Mercury 350 MAG MPI Horizon User Manual (English)

Questions and Answers:

V
veronica28Aug 15, 2025
What to do if the starter motor will not crank the Mercury 350 MAG MPI Horizon Engine or cranks slowly?
- T
  Tyler MaysAug 15, 2025
  If the starter motor of your Mercury Engine won't crank the engine or cranks slowly, here's what you can check: Ensure the battery switch is turned on. Make sure the remote control is in the neutral position. Inspect and reset the circuit breaker, or replace the fuse if it's blown. Examine all electrical connections and wires, paying special attention to the battery cables; clean and tighten any faulty connections. Finally, test the battery and replace it if it's bad.
G
Gina HammondAug 18, 2025
Why Mercury Engine will not start, or is hard to start?
- B
  barry03Aug 18, 2025
  If your Mercury Engine won't start or is hard to start, consider the following: Ensure you are following the correct starting procedure. Check that the fuel tank isn't empty and the fuel shutoff valve is open. If the engine is flooded, wait at least 5 minutes before attempting to start it again. Other potential issues include a faulty ignition system component (which needs servicing), clogged fuel filters (replace them), stale or contaminated fuel (drain and refill with fresh fuel), a kinked or clogged fuel line or tank vent line (replace or blow out with compressed air), a fault in the EFI system (have it checked by a dealer), or faulty wire connections (inspect them).
J
Jessica BoyerAug 21, 2025
What to do if Mercury Engine runs rough, misses and/or backfires?
- R
  Robert CoxAug 21, 2025
  If your Mercury Engine is running rough, missing, and/or backfiring, there are several potential causes. The idle speed might be too low, requiring a dealer check for EFI models. There could be an ignition system fault needing service, a clogged fuel filter needing replacement, or stale/contaminated fuel requiring a tank drain and fresh fuel. Check for kinked or clogged fuel lines (replace/clear them), a plugged flame arrestor (clean it), or an EFI system fault (dealer check). Also, ensure you have enough fuel and check the engine timing.
J
Jessica CampbellAug 25, 2025
Why is there excessive temperature in my Mercury 350 MAG MPI Horizon?
- D
  Dustin BuckAug 25, 2025
  If your Mercury Engine is overheating, here's what to check: Ensure the water inlet valve is fully open. Inspect the drive belt for looseness or poor condition, and replace or adjust as needed. Clear any obstructions from the seawater pickups. Replace a faulty thermostat. For closed cooling systems, check and repair any causes of low coolant, then refill with the proper solution. Clean heat exchanger cores, check for leaks in the cooling system, and inspect/test the pressure cap. Finally, straighten or replace any kinked, collapsed, or broken hoses, and flush the transmission fluid cooler and/or engine oil cooler to remove any foreign material.
J
Jesse SmithAug 27, 2025
What to do if Mercury 350 MAG MPI Horizon Engine RPM decreases, will not rise above 2000 RPM?
- K
  Katherine WalkerAug 27, 2025
  If your Mercury Engine RPM decreases and will not rise above 2000 RPM, check the trouble codes to determine the fault, as it may be in Guardian Mode.
C
christopher37Aug 30, 2025
What causes insufficient temperature in Mercury Engine?
- N
  Natalie TaylorAug 30, 2025
  If your Mercury Engine temperature is insufficient, replace the faulty thermostat.
J
Jeffrey HarrisSep 2, 2025
What to do if there is excessive transmission fluid temperature or slipping transmission in Mercury 350 MAG MPI Horizon Engine?
- T
  tcookSep 2, 2025
  If your Mercury Engine's transmission fluid temperature is too high or the transmission is slipping, make sure the fluid level is correct. Check the transmission shift lever positioning and inspect the shift linkages. Straighten any kinked fluid cooler hoses, or replace them if they're damaged.
A
Ashley HallSep 5, 2025
Why is there low oil pressure in my Mercury 350 MAG MPI Horizon Engine?
- A
  amillerSep 5, 2025
  If your Mercury Engine's oil pressure is low, first check the oil level in the crankcase and add oil if needed. If there's too much oil (causing aeration), remove the excess and find out why it's overfilled. Inspect the engine oil cooler hose for kinks and straighten or replace if necessary. If the oil is diluted or has the wrong viscosity, change the oil and filter using the correct grade and determine the cause of the dilution.
J
Julie SchultzSep 8, 2025
How to fix a Mercury 350 MAG MPI Horizon battery that will not come up on charge?
- K
  Kristen FrancoSep 8, 2025
  If the battery on your Mercury Engine isn't charging, try these steps: Reduce the current draw by turning off unnecessary accessories. Inspect all electrical connections and wires, especially the battery cables, and clean/tighten any faulty connections, repairing or replacing damaged wiring. Check the alternator drive belt for looseness or wear, replacing or adjusting as needed. Finally, test the battery to assess its condition.
V
vaughanjenniferSep 12, 2025
Why does the Mercury 350 MAG MPI Horizon remote control operate hard, bind, has excessive free-play or makes unusual sounds?
- L
  Linda PorterSep 12, 2025
  If your Mercury Engine's remote control is hard to operate, binds, has excessive free-play, or makes unusual sounds, start by lubricating the shift and throttle linkage fasteners. Check for loose or missing shift and throttle linkages. Remove any obstructions in the shift or throttle linkages. If the shift or throttle cable is kinked, straighten it or have a dealer replace it if the damage is beyond repair.

