MerCruiser Engine Runs Poorly at High RPM 2001-2006

The following information contains troubleshooting testing procedures for engine runs poorly at high rpm problems on the MerCruiser 5.oL, 5.7L and 6.2L MPI gasoline 2001 2002 2003 2004 2005 2006 engines. Testing and diagnosis helps determine problems and possible causes. Its recommended you use a factory manual when performing repairs on your inboard engine, additional safety precautions, or technical data maybe included. 

SYMPTOMS: Engine runs poorly at high rpm

CAUSES

1.0 Crankcase overfilled with oil

1.1 Anti-siphon valve restricting fuel supply

1.2 Plugged fuel tank vent 

1.3 Improper fuel supply

1.4 Incorrect rotor location

1.5 Low grade of fuel or water in the fuel

1.6 Spark plugs fouled, burned, cracked porcelain or incorrect heat range

1.7 Spark plug wires broken or poorly insulated

1.8 Distributor cap or rotor dirty or cracked

1.9 Coil damaged or malfunctioning

1.10 Excessive play in distributor shaft

1.11 Engine overheating

1.12 Low compression - worn valves, rings or seals

1.13 Restricted exhaust 

ACTIONS NEEDED

1.0 Check the oil level with the boat at rest in the water. Drain the oil if necessary to obtain the correct level.

1.1 Check fuel flow rates and repair, replace or remove anti-siphon valve as necessary.

1.2 Clean the fuel tank vent. 

1.3 Inspect fuel supply system. Repair or replace as necessary. 

1.4 Reset rotor location.

1.5 Drain the fuel tank and refill with the correct fuel.

1.6 Inspect the spark plugs and service or replace.

1.7 Replace the spark plug wires.

1.8 Inspect the distributor cap for cracks or marks. Replace the distributor cap and the rotor if necessary.

1.9 Replace the coil. 

1.10 Replace distributor.

1.11 Check cooling water supply or coolant level.

1.12 Perform the cylinder compression check to determine which cylinder has low compression. Replace the rings, the valves or the pistons.

1.13 Clean the exhaust system.

MerCruiser RPM Information

• mercruiser lifter noise high 3500 rpm
• 1995 3l mercruiser engine runs rough at 3000 rpm
• 1999 mercruiser 4.3 high rpm after changing belt and pulley
• 2001 mercruiser 5.0 litre runs hot at 4000 rpms
• 2003 mercruiser 5.7 bravo 2 overheating at high rpms or under load
• 2005 mercruiser 4.3 not reaching wot rpm
• 2005 mercruiser 5.7 mpi wont turn rpms
• 3.0 4 cyl mercruiser overheating at high rpm/hot water going out intake
• 3.0 liter mercruiser running rough at high rpm
• 3.0 mercruiser high rpm wide open and over heat
• 350 magnum fuel cooler engine running hot at 4500 rpm
• 350 magnum mercruiser overheats only at low rpms
• 350 mercruiser boat overheating at high rpms
• 350 mercruiser rough running at 3600 rpm
• 5.7 mercruiser engine miss at hi rpm
• 5.7 mercruiser overheats above 2000 rpms

These service procedures apply to: 2001 2002 2003 2004 2005 2006 Mercury MerCruiser 5.0L, 5.7L, 6.2L MPI Multiport Injection Engine. 5.0L MPI Alpha and Bravo, 350 Alpha and Bravo, 350 MAG MPI Horizon, MX 6.2 MPI, MX 6.2 MPI Horizon, Tow Sports (MIE), 350 MAG MPI, 350 MAG MPI Horizon Inboard, MX 6.2 MPI Inboard, MX 6.2 MPI Horizon Inboard and 350 MAG MPI Tow Sports.

The following information contains troubleshooting testing procedures for engine surges problems [i.e. engine surges, low or lack of top speed power, poor performance when engine is cold, engine stalls or engine misses] on the MerCruiser 5.oL, 5.7L and 6.2L MPI gasoline engines. Testing and diagnosis helps determine problems and possible causes. Its recommended you use a factory manual when performing repairs on your inboard engine, additional safety precautions, or technical data maybe included. 

