastic Side Mounted Flame Arrestor . 1B-3855
  - Plastic Front Mounted Flame Arrestor 1B-3956
  - Metal Side Mounted Flame Arrestor56
- Serpentine Drive Belt57
  - Component Location57
  - Inspecting59
  - Replacing And/Or Adjusting Tension59
- Ignition Timing60
- Cold Weather or Extended Storage61
  - Precautions61
  - Power Package Layup62
Important Information 184
- Precautions85
- Used Spark Plug Analysis86
- Spark Plug Analysis87
  - Normal Condition87
  - Chipped Insulator87
  - Wet Fouling (Oil Deposits)87
  - Cold Fouling88
  - Overheating88
  - High Speed Glazing88
  - Scavenger Deposits89
  - Pre-Ignition Damage89
  - Reversed Coil Polarity89
  - Splashed Deposits90
  - Mechanical Damage90
- Poor Boat Performance And/Or Poor Maneuverability91
- Improper Full Throttle Engine RPM92
  - RPM too High92
  - RPM too Low92
- Engine Cranks over but will Not Start or Starts Hard93
  - Important Information93
  - Fuel System Rich93
  - Fuel System Lean93
  - Miscellaneous93
- Charging System Inoperative94
- Engine will Not Crank over94
- Instrumentation Malfunction95
- Noisy Alternator95
- Radio Noise95
- Engine Runs Poorly at Idle96
- Poor Fuel Economy96
- Engine Acceleration Is Poor97
- Engine Runs Poorly at High RPM97
- Engine Noise98
  - Important Information98
  - Valve Cover Area99
  - Cylinder Area99
  - Camshaft Area100
  - Crankshaft Area100
  - Miscellaneous101
- Troubleshooting with Vacuum Gauge98
- Oil Pressure102
  - Miscellaneous102
  - Low Oil Pressure103
  - High Oil Pressure103
- Excessive Oil Consumption104
- Water in Engine105
  - Important Information105
  - Water on Top of Pistons106
  - Water in Crankcase Oil106
- Engine Overheats107
  - Mechanical107
- Engine Overheats108
  - Cooling System108
  - Insufficient Water Flow from Belt Driven Seawater Pickup Pump109
- Power Steering110
  - Poor, Erratic or no Assist110
  - Noisy Pump111
  - Fluid Leaks111
Removal and Installation 2113
- Lubricants / Sealants / Adhesives114
- Tools114
- Torque Specifications114
- Preparation115
- Removal115
- Installation116
Removal and Installation 2155
- Preparation157
- Removal157
- Installation158
  - Engine Installation and Initial Alignment158
  - Engine Final Alignment160
- Engine Connections163
- Seawater Hose Connection163
- Electrical Connections164
- Fuel Supply Connections164
- Exhaust Hose Connection165
- Throttle Cable Installation and Adjustment165
- Shift Cable Installation and Adjustment171
- Velvet Drive Transmissions171
Removal and Installation 2181
- Torque Specifications182
- Preparation183
- Removal183
- Installation184
  - Engine Installation and Initial Alignment184
  - Engine Final Alignment187
- Engine Connections191
- Shift Cable Installation and Adjustment199
Engine205
- Mag / 502 Mag / 8.2L211
- General Information220
  - Engine Identification220
  - Cylinder Head Identification221
  - Engine Rotation221
  - Crankshaft222
  - Piston and Connecting Rods222
  - Camshaft and Drive222
  - Cylinder Head222
  - Valve Train222
  - Lubrication System222
- Bearing Failures224
- Piston Failures226
  - Pre-Ignition226
  - Detonation227
- Engine Mounts229
- Rocker Arm Cover232
  - Removal232
  - Installation232
- Intake Manifold233
  - Removal233
  - Cleaning and Inspection233
  - Installation234
- Rocker Arm/Push Rod236
  - Removal236
  - Cleaning and Inspection236
  - Installation236
  - Rocker Arm Stud Kit237
- Hydraulic Roller Valve Lifters237
  - Locating Noisy Lifters238
  - Removal239
  - Installation239
- Valve Adjustment237
- Valve Stem Oil Seal / Valve Spring240
  - Removal - Head Installed240
