Mercury MerCruiser GM V8 454 CID (7.4L) / 502 CID (8.2L) Marine Engine Service Manual No. 23 (P/N 90-861326--1)

Mercury Marine Service Manual Number 23, P/N 90-861326--1, March 1999 edition, covers GM V8 454 CID (7.4L) and 502 CID (8.2L) marine engines in the listed Gen. VI and L-29 sterndrive and inboard configurations.

• Engine overhaul: Cylinder head and valve conditioning, crankshaft and bearing inspection, piston and connecting rod service, camshaft replacement, and timing chain installation.

• Starting, ignition, and charging: Direct-drive, Delco PG260, PG260F1, and Mando starters; Delco EST ignition; alternator testing and repair; instrumentation and sender checks.

• MPI fuel service: Cool Fuel system repair, water separating fuel filters, throttle bodies, fuel rails, injector replacement, and sensor and idle air control valve service.

• Electronic diagnostics: MEFI 1, 2, and 3 connector charts, scan and non-scan troubleshooting, injector balance testing, and diagnostic trouble code procedures.

• Cooling and exhaust: Seawater pump rebuilding, thermostat testing, heat exchanger leak checks, and service procedures for exhaust manifolds, elbows, risers, and collectors.

• Installation and drivetrain: Engine removal and final alignment, shift cable installation and adjustment, transmission pressure checks, driveshaft repair, and propeller shaft inspection.

• Power steering: Pump and pulley replacement, hydraulic hose and fluid cooler service, serpentine belt adjustment, and system filling and air bleeding.

Written for dealer mechanics and Mercury Marine service personnel, this manual assumes training in marine service procedures and access to the specified tools.

Engine Overhaul and Installation

The engine section goes beyond removal and replacement. It covers valve guide and seat work, hydraulic roller lifters, piston ring inspection, bearing clearances, crankshaft installation, and cylinder block inspection, with component-specific torque tables and wear specifications.

Cylinder head identification distinguishes the rounded intake ports of the L-29 from the rectangular ports used on the Mag and 8.2L engines. The troubleshooting section also explains the higher oil consumption associated with forged pistons in the 454 and 502 Mag MPI engines, alongside checks for leaks, valve guide wear, and ring problems.

Separate installation procedures address conventional sterndrive layouts, driveshaft extensions, and inboard packages. The alignment instructions specify Quicksilver tool 91-805475A1 and explain how forcing the tool or moving the engine with it inserted can damage the gimbal bearing or engine coupler.

MEFI Fuel and Electrical Diagnosis

Serial-based ECM identification helps you select the matching connector chart and diagnostic procedure before testing a sensor or replacing a module. The Diagnostic Circuit Check, circuit/symptom charts, and scan tool reference data support fault isolation for hard starting, unstable idle, hesitation, and loss of power.

The manual documents trouble code reading and clearing, sensor circuit tests, ignition control checks, and injector balance testing. Typical scan values are identified as reference readings from normally operating vessels, with the operating conditions stated.

Starting and Charging System Wiring Harness Diagrams, MEFI wiring diagrams, and ECM connector charts show the electrical connections. The Fuel System Flow Diagram and distributor assembly exploded views support fuel and ignition service.

Fuel and ignition work requires thorough engine-compartment ventilation and the manual’s marine fire and explosion precautions.

Cooling, Exhaust, and Steering Service

Cooling coverage includes seawater pump output testing and rebuilding, thermostat opening tests, and closed cooling pressure tests. Heat exchanger checks distinguish internal leakage from blockage, while the Closed Cooling System Water Flow Diagram shows circuit routing.

Maintenance schedules and flushing procedures distinguish standard and Horizon applications. Exhaust coverage extends to shutters, risers, collectors, and Silent Choice components.

Power steering procedures cover pulley installation, hose routing, fluid cooler connections, belt tension, and bleeding. Further steering-system testing and repair is referred to the appropriate sterndrive service manual.

Transmission Service and Shift Controls

Velvet Drive sections cover fluid service, removal and installation, shift lever adjustment, and pressure testing. Internal transmission repair is referred to separate Velvet Drive service literature.

Hurth coverage includes shift cable setup and functional testing. Walter V-Drive service addresses its separate lubricant supply; the Walter unit and an associated Velvet Drive transmission do not share fluid.

The shift-control sections explain how insufficient cable travel can cause transmission slippage and eventual failure, including the related Velvet Drive warranty restrictions.

Cooling and Steering Specification Preview

These are a small selection of the reference values provided in the cooling and steering sections.

For closed-cooled engines, the pressure cap specification is 16 psi (110 kPa), alongside separate cap and cooling-system testing procedures.

The power steering torque table lists 23 lb-ft (31 N·m) for hose fittings. The hose procedures distinguish these connections from pump-side fittings described separately.

Searchable Text and Wiring Diagram Detail

Text search and diagram access: The PDF contains searchable text. Its bookmark list has no working section destinations. Small wire-color codes and connector labels in dense harness diagrams are readable when enlarged.

One printable 1,027-page PDF contains Sections 1–9 of this service publication. Although the cover identifies Book 1 of 2, the file continues beyond Sections 1–4. The total already includes the cover, six preliminary pages, and 89 intentionally blank pages within the numbered sections.