Mercury 350 MAG MPI Horizon Specifications

General

Brand	Mercury
Model	350 MAG MPI Horizon
Category	Engine
Language	English

Summary

Horizon Inboard Models Overview

Identification Record

Vehicle Identification Details

Record engine model, serial number, transmission, propeller, HIN, and boat manufacturer details.

Welcome and Warranty Introduction

Manual Purpose and Safety Notices

Explains the manual's content and highlights important safety notices.

Warranty Message and Coverage Overview

Provides information on the product's limited warranty and coverage.

Comprehensive Maintenance Guide

Maintenance Aids, Fluids, and Capacities

Lists maintenance aids, types of fluids, and their capacities for engine systems.

Maintenance Schedule and Fluid Changes

Outlines the maintenance schedule and procedures for changing fluids.

Cooling System Flushing and Belt Care

Instructions for flushing the cooling system and maintaining the serpentine drive belt.

Storage, Winterization, and Recommissioning

Procedures for preparing the power package for storage and bringing it back to service.

Troubleshooting and Service Assistance

Guidance for diagnosing engine problems and obtaining service.

Warranty Registration and Policies

Owner Warranty Registration (US & Canada)

Steps for registering the product warranty in the United States and Canada.

International Warranty Registration

Mercury MerCruiser Limited Warranty Policies

Gasoline Engine Warranty Terms (US & Canada)

Detailed limited warranty terms for gasoline engines in the US and Canada.

International Gasoline Engine Warranty

Corrosion Warranty and Transferability

3-Year Limited Corrosion Warranty

Terms of the 3-year limited warranty against corrosion failure.

Owner Responsibilities and Warranty Transfer

Owner's role in corrosion prevention and how to transfer the warranty.

Transferable Warranty and Protection Plans

Direct Sale Warranty Transfer

Process for transferring warranty when selling directly to a new owner.

Mercury Product Protection Plan

Information about optional extended warranty plans.

Manual Navigation and Safety Alerts

Reading the Manual Thoroughly

Emphasizes the importance of reading and understanding the manual for safe operation.

Understanding Safety Warning Symbols

Explains the meaning and use of WARNING, CAUTION, and IMPORTANT notices.

General Information and Lanyard Switch

Lanyard Stop Switch Functionality

Explains the purpose and operation of the lanyard stop switch for engine shutdown.

Exhaust Emissions and CO Safety

Carbon Monoxide Poisoning Risks

Details risks, symptoms, and prevention of carbon monoxide poisoning.

Safe Boating Practices

Water Safety and Operation Precautions

Protecting People in the Water

Safety measures to protect individuals in the water from the boat.