SYMPTOMS: Engine Surges

CAUSES

1.0 Leaking idle air control (IAC), intake manifold or throttle body gaskets

1.1 Throttle valves worn or damaged

1.2 Worn or leaking IAC hose or vacuum hose to the pressure regulator 

ACTIONS NEEDED

1.0 Find and replace the leaking gaskets.

1.1 Inspect the throttle valves. Replace the throttle body if necessary.

1.2 Find and replace the leaking hose.

MerCruiser Low Top Speed/Lack Of Power

SYMPTOMS: Low top speed or lack of power 

CAUSES

Leaking idle air control (IAC), intake manifold or throttle body gaskets 

ACTIONS NEEDED

Find and replace the leaking gaskets. 

MerCruiser Poor cold engine operation

SYMPTOMS: Poor cold engine operation   

CAUSES

3.0 Engine is not reaching normal operating temperature

3.1 Faulty engine temperature sensor (ECT)

3.2. Fuel PSI too high or too low 

3.3 Throttle valves worn or damaged

3.4 Engine flooding 

ACTIONS NEEDED

3.0 Replace thermostat or thermostat housing gasket.

3.1 Replace the ECT.

3.2 Determine the cause and repair.

3.3 Inspect the throttle valves. Replace the throttle body if necessary.

3.4 Check the fuel pressure. If too high, find cause and correct. Clean the flame arrestor. Perform the cylinder balance test. 

MerCruiser Engine stalls

SYMPTOMS: Engine Stalls   

CAUSES

4.0 Idle rpm too low

4.1 Engine flooding

4.2 Leaking idle air control (IAC), intake manifold or throttle body gaskets 

ACTIONS NEEDED

4.0 Clean the flame arrestor. Test the IAC to see if it opens and closes properly.

4.1 Check the fuel pressure. If too high, find cause and correct. Inspect the fuel pressure regulator. Inspect the injectors.

4.2 Find and replace the leaking gaskets.

MerCruiser Engine Misses

SYMPTOMS: Engine Misses

CAUSES

Fault in the ignition system

ACTIONS NEEDED

Check the spark plugs. Check the spark plug wires. Check the distributor cap for cracks or marks. Check the ignition coil operation. Replace the faulty component. 

These service procedures apply to: 2001 2002 2003 2004 2005 2006 Mercury MerCruiser 5.0L, 5.7L, 6.2L MPI Gasoline Multiport Injection Engine.5.0L MPI Alpha and Bravo, 350 MAG MPI Alpha and Bravo, 350 MAG MPI Alpha and Bravo Horizon, MX 6.2 MPI, MX 6.2 MPI Horizon, Inboard and Tow Sports (MIE), 350 MAG MPI Inboard, 350 MAG MPI Horizon Inboard, MX 6.2 MPI Inboard, MX 6.2 MPI Horizon Inboard and 350 MAG MPI Tow Sports.

MerCruiser Overheating - Merc Cruiser Troubleshooting Guide

Troubleshooting can determine faults and help correct mechanical or cooling system problems. This troubleshooting guide will focus on actions needed to correct engine overheating issues for MerCruiser Inboard motors. 

Components my differ depending on MerCruiser year, these guidelines are typical for most model MerCruisers for 1998 1999 2000 2001 2002 2003 2004 2005 2006. Since these are general guidelines, the following years may be relevant: 1989 1990 1991 1992 1993 1994 1995 1996 1997.

The following is a check list of probable causes, but keep in mind, more than one of these conditions could be present. 

Engine Overheats Mechanical:

IMPORTANT: First, verify if the engine is actually overheating, the temperature gauge or sender could be faulty.

IMPORTANT: Best way to test the gauge or sender is just to replace it. 

Key features of interest will be focused on overheating due to mechanical problems:

• Engine laboring
• Timing
• Distributor
• Spark plug wires
• Fuel mixture
• Spark plugs
• Exhaust
• Valve timing
• Head-gasket
• Lubrication

1. The engine rpm is below specifications at WOT (wide open throttle), this could be do to a damaged or incorrect propeller. Check for excessive growth on the bottom of the boat. Is bilge working? Check to see if the hull is filled with water causing excessive drag (engine laboring). Also, excessive air bubbles due to growth on bottom hull could cause the sea water pick-up insufficient water for cooling (if equipped), see engine overheats cooling system. These issues can cause an engine to overheat.