  - Valve Assembly (Exploded View)241
  - Valve Installation - Head Installed242
- Cylinder Head243
  - Removal243
  - Cleaning243
  - Inspection243
- Cylinder Head and Valve Conditioning245
  - Disassembly245
  - Cleaning245
  - Inspection246
  - Valve Guide Bore Repair247
  - Valve Springs - Checking Tension248
  - Valve Seat Repair248
  - Valve Grinding249
  - Reassembly250
- Crankcase Oil Dipstick Specifications252
  - All Engines252
- Oil Pan253
  - Removal253
  - Installation253
- Oil Pump255
  - Removal255
  - Disassembly256
  - Cleaning256
  - Reassembly256
  - Installation257
- Torsional Damper257
  - Removal257
  - Installation258
- Front Cover / Oil Seal259
  - Oil Seal Replacement (Without Removing Front Cover)259
- Front Cover260
  - Removal260
  - Cleaning260
  - Inspection260
  - Installation260
- Flywheel262
  - Removal262
  - Inspection263
  - Installation263
- Rear Main Oil Seal264
  - Removal264
  - Cleaning and Inspection264
  - Installation265
- Main Bearings265
  - Inspection265
  - Checking Clearances265
  - Replacement267
- Connecting Rod Bearings269
  - Inspection and Replacement269
- Connecting Rod / Piston Assembly271
  - Removal271
- Crankshaft278
  - Removal278
  - Cleaning and Inspection278
- Timing Chain and Sprocket280
  - Removal280
  - Cleaning and Inspection280
  - Installation280
- Crankshaft Sprocket281
  - Removal281
  - Installation281
  - Checking Timing Chain Deflection281
- Camshaft282
  - Measuring Lobe Lift282
  - Removal282
  - Inspection283
  - Installation283
- Camshaft Bearings283
  - Removal283
  - Inspection284
  - Installation284
- Cylinder Block285
  - Cleaning285
  - Inspection285
- Compression Test 3A-85289
- Oil Filter By-Pass Valve and Adaptor 3A-86290
  - Inspection and / or Replacement 3A-86290
Electrical Systems292
- Starting System Components293
- Direct Drive Starter Motor294
  - Identification294
  - Electrical Circuit Description294
- Lubricants / Sealants / Adhesives295
- Specifications295
- Direct Drive Starter Motor Exploded View 4A-6297
- Delco PG260 Starter Motor307
  - Identification307
  - Electrical Circuit Description307
- Specifications308
- Lubricants / Sealants / Adhesives309
- PG 260 Starter Motor Exploded View310
- PG 260 Starter Motor Repair311
- PG260F1 Starter Motor325
- Starter Motor Repair328
- Mando Starter Motor339
  - Identification339
  - Electrical Circuit Description339
- Description341
  - Exploded View341
Electrical System342
- Lubricants / Sealants / Adhesives343
- Specifications343
- Torque Specifications343
- Ignition Control System Components344
- Ignition Timing354
  - Checking/Adjusting354
Electrical System356
- Description357
  - Multiple EFI Engine Battery Precautions361
- Mando 65 Amp Alternator357
  - Identification357
- Replacement Parts Warning357
- Specifications357
- EFI Electrical System Precautions361
- Drive Belt Tension Adjustment362
- Periodic Maintenance362
- System Components362
- Troubleshooting Tests363
- Exploded View369
- Alternator Repair370
- Installation383
  - Battery Isolator Diagram384
Electrical System390
- Identification391
- Special Information392
  - Lighting Options392
- Gauges394
- Senders398
- Switches404
- Audio Warning System406
Electrical System412
- Wiring Colors for Mercruiser413
- Starting and Charging System Wiring Harness Diagrams415
- Fuel Injection Wiring Harness Diagrams423
- MEFI 2 MIE 7.4L MPI Model425
- MEFI 3 MCM 7.4L MPI Bravo and MIE 7.4L Inboard428
- Instrumentation Harness Diagrams429
- MCM (Sterndrive)429
- MIE (Inboard)430
- Dual Station Wiring431
Fuel System438
- Lubricants / Sealants / Adhesives439
