Bi-Directional Water Pump for MerCruiser Sterndrive 8503991

Bi-Directional Water Pump – Fits MerCruiser V6/V8 Small Block (Replaces 8503991)

This engine-mounted, bi-directional water circulating pump is designed for a wide range of marine engines, including MerCruiser, Volvo Penta, OMC, and other brands using GM small block V6 and V8 engines. It ensures reliable coolant circulation for optimal engine temperature.

This component is a direct replacement for MerCruiser part numbers 8503991 and 8M0113734, Volvo Penta 3853850, Sierra 18-3599-1, and others, offering a precise fit for compatible models.

• Installation: Engine mounted for secure, direct integration.
• Material: Features a durable cast steel housing with a stainless steel backing plate for corrosion resistance.
• Compatibility: Designed for both steel and composite timing chain covers.

Part Number Cross-Reference

Use this table to confirm equivalent part numbers across OEM and aftermarket brands.

Compatible Engine Models

Confirm your engine model/year in the table below. For Mercury owners, here’s a guide to help you identify your engine’s serial/model: Mercury Serial Number Guide. You can also review exploded diagrams to verify part numbers: Mercury Parts Diagrams & Catalog.

Confirm your engine model/year in the table below.

Technical Specifications

Package Contents

• 1 x Water Circulation Pump (8503991/8M0113734)

Explosion Diagram

Installation Guide

1. Disconnect Battery: Ensure the engine is off and disconnect the negative battery terminal.
2. Drain Coolant: Place a catch pan under the engine and drain the cooling system.
3. Loosen Belt: Loosen the tensioner to remove the serpentine belt from the water pump pulley.
4. Remove Hoses: Carefully disconnect the inlet and outlet hoses from the water pump.
5. Unbolt Old Pump: Remove the mounting bolts securing the old water pump to the engine block.
6. Clean Surface: Thoroughly clean the engine block's mounting surface to remove any old gasket material or sealant.
7. Install New Pump: Position the new gasket and water pump onto the engine block and hand-tighten the bolts.
8. Torque Bolts: Tighten the mounting bolts to the manufacturer's recommended torque specifications.
9. Reconnect Components: Reattach the hoses and reinstall the serpentine belt, ensuring proper tension.
10. Refill System: Fill the cooling system with the appropriate coolant mixture.
11. Check for Leaks: Reconnect the battery, start the engine, and carefully check for any leaks around the pump and hoses.

Troubleshooting & FAQ

How do I confirm this fits my engine?
This pump is designed for a wide range of MerCruiser, Volvo Penta, and OMC engines. A critical exception is that it will NOT fit 2007 and newer MerCruiser V6 4.3L engines that lack a water bypass hole. Please verify your engine model, year, and part number against the compatibility tables above.

What are the signs of a failing water pump?
Common symptoms include coolant leaks from the pump itself, an engine that is overheating, a loose or noisy water pump pulley, or steam coming from the engine compartment.

What does "bi-directional" mean?
A bi-directional pump can operate correctly regardless of the engine's rotational direction (clockwise or counter-clockwise), making it versatile for different engine setups.

OEM vs. Aftermarket?
This listing is an aftermarket replacement part (not an OEM component). We build to OEM specifications using comparable-grade materials. Production is done in an ISO9002-certified facility, and each batch is inspected for consistent performance.

How fast do you ship and how much does it cost?
We offer fast global shipping—typically 5–15 days worldwide—and standard shipping is free. Expedited options are available at checkout depending on your location.

What is your warranty and return policy?
This product includes a 1-year warranty against manufacturing defects under normal use. If something goes wrong, contact our support team with your order details. Returns are accepted within our standard return window (unused & in original condition); please see our policy page or reach out for instructions.

Important Notes

• Verify that the product images and specifications match your original part. If discrepancies exist—even if your model is listed—do not proceed with the purchase.
• This product is an aftermarket replacement part and is not an OEM component.
• Professional installation is recommended to ensure safety and optimal performance.

Why Choose JLM MARINE?

• Quality Assured: As a trusted manufacturer for over 20 years, we supply top marine brands globally.
• Fast Global Shipping: Free delivery worldwide in 5–15 days.
• Direct from Manufacturer: Competitive pricing without compromising quality.