2. Worn down ignition timing, check to see if the timing is retarded or possibly to far advanced.

3. Inspect the distributor advance weights to see if there sticking.

4. Spark plug wires can be transversed (wrong firing order), make sure spark plug wires are in correct order.

5. Lean fuel mixture.

6. Incorrect heat range for spark plugs.

7. Restriction in exhaust system.

8. Is the timing off? Possibly the timing chain has jumped or installed improperly.

9. Blown head gasket, a compression test will not reveal if the head gasket is blown. Typically, an engine will not overheat until the engine has reached speeds above 3000 rpm. For engines that are sea water cooled: use a clear plastic hose, inspect for air bubbles between seawater pump and engine. If you do not notice bubbles, install the hose between the thermostat housing and manifold(s). Any bubbles detected at higher rpm is an indication of a blown head gasket. 

10. Lubrication issues for moving parts. A fouled oil pun, blocked oil passages or low oil level may be the problem.

Engine Overheats Cooling System

Key features of interest will be focused on overheating due to cooling system problems:

• Drive belt
• Seawater shutoff valve (if equipped)
• Sea strainer
• Seawater pick-up (models with belt driven seawater pump only)
• Seawater kinks or collapsed hose
• Seawater blockage 
• Hull obstructions
• Thermostat
• Exhaust elbow
• Faulty seawater pump
• Obstruction in cooling system
• Engine circulating pump faulty
• Coolant
• Antifreeze
• Heat exchanger
• Water hoses
• Drive belt
• Seawater shutoff valve

1. Possible broken or loose drive belt.

2. The seawater shutoff valve may be partially or completely closed (if equipped).

3. Incorrectly installed sea water strainer

4. Loose hose connections between seawater pickup and pump inlet, this applies only to models equipped with belt driven seawater pumps). Note: the pump will suck air. The pump may also fail to prime or will force bubbles into the cooling system.

5. The seawater inlet hose could possibly be kinked or collapsed, this may or may not be noticeable until under load. 

6. Blocked or clogged sea water pick-up.

7. Obstructions on the bottom hull of boat causing to much water turbulence. If there are obstructions in front of the seawater pick-up, it could cause air bubbles to be forced into the cooling system.

8. Check thermostat for proper operation.

9. Make sure the exhaust elbow water outlet in not blocked, clogged or plugged. 

10. Seawater pump may have a worn pump impeller, replace if necessary.

11. Possible obstructions in cooling system. Check items such as casting flash, salt, sand, rust, etc. Check your service manual for water flow diagram.

12. Engine circulating pump could be faulty. 

13. Check coolant level.

14. Is the antifreeze mixed correctly? A good mix ratio should be 50/50 or maximum 60/40 (antifreeze = 60%, water = 40%)

15. Inspect heat exchanger for plugged cores.

16. Water hoses transversed, make sure water hoses are not reversed at the water distribution block.

17. Broken, worn or loose drive belt. 

18. Seawater shut off valve fully or partially closed. 

Service procedures apply to some of the following models:

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• 496 8.1 mercruiser check valves glogged causing overheating
• 82 mercruiser 350 engine overheating problem
• 89 3.0 l mercruiser alpha 1 overheating problem

MerCruiser Inboard Engine Model

• 496 Mag HO 8.1S Horizon HO
• GM V8 454 cid 7.4L 502 cid 8.2L
• 4.3L MPI Alpha and Bravo
• 3.0L & 3.0LX 
• 4 Cylinder
• 305 CID 5.0L 350 CID 5.7L 6.2L
• GM V8 305-350-454-482-540 CID
• D1.7L DTI
• In-ine Diesel D2.8L D4.2L D-Tronic Manual
• GM 6-Cylinder Engines

While this guide was written to help you diagnose problems. Its recommended you refer to the factory service manual before performing any work. Improper service or repairs can magnify existing issues. Also, MercCruiser repair manuals may contain more detail information including important safety notations or warnings. 

MerCruiser Rocker Arm & Valve Pushrod Service Procedures 2001-2006

The following information contains factory service procedures for Rocker Arm & Valve Pushrods for the MerCruiser 5.oL, 5.7L and 6.2L MPI gasoline engines. Its recommended you use a factory manual when performing repairs on your inboard engine, additional safety precautions, or technical data maybe included.

REMOVAL

NOTE: When performing service to just one cylinder's rocker arms, you must bring that cylinder's piston to TDC (Top Dead Center) before you remove the rocker arms. If your performing service to all of the rocker arms, you must bring cylinder number 1 up to TDC before you remove the rocker arms. 

1. Remove the valve covers.

2. Remove the rocker arm assemblies and the valve pushrods. 

IMPORTANT: You must organize the rocker arm assemblies and valve pushrods in a rack in order to reassemble them in their original and exact locations. 