- Specifications439
- Tools439
- Torque Specifications439
- Fuel Supply Connections440
- Precautions440
- Replacement Parts Warning440
- Fuel Delivery System441
  - Recommendations441
- Special Tools443
  - Rinda Technologies Special Tools444
- Cool Fuel System Exploded View445
- Fuel System Flow Diagram447
- Water Separating Fuel Filter448
- Cool Fuel System Repair449
- Vacuum Line Routing457
Fuel System460
- Identification461
  - Electronic Control Module (ECM)461
- General Information462
- Abbreviations465
- ECM Self-Diagnostics466
- Special Tools471
- Precautions472
- Electronic Control Module (ECM) and Sensors474
- Spark Management486
- Fuel Metering System492
Fuel System502
- Precautions503
- Exploded Views504
- 454 / 502 Mag MPI Multi-Port Components509
- Fuel Pressure Relief Procedure509
- 454 / 502 MPI and 8.2L MPI - Sensor and Module Servicing523
- Vacuum Line Routing536
  - 502 MPI and 8.2L MPI Models 5C-35536
- Exploded View537
  - 7.4L MPI Intake Manifold, Plenum and Fuel Rail537
  - 7.4L MPI Throttle Body and Adapter Components539
- 7.4L MPI Components540
- Fuel Pressure Relief Procedure540
- Vacuum Line Routing557
  - 7.4L MPI Models557
Fuel System560
- Diagnostic Trouble Codes for the MEFI 1 454 / 502 Mag MPI and 8.2L MPI Engines561
  - MEFI 2 7.4L MPI Engines561
  - (8.2L) Engines561
- Troubleshooting561
- Scan Tool Normal Specifications564
  - Neutral)564
- Important Preliminary Checks565
- Visual/Physical Check565
- Hard Start Symptom567
- Surges And/Or Chuggles Symptom570
- Lack of Power, Sluggish or Spongy Symptom572
- Detonation/Spark Knock Symptom575
- Hesitation, Sag, Stumble Symptom578
- Cuts Out, Misses Symptom580
- Rough, Unstable, or Incorrect Idle, Stalling Symptom582
- Poor Fuel Economy Symptom586
- Backfire Symptom589
Fuel System594
- Diagnostic Circuit Check595
  - Scan Tool Normal Specifications (Idle / Warm Engine / Closed Throttle / Neutral)595
- MEFI 1 - ECM Connector Chart for 454/502 Mag MPI and 8.2L MPI596
- Diagnostic Trouble Codes598
- Special Tools600
- MEFI 1 - 454/502 Mag MPI and 8.2L MPI Symptom Chart (J-1 Circuits)604
- MEFI 1 - 454/502 Mag MPI and 8.2L MPI Symptom Chart (J-2 Circuits)606
- MEFI 2 - ECM Connector Chart for 7.4L MPI616
- MEFI 2 - 7.4L MPI Symptom Chart (J-1 Circuits)619
- MEFI 3 - ECM Connector Chart633
- Test Example643
- General Diagnostic Tests644
Fuel System746
- Clearing Trouble Codes748
  - Clearing Codes Using Diagnostic Code Tool (Non-Scan)748
  - Clearing Codes Using Scan Tool (Scan)748
- Diagnostic Testing Without a Scan Tool749
  - Voltage Low756
  - Temperature Indicated759
- Code 41 (1 of 3): Ignition Control (IC)770
  - Open Bypass Circuit, or Grounded773
  - Bypass Circuit773
- Code 44 (1 of 3): Knock Sensor (KS)778
  - Code 51 (1 of 2): Calibration Memory Failure (Non-Scan)781
- Diagnostic Testing Using a Scan Tool783
  - Temperature Indicated783
- Code 15 (1 of 3): Engine Coolant785
- Code 21 (1 of 2): Throttle Position (TP) Sensor Circuit (Scan) High Signal Voltage788
- Code 51 (1 of 1): Calibration Memory Failure (Scan)813
- Code 52 (1 of 1): EEPROM Failure (Scan)814
Cooling System826
- Tools827
  - Special Tools827
- Torque Specifications827
- Lubricants / Sealants / Adhesives828
- Specifications828
  - Cooling System Capacity828
  - Thermostat828
- Seawater Inlet Recommendations829
- Seawater Pickups830
  - Through-Hull Mounted830
- Sea Strainer833
  - Removal833
  - Installation834
- Seawater Pickup Pump835
  - Seawater Pump Disassembly836
  - Seawater Pump Reassembly837
- Seawater Pump Bearing Housing838
- Water Circulating Pump844