Boat Operation and Performance Factors

High-Speed Boat Operation

Guidance for operating high-speed or unfamiliar boats safely.

Weight Distribution and Hull Condition

How weight distribution and boat bottom condition affect performance.

Propeller Selection for Performance

Performance Factors and Maintenance Responsibilities

Impact of Elevation and Climate

How environmental factors like elevation and climate affect engine performance.

Owner Responsibilities for Operation & Maintenance

Outlines the operator's responsibilities for safe operation and maintenance.

Operational Precautions and Freezing Prevention

Freezing Temperature Operation Warnings

Critical precautions to prevent freezing damage to the power package.

Post-Submersion Procedures

Actions required immediately after the power package is submerged.

Boat Launching and Component Care

Launching and Operation Care

Advice on careful launching and boat operation.

Replacement Service Parts Guidance

Importance of using genuine Mercury MerCruiser parts for service.

DIY Maintenance and Service Tips

EFI System Considerations and Troubleshooting

Multiple EFI Engine Battery Precautions

Battery precautions for multi-engine EFI systems.

Diagnosing EFI System Faults

How to diagnose problems within the EFI system.

Water Separating Fuel Filter Caution

Important caution regarding the installation and function of the fuel filter.

Engine Break-In and Post-Period Recommendations

20-Hour Break-In Period Guidelines

Essential rules for the initial 20-hour engine break-in period.

Post Break-In and First Season Checkup

Recommendations for operation after break-in and end-of-season service.

Engine Operation and Instrumentation

Understanding Boat Instruments

Explanation of common boat instruments and their functions.

Audio Warning System Alerts

Electrical System Protection

Remote Control System Operation

Control Handle and Neutral Lever Functions

Operation of single/dual control handles and the neutral release lever.

Lanyard Safety Switch and Other Controls

Explanation of lanyard switch safety and other controls like trim and ignition.

Kiekhaefer Remote Control Features

Engine Starting, Shifting, and Stopping

Operation Chart for Engine Management

Engine Guardian Strategy

Engine Specifications and Fuel

Fuel Recommendations and Ratings

Guidelines on recommended fuel types, octane, and oxygenates.

Detailed Engine Specifications

Maintenance and Fluid Capacities

Maintenance Aids and Fluid Types

Lists maintenance aids, types of fluids, and their capacities for engine systems.

Engine Fluid Capacities

Specific fluid capacities for crankcase oil, cooling systems, and transmission.

Transmission and Gas Inboard Maintenance

Transmission Fluid Specifications

Details on fluid types and capacities for Velvet Drive and ZF transmissions.

Gas Inboard Routine Maintenance Schedule

Schedule for routine maintenance tasks for gas inboard engines.

Gas Inboard Scheduled Maintenance

Engine Fluid Level Checks

Checking Oil, Transmission, and Coolant

Procedures for checking crankcase oil, transmission fluid, and engine coolant levels.

Changing Engine Fluids

Changing Crankcase Oil and Filter

Step-by-step guide to changing the engine oil and oil filter.

Changing Water Separating Fuel Filter

Cleaning Flame Arrestor and Components

Lubrication of Control Cables

Cooling System Flushing (350 MAG MPI)

Cooling System Flushing (8.1 S Horizon)

Serpentine Drive Belt Inspection

Miscellaneous Maintenance and Inspection

Battery Maintenance and Handling

Precautions for battery maintenance, charging, and handling.

General Power Package Inspection

Routine inspection of the power package for potential issues.

Cold Weather and Extended Storage Procedures

Power Package Lay Up for Storage

Steps to prepare the power package for storage, including fuel and cooling systems.

Draining Cooling System Instructions

Power Package Recommissioning After Storage

Troubleshooting Engine Start and RPM Issues

Starter Motor and Engine Start Problems

Causes and solutions for starter motor and engine start issues.

Engine RPM Performance Diagnosis

Troubleshooting low engine RPM or performance degradation.

Troubleshooting Engine Runability and Performance

Rough Running, Misfires, and Backfires

Causes and remedies for engines running rough, missing, or backfiring.

