Mercury MerCruiser 377HP 4-Stroke Marine Engine Parts

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This page is for Mercury MerCruiser 377HP 4‑Stroke sterndrive/inboard engines. Use it to find compatible parts and practical service information. Below you’ll find maintenance tips, common replacement parts, and troubleshooting guidance. You can also browse the main MerCruiser collection. Scroll to the bottom to start browsing parts for this engine, and remember that on the collection page you can use the Filter & Sort controls to narrow results by horsepower, year range, and part category.

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How to Identify Your 377HP Model

Locate serial numbers: the engine serial tag is typically on the starboard side of the block or on a decal near the flame arrestor; the drive and transom assembly have separate serial tags on the gimbal housing/transom bracket.
Model coding: many 377 cid engines are referred to as “377 MAG/MPI.” Suffixes may indicate drive family (Alpha/Bravo), cooling type (raw/closed), and emissions package. Record the full code and any suffix letters.
Platform clues (4‑Stroke V8): multi‑port fuel injection (fuel rails, injectors) and an ECM with multi‑pin connectors distinguish MPI setups from older carbureted versions.
Year/controls: harness labels and ECM family (e.g., ECM 555 era) help identify parts like ignition components and sensors. Always confirm by engine and drive serial numbers.

Engine Overview

Small‑Block V8 377 cid (6.2L) Platform

Feature	Description
Configuration	4‑Stroke V8, 377 cid small‑block GM base, multi‑port EFI with serpentine accessory drive
Common Models	377 MAG/MPI variants used with Bravo sterndrives and inboard installations; ECM‑managed ignition/fuel systems
Advantages	Strong mid‑range torque, reliable EFI starting, good parts availability, robust cooling and charging systems
Notes	Many service items overlap with other GM small‑block MerCruiser V6/V8 families (5.0/5.7/6.2). Confirm fitment with engine and drive serials, and note ECM family when selecting ignition and sensor components.

Routine Maintenance & Service Parts

Intervals below are general guidelines for typical recreational use. Always follow the official MerCruiser service manual for your exact model and operating environment.

Item	Interval*	Remarks	Parts Link
Engine oil + filter	100 hours or annually	Use FC‑W marine‑rated oil; warm engine before draining; replace drain plug gasket if required	MerCruiser Oil Filters
Gear oil (sterndrive)	100 hours or annually	Drain/fill from the drive; inspect for water or metal; replace gaskets/seals as needed	MerCruiser Collection
Spark plugs	300 hours or 3 years	Verify heat range and gap per manual; inspect wires and boots	Spark Plugs
Fuel filter / water‑separating filter	100 hours or annually (more often with ethanol fuel)	Drain bowl (if equipped) and check for water/debris; replace elements	MerCruiser Fuel Filters
Raw/sea water pump impeller or kit	Every 2 seasons or 200 hours (inspect annually)	Replace sooner after overheating, sand/silt operation, or low flow	MerCruiser Impellers
Anodes (engine/drive)	Inspect monthly; replace when >50% depleted	Match alloy to water type (zinc, aluminum, or magnesium)	MerCruiser Anodes
Serpentine belt	Inspect each season; replace 2–3 years or on wear/noise	Check tensioner/idler bearings; look for glazing or cracking	Belts
Thermostat	3–5 years, or if overheat/temperature instability occurs	Use correct temperature spec for your cooling system	MerCruiser Collection

Frequently Used Parts Categories

Routine Service Kits – MerCruiser Service & Maintenance Kits
Cooling System – MerCruiser Impellers
Fuel System – MerCruiser Fuel Filters
Ignition / Electrical – Electrical Items
Charging & Starting – Alternators and Starters
Corrosion Protection – MerCruiser Anodes
Propellers & Driveline – MerCruiser Propellers

Common Symptoms & Quick Checks

Symptom	Check First	Related Parts
Overheat alarm / high engine temperature	Inspect sea strainer and intake screens, raw‑water pump impeller, belt tension, and thermostat operation	Water pump impeller kit, thermostat, serpentine belt
Poor acceleration / power loss	Check for damaged/fouled prop, restricted fuel filter, stale fuel, and fouled spark plugs	Propeller, fuel filter, spark plugs
Hard starting / rough idle	Verify battery voltage and grounds, replace water‑separating fuel filter, inspect plugs and ignition leads	Fuel filter, spark plugs, air filter
Gearcase noise or metal in gear oil	Confirm correct oil level, check for water intrusion, inspect prop shaft seal after line/rope strikes	Lower unit seal kit, drain/fill gaskets, gear lube

FAQs

Are parts interchangeable across different years or platforms?

Many 377 cid small‑block service parts overlap with other MerCruiser GM‑based V6/V8 engines, but sensors, ignition components, and cooling parts can vary by ECM family, year, and drive package. Always confirm with both engine and drive serial numbers.

When should I replace the raw‑water pump impeller?

Inspect annually and replace every two seasons or 200 hours. Replace immediately after an overheat event or operation in sandy/silty water.

How often should sterndrive gear oil be changed?

As a general rule, change every 100 hours or annually, and any time the drive experiences an impact or you see signs of water contamination in the oil.

What information do you need to confirm part compatibility?

Provide the engine serial number, drive/transom serials, full model designation (including suffixes), cooling type (raw vs closed), and any key components (ECM family, ignition style). Clear photos of the nameplate and the part to be replaced are helpful.

Ready to find the right components? Browse the main MerCruiser collection by horsepower, year, and model to locate compatible parts for your 377HP engine.

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If you’re unsure about your model, send us a clear photo of the nameplate and the part you need and we’ll help verify compatibility.