CLEANING

1. Use a designated cleaner to clean the rocker arm assemblies and pushrods. 

2. Clean the valve pushrod oil passages.

3. Use safety glasses before proceeding to the next step.

4. You compressed air to dry parts. 

INSPECTION

1. Carefully inspect all contact surfaces for excessive wear, damage or scoring. 

2. Ensure the valve pushrod oil passage is not restricted.

3. Ensure the valve pushrods do not contain bends by rolling the pushrod on a flat surface. 

4. Replace any damaged parts or parts that do not meet factory specifications. 

INSTALLATION

1. Lubricate the ends of the valve pushrods and the outer surfaces.

2. Reinstall the valve pushrods in their exact location they were in upon removal. Make sure the pushrods seat in the lifter socket. 

3. Apply lubrication to the rocker arm ball contact surfaces and the rocker arm. 

4. Apply lubrication to the threads of the valve rocker arm studs. 

5. Reinstall the rocker arms, rocker arm balls and rocker arm nuts in their original and exact locations they were in upon removal. 

6. Last, adjust the valve clearance. 

LUBRICANTS REQUIRED

A. Johnson EP lube or equivalent, the EP lube is used for lubricating valve pushrods, rocker arm and rocker arm ball contact surfaces. 

B. 80W gear lube, this lube is used to lubricate the rocker arm ball stud, this material can be obtained at local marine service centers. 

These service procedures apply to: 2001 2002 2003 2004 2005 2006 Mercury MerCruiser 5.0L, 5.7L, 6.2L MPI Engine.5.0L MPI Alpha and Bravo, 350 MAG MPI Alpha and Bravo, 350 MAG MPI Alpha and Bravo Horizon, MX 6.2 MPI, MX 6.2 MPI Horizon, Inboard and Tow Sports (MIE), 350 MAG, 350 MAG Horizon Inboard, MX 6.2 MPI Inboard, MX 6.2 MPI Horizon Inboard and 350 MAG Tow Sports.

MerCruiser Runs Rough or Poor Performance V8 305 350 454 482 540 CID

Troubleshooting can determine faults and help correct mechanical or electronic system problems. This troubleshooting guide will focus on actions needed to correct engine runs poorly issues for MerCruiser Inboard motors. 

Components my differ depending on MerCruiser year, these guidelines are typical for most model MerCruiser for 1998 1999 2000 2001 2002 2003 2004 2005 2006. Since these are general guidelines, the following years may be relevant: 1989 1990 1991 1992 1993 1994 1995 1996 1997. 

Key features of interest will be focused on:

• Oil
• Anti-Siphon (if equipped) 
• Fuel vent tank
• Fuel
• Timing
• Fuel contamination
• Spark plugs
• Spark plug wires
• Distributor cap or rotor
• Overheating
• Compression
• Exhaust

While this guide was written to help you diagnose problems. Its recommended you refer to the factory service manual before performing any work. Improper service or repairs can magnify existing issues. Also, MerCruiser repair manuals may contain more detail information including important safety notations or warnings. 

The following is a check list of probable causes, but keep in mind, more than one of these conditions could be present. 

Troubleshooting Checklist:

• Check crankcase to see if its overfilled with oil. Its best to check the oil level while the boat is in the water.
• Anti-Siphon valve (if equipped), possibly restricting or blocking the fuel supply. Inspect to make sure its not clogged. 
• Possible plugged fuel tank vent
• Completely check all fuel supply components and lines.
• Ignition timing may be incorrect.
• Low grade fuel, check for contaminations. Is fuel contaminated with water?
• Broken, burnt, cracked porcelain, fouled, spark plugs. Make sure plugs are not covered in oil.
• Broken spark plug wires or poorly insulated wires.
• The distributer cap or rotor may be dirty or cracked. Replace if necessary.
• Check coil. 
• Check for excessive shaft play on distributor. 
• Engine running hot, check for overheating issues.
• Possible worn valves, cylinders, piston rings, etc. Do a compression check.
• Check the exhaust system for possible restrictions.