Addressing Poor Overall Performance

Troubleshooting steps for general poor engine performance.

Engine Temperature and Oil Pressure Issues

Overheating and Low Temperature Problems

Causes and remedies for excessive or insufficient engine temperatures.

Low Engine Oil Pressure Diagnosis

Causes and remedies for low engine oil pressure.

Transmission, Battery, and Control System Issues

Transmission Fluid and Battery Charging Problems

Causes and remedies for transmission fluid issues and battery charging problems.

Remote Control Operation and Sound Issues

Troubleshooting remote control operation and unusual sounds.

Owner Service Assistance and Support

Getting Local Repair Service

Importance of using local authorized dealers for optimal service.

Parts, Accessories, and Out-of-Area Service

Information on ordering parts and obtaining service when away from home.

Resolving Product Problems

Mercury Marine Service Office Locations

Customer Service Literature and Manuals

Ordering English and Other Language Manuals

How to obtain service literature and manuals in various languages.

Ordering Additional Product Literature

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Mercury 350 MAG MPI Horizon User Manual

Table of Contents

Questions and Answers:

Mercury 350 MAG MPI Horizon Specifications

Summary

Horizon Inboard Models Overview

Identification Record

Vehicle Identification Details

Welcome and Warranty Introduction

Manual Purpose and Safety Notices

Warranty Message and Coverage Overview

Comprehensive Maintenance Guide

Maintenance Aids, Fluids, and Capacities

Maintenance Schedule and Fluid Changes

Cooling System Flushing and Belt Care

Storage, Winterization, and Recommissioning

Troubleshooting and Service Assistance

Warranty Registration and Policies

Owner Warranty Registration (US & Canada)

International Warranty Registration

Mercury MerCruiser Limited Warranty Policies

Gasoline Engine Warranty Terms (US & Canada)

International Gasoline Engine Warranty

Corrosion Warranty and Transferability

3-Year Limited Corrosion Warranty

Owner Responsibilities and Warranty Transfer

Transferable Warranty and Protection Plans

Direct Sale Warranty Transfer

Mercury Product Protection Plan

Manual Navigation and Safety Alerts

Reading the Manual Thoroughly

Understanding Safety Warning Symbols

General Information and Lanyard Switch

Lanyard Stop Switch Functionality

Exhaust Emissions and CO Safety

Carbon Monoxide Poisoning Risks

Safe Boating Practices

Water Safety and Operation Precautions

Protecting People in the Water

Boat Operation and Performance Factors

High-Speed Boat Operation

Weight Distribution and Hull Condition

Propeller Selection for Performance

Performance Factors and Maintenance Responsibilities

Impact of Elevation and Climate

Owner Responsibilities for Operation & Maintenance

Operational Precautions and Freezing Prevention

Freezing Temperature Operation Warnings

Post-Submersion Procedures

Boat Launching and Component Care

Launching and Operation Care

Replacement Service Parts Guidance

DIY Maintenance and Service Tips

EFI System Considerations and Troubleshooting

Multiple EFI Engine Battery Precautions

Diagnosing EFI System Faults

Water Separating Fuel Filter Caution

Engine Break-In and Post-Period Recommendations

20-Hour Break-In Period Guidelines

Post Break-In and First Season Checkup

Engine Operation and Instrumentation

Understanding Boat Instruments

Audio Warning System Alerts

Electrical System Protection

Remote Control System Operation

Control Handle and Neutral Lever Functions

Lanyard Safety Switch and Other Controls

Kiekhaefer Remote Control Features

Engine Starting, Shifting, and Stopping

Operation Chart for Engine Management

Engine Guardian Strategy

Engine Specifications and Fuel

Fuel Recommendations and Ratings

Detailed Engine Specifications

Maintenance and Fluid Capacities

Maintenance Aids and Fluid Types

Engine Fluid Capacities

Transmission and Gas Inboard Maintenance

Transmission Fluid Specifications

Gas Inboard Routine Maintenance Schedule