Water Exhaust Shutter – Fits MerCruiser V6/V8 Stern Drives (Replaces 99370A2, 18-2733, 89170)
Water Exhaust Shutter – Fits MerCruiser V6/V8 Stern Drives (Replaces 99370A2, 18-2733, 89170) is backordered and will ship as soon as it is back in stock.
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Worry-free returns
You may return most new, unopened items within 7 days of delivery for a full refund. We'll also pay the return shipping costs if the return is a result of our error (you received an incorrect or defective item, etc.).
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Delivery and Shipping
Delivery and Shipping
Free standard shipping on all orders under 1 KG
Water Exhaust Shutter – Fits MerCruiser V6/V8 Stern Drives (Replace...
Product Details
This Water Exhaust Shutter is a crucial replacement part designed to prevent water from flowing back into the engine through the exhaust system. It is specifically engineered to fit MerCruiser 4.3L V6, 5.0L V8, and 5.7L V8 engines equipped with center riser manifolds, covering models from 1982–1987.
It serves as a direct interchange for MerCruiser 99370A2, Sierra 18-2733, and GLM 89170, ensuring a precise fit and reliable performance.
- Installs directly into the exhaust Y-pipe.
- Constructed from durable rubber with a steel rod for long-lasting service.
- Features a pin/hinge design for a secure, direct-fit installation.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket.
| Brand | Part Number |
|---|---|
| MerCruiser | 99370A2 |
| Sierra | 18-2733 |
| GLM | 89170 |
Engine & Model Compatibility
Confirm your engine model/year in the table below.
These shutters are for MerCruiser V6 and V8 engines from 1982–1987 equipped with center riser manifolds.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. | Notes |
|---|---|---|---|---|---|---|
| MerCruiser | V6 | 4.3L | 4-Stroke | 1982–1987 | 99370A2 | Used on center riser manifolds |
| MerCruiser | V8 | 5.0L | 4-Stroke | 1982–1987 | 99370A2 | Used on center riser manifolds |
| MerCruiser | V8 | 5.7L | 4-Stroke | 1982–1987 | 99370A2 | Used on center riser manifolds |
Technical Specs
- Product Type
- Water Exhaust Shutter/Flapper
- Installation
- Installs in exhaust Y-pipe
- Connection
- Pin/hinge design
- Housing Material
- Rubber with steel rod
- Housing Color
- Black
- Dimensions
- 3-inch design
- Fitting Type
- Direct-fit replacement
- Other Technical Attributes
- Prevents water backflow into exhaust manifold and engine
What’s in the Box
- 1 x Water Exhaust Shutter (99370A2)
Installation Notes
- Ensure the engine is off and completely cool. Disconnect the boat's battery for safety.
- Locate the exhaust Y-pipe where the old shutter is housed. Access may require removing other exhaust components.
- Carefully remove any clamps or hoses to gain access to the shutter assembly.
- Note the orientation of the old shutter before removing it from its pin or hinge.
- Remove the old, worn shutter. Be prepared for it to be brittle or broken.
- Clean the inside of the Y-pipe mounting area, removing any old sealant, rubber fragments, or corrosion.
- Install the new shutter in the same orientation, ensuring the flapper moves freely to allow exhaust out.
- Reassemble all exhaust components, ensuring clamps are secure and properly tightened.
- Reconnect the battery, start the engine, and meticulously check for exhaust or water leaks at idle.
Troubleshooting & FAQ
How do I know if this shutter fits my engine?
This shutter fits MerCruiser 4.3L V6, 5.0L V8, and 5.7L V8 engines from 1982–1987 with center riser manifolds. The best way to confirm is to match your existing part number to one of the direct replacements: MerCruiser 99370A2, Sierra 18-2733, or GLM 89170.
What are the signs of a failing exhaust shutter?
Common symptoms include a distinct rattling or clanking noise from the exhaust at idle, finding broken plastic or rubber pieces in the exhaust stream, or, in severe cases, water ingestion into the engine, which can cause catastrophic failure (hydrolock).
How often should these be inspected or replaced?
Exhaust shutters are a critical maintenance item. We recommend inspecting them annually for signs of cracking, brittleness, or damage. Proactive replacement every 3-5 years is advised to prevent engine damage, especially for boats operated in saltwater.
Why is this part so important?
The shutter acts as a one-way valve. It opens to let exhaust gases out when the engine is running but closes to prevent water from surging back up the exhaust pipe and into the engine cylinders when the boat is decelerating or sitting stationary in the water.
Can a broken shutter cause engine damage?
Yes. If the shutter fails to close, water can enter the exhaust manifolds and potentially seep into the engine cylinders, leading to corrosion and severe internal damage known as hydrolock.
OEM vs. Aftermarket?
This listing is an aftermarket replacement (not OEM). Built to OEM-level specifications with comparable materials. Manufactured in an ISO 9002 facility, batch-inspected for consistency.
Shipping speed & cost
We ship worldwide quickly—typical transit 5–15 days. Standard shipping is free; expedited options appear at checkout where available.
Warranty & returns
Covered by a 1-year warranty against manufacturing defects under normal use. If issues arise, contact support with your order details. Returns are accepted within our standard window (unused & in original condition). See our policy page or reach out for instructions.
Safety & Important Info
- Verify images/specifications vs your original part. If anything differs—even if your model appears compatible—do not purchase.
- This is an aftermarket replacement, not an OEM component.
- Professional installation is recommended for safety and performance.
About Sea Sierra
- Pro-Grade Standards — We supply marine parts engineered to OEM-level tolerances and tested for real-world conditions.
- Fast Global Fulfillment — Free standard delivery to most regions in 5–15 days.
- Manufacturer-Direct Value — Fair pricing without compromising on materials or QC.
Payment & Security
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