Common Terminology

• Poor boat performance
• Mer Cruiser  Mercury low power
• MerCruiser Troubleshooting
• Troubleshooting MerCruiser Inboard

MerCruiser Inboard Engine Models

• 496 Mag HO 8.1S Horizon HO
• GM V8 454 cid 7.4L 502 cid 8.2L
• 4.3L MPI Alpha and Bravo
• 3.0L & 3.0LX 
• 4 Cylinder
• 305 CID 5.0L 350 CID 5.7L 6.2L
• GM V8 305-350-454-482-540 CID
• D1.7L DTI
• In-ine Diesel D2.8L D4.2L D-Tronic
• GM 6-Cylinder

MerCruiser Valve Cover Removal Cleaning Installation 5.0L 5.7L 6.2L MPI

The following information contains factory service procedures for valve cover removal, cleaning and installation for the MerCruiser 5.oL, 5.7L and 6.2L MPI gasoline engines. Its recommended you use a factory manual when performing repairs on your inboard engine, additional safety precautions, or technical data maybe included. 

REMOVAL

NOTE: On some models, you may need to remove the exhaust manifold before removing the valve cover. 

1. Disconnect crankcase ventilation hoses.

2. Remove all items that may interfere while removing the valve cover.

3. Remove the valve cover.

CLEANING AND INSPECTION

1. Clean the gasket material from all of the sealing surfaces.

2. Use a degreaser to clean sealing surfaces on cylinder head and valve cover.

NOTE: Use safety glasses to protect your eyes from cleaners and small debris.

INSTALLATION

1. Apply the new valve cover gasket to the valve cover. 

2. Install the valve cover and torque bolts.

Valve Cover Bolt Torque Specifications: Nm 12, lb-in. 106

3. Replace any components removed during removal process.

4. Re-connect the crankcase ventilation hoses.

5. Supply cooling water to the power package. 

6. Start engine and inspect for any oil leaks. 

These service procedures apply to: 2001 2006 2002 2005 2003 2004 MerCruiser 5.0L, 5.7L, 6.2L MPI, 5.0L MPI Alpha and Bravo, 350 MAG MPI Alpha and Bravo, 350 MAG MPI Alpha and Bravo Horizon, MX 6.2 MPI, MX 6.2 MPI Horizon, Inboard and Tow Sports (MIE), 350 MAG MPI, 350 MAG MPI Horizon Inboard, MX 6.2 MPI Inboard, MX 6.2 MPI Horizon Inboard and 350 MAG MPI Tow Sports.

MerCruiser Valve Cover Information

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MerCruiser Wiring Diagrams 5.0L 5.7L Inboard Engine 2001-2006

5.0L (305 cid)
5.7L (350 cid)

Model Years: 2001 2002 2003 2004 2005 2006

General Information

Connect Wires Together with Screw and Hex Nut; Apply Liquid Neoprene to Connection and Slide Rubber Sleeve over Connection. Power for a Fused Accessory Panel May Be Taken from This Connection. Load Must Not Exceed 40 Amps. Panel Ground Wire Must Be Connected to Instrument Terminal That Has an 8-Gauge Black (Ground) Harness Wire Connected to it. Lanyard stop switch lead and neutral safety switch leads must be soldered and covered with shrink tube for a water proof connection. If an alternate method of connection is made, verify connection is secure and sealed for a water proof connection.

1 Audio Warning Alarm
2 Tachometer
3 Oil Pressure
4 Water Temperature
5 Battery Meter
6 Ignition Switch
7 Trim Indicator
8 Read/Observe NOTE 1 and 2.
9 Read/Observe NOTE 3.
10 To Engine Wiring Harness

Power Trim System

1 - 20 Amp Fuse
2 - Ground Bolt (Floor Mount)
3 - UP Solenoid
4 - 110 Amp Fuse
5 - DOWN Solenoid
6 - Trailer Switch
7 - Neutral Switch to Instrument Wiring Harness
8 - Trim Limit Switch

Wire Colors

BLK = BLACK
BLU = BLUE
BRN = BROWN
GRY = GRAY
GRN = GREEN
ORN = ORANGE
PNK = PINK
PUR = PURPLE
RED = RED
TAN = TAN
WHT = WHITE
YEL = YELLOW
LT OR LIT = LIGHT
DRK = DARK

MerCathode System Wiring Diagram

1 - Controller
2 - 20 Amp Fuse
3 - Electrode
4 - BLACK Wire - With Engine Harness or Separate

Wire Colors

BLK = BLACK
BLU = BLUE
BRN = BROWN
GRY = GRAY
GRN = GREEN
ORN = ORANGE
PNK = PINK
PUR = PURPLE
RED = RED
TAN = TAN
WHT = WHITE
YEL = YELLOW
LT OR LIT = LIGHT
DRK = DARK

MerCruiser Wire Information

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