Mercury MerCruiser Ignition Coil 8M0178581 — 496 MAG / 8.1L | Design II
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Mercury MerCruiser Ignition Coil 8M0178581 — 496 MAG / 8.1L | Design II
This genuine OEM ignition coil is the later Design II unit used in the cylinder-head ignition system on selected MerCruiser 496 MAG and 8.1L inboard engines. Each coil fires one cylinder directly, so a full V8 application requires 8 coils. If your engine is misfiring, running rough on a specific cylinder, or throwing an ignition-related fault, this is the OEM replacement to reach for — once the affected cylinder and coil have been properly diagnosed.
Part Information
- OEM Part Number: 8M0178581
- Brand: Mercury / MerCruiser
- Official Product Type: Ignition coil (Design II)
- Supersedes: 889925 (Mercury reference: 392-889925)
- Configuration: Individual coil-on-plug — one per cylinder
- Quantity for Full V8 Set: 8 coils required
- System: Cylinder-head ignition system
- GTIN / UPC: 745061570443
Compatible Applications
- MerCruiser 496 MAG — serial 0M376088 and up (Design II)
- MerCruiser 496 MAG HO — serial 0M376088 and up (Design II)
- MerCruiser MIE 8.1L
- MerCruiser MIE 8.1L HO
- Selected MerCruiser 8.1L Inboard applications
- Selected MerCruiser 8.1L TowSport applications
- Important: 496 MAG engines through serial 0M376087 use a different Design I coil — confirm your engine serial number before ordering
Serial Number Fitment Notice
For the 496 MAG family, the Design II coil (8M0178581) begins at serial number 0M376088. Engines through serial 0M376087 use a different Design I ignition coil and require a different part number. Always verify the serial number before ordering.
When to Replace
- Engine misfires on a specific cylinder confirmed by diagnosis
- Rough running, hesitation, or loss of power traced to one cylinder
- Difficult starting associated with an ignition fault code
- Coil shows physical damage, cracking, or failed electrical connector
- Coil resistance or output is outside Mercury service specification on testing
Installation Notes
- Disconnect battery power before beginning any ignition system service
- Confirm the affected cylinder through proper ignition-system diagnosis before replacing coils
- Inspect the spark plug and plug connector on the affected cylinder while the coil is removed
- Check the coil electrical connector and wiring harness for corrosion or damage
- Secure the replacement coil with two retaining screws per the Mercury service diagram for these V8 assemblies
- Reconnect power and verify correct operation before returning to service
Technical FAQ
What is the difference between Design I and Design II coils on the 496 MAG?
Mercury updated the cylinder-head ignition system on the 496 MAG at serial number 0M376088. Engines at that serial and above use the Design II coil (8M0178581). Earlier engines through 0M376087 use a different Design I coil. The two designs are not interchangeable — always confirm the serial number first.
How many coils does a V8 application require?
One coil per cylinder. A full 8-cylinder engine requires 8 coils (8 x 8M0178581). If only one cylinder is misfiring, only that coil needs replacement — but it is good practice to inspect all coils for physical damage or corrosion during service.
What did 8M0178581 replace?
8M0178581 supersedes 889925 (Mercury reference 392-889925). If you have been ordering 889925 for a Design II application, 8M0178581 is the current OEM replacement.
Parts Desk Insights
The most common ordering mistake with this coil is not checking the 496 MAG serial number — Design I and Design II coils look similar but are not interchangeable, and ordering the wrong one is a very easy mistake to make on this engine family. When a customer reports a cylinder-specific misfire on a 496 MAG or 8.1L, this coil is the first replacement candidate after the spark plug has been ruled out. It’s also worth checking the plug connector and wiring at the same time, since corrosion at the coil connector is a common cause of repeat failures after a coil swap. Browse the full MerCruiser Marine Engine Parts collection for related ignition and engine components.
Shipping information
In-stock and OEM parts: ship same or next business day.
Locally warehoused items: ship within 1-3 business days.
Special-order items: timeline confirmed after order.
Returns & warranty
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We stand firmly behind the quality of every product we sell. Your confidence matters to us — and we back it with a straightforward 30-Day Satisfaction Guarantee.
If you are not fully satisfied, you may request a return within 30 days of delivery. All returns must receive pre-approval prior to being sent back. Unauthorized returns will not be accepted.
To qualify for a return:
Item must be in new, unused condition
Product must be in its original packaging
All parts, components, hardware, manuals, and accessories must be included
Packaging must be undamaged and suitable for resale
Important:
Electronics are strictly inspected. No returns on open-box electronics. Exceptions may apply only with prior written approval. Any electronic unit that has been installed, powered, activated, or altered may not be eligible for return.
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