- Drive Belt Tension Adjustment845
- Flushing Seawater Cooling System846
  - 454/502 Mag MPI and 8.2L MPI846
  - 454 Mag MPI Horizon Models847
- Seawater Strainer848
- Thermostat849
- Auxiliary Hot Water Heater Installation852
- Water Flow Diagram853
- Water Tap Locations for Propshaft Coolers853
Cooling System856
- Cooling856
- Lubricants / Sealants / Adhesives857
- Specifications857
- Torque Specifications857
- Description858
- Maintaining Coolant Level858
- Pressure Cap Maintenance859
- Heat Exchanger Repair860
- Seawater Pickup Pump Maintenance860
- Testing Closed Cooling System860
- Thermostat865
- Changing Coolant868
- Cleaning System869
  - Closed Cooling Section869
  - Seawater Section870
- Filling Closed Cooling Section871
- Auxiliary Hot Water Heater Installation873
- Heat Exchanger Bracket Hardware874
- Heat Exchanger Hose Connections875
- Closed Cooling System Water Flow Diagram876
  - MCM / MIE Models876
- (Coolant Section of System)877
Exhaust System880
- Exhaust System Requirements881
- Exhaust Tube Requirements883
Exhaust System886
- Lubricants / Sealants / Adhesives887
- Removal887
- Torque Specifications887
- Cleaning888
- Inspection888
- Installation889
Exhaust System891
- Description892
- Preparation892
- Sealants892
- Torque Specifications892
- Exhaust Risers893
- Cast Iron Elbows with 3 Inch Riser895
- Installation Diagrams895
- Maintenance Instructions895
- Cast Iron Elbows with 3 Inch Riser and Muffler Spacer896
- Cast Iron Elbows with 6 Inch Riser897
- Cast Iron Elbows with 6 Inch Riser and Mufflers898
Exhaust System900
- Lubricants/Sealants/Adhesives901
- Torque Specifications901
- Shutter Replacement902
- Component Replacement903
- Through-Transom Exhaust904
  - Shutter Replacement904
  - Component Replacement905
- Below Swim Platform Exhaust Pipe905
- Silent Choice Exhaust System906
- Exhaust Tube Installation907
- Air Tube Routing909
- Maintenance910
- Exhaust Muffler Kit911
- Cleaning and Inspection912
- Installation913
- Maintenance Instructions913
Drives916
- Velvet Drive In-Line (72C) Specifications917
- Important Information953
- Shift Cable Adjustment955
- Checking Transmission Fluid Level960
- Changing Transmission Fluid961
  - Draining Transmission961
  - Filling Transmission962
- Removal and Installation962
- Shift Lever Installation964
- Pressure Test965
- Transmission Repair965
Drives993
- Lubricants/Sealants994
- Tools994
- Flywheel Housing/Output Shaft Housing Repair (MCM Sterndrive Models)995
  - Disassembly995
  - Reassembly995
- Drive Shaft Repair (MCM Sterndrive Models)997
- Installation1000
- Bearing Support Repair (MCM Sterndrive Models)1002
- Propeller Shaft (MIE Inboard Models)1006
Power Steering1012
- Lubricants / Sealants1013
- Precautions1013
- Tools1013
- Power Steering Pump and Components (Exploded View)1014
- Belt Routing1015
- Serpentine Belt Adjustment1015
- Important Service Information1016
- Pump Pulley Replacement1016
- Checking Pump Fluid Level1017
  - Filling and Air Bleeding System1017
- Testing and Repair1017
- Pump Assembly1018

Questions and Answers:

J
James MitchellAug 9, 2025
Why does my Mercury MerCruiser MCM 502 Mag MPI Engine fluctuate rapidly at idle and then steadies as rpm is increased?
- M
  Monica WalkerAug 9, 2025
  If your Mercury Engine reading fluctuates rapidly at idle but steadies as the rpm increases, the valve guides may be worn.

Mercury MerCruiser MCM 502 Mag MPI Specifications

General

Engine Type	V8
Fuel System	Multi-Port Fuel Injection (MPI)
Alternator	65 amp/917 watt
Displacement	8.2L
Bore x Stroke	4.47 in x 4.00 in
Cooling System	Closed Cooling