Replace Your Mercruiser Spark Plug Wires

Replacing your spark plug wires can be easy if you take your time and know your engine information.

Steps

  1. Read your manual and all the safety tips.[1]
  2. Read all warnings about fuel, fuel vapors and dangerous moving parts.
  3. Understand that you are working with explosive vapors and sparks so extreme caution must be taken to avoid explosion and or fire.
  4. Determine your engine rotation (see tips).[2]
  5. Write down your firing order for your model engine. This information will be listed in your factory Mercruiser Service Manual.[3]
  6. Get familiar with your cylinder numbering system. GM engines do not have the same cylinder number layout as Ford engines.[4]
  7. Label each cylinder position such as #1, #2, #3, etc.
  8. Make sure all your spark plugs are the correct type and that they have been properly installed and tightened.
  9. Lay out each spark plug wire in order of length.
  10. Plan ahead so you know which plug wire will reach which cylinder.
  11. Only install good clean spark plug wires.
  12. Determine your distributor rotor rotation.
  13. Label each distributor plug wire pole with White-Out if not already labeled from the factory.
  14. Refer to your firing order to properly label the distributor cap poles.[5]
  15. Double check that you have the cap properly labeled as this is where many people get confused.
  16. Locate the #1 spark plug wire pole on your distributor cap. Your manual can help you ID which pole is #1.
  17. Install the #1 plug wire to the #1 pole and neatly run the plug wire down to the #1 spark plug.
  18. Make sure the plug wire snaps firmly onto the spark plug.
  19. Select the correct length wire for the next cylinder number in the firing order and install it.
  20. Install each and every plug wire one at a time double checking it's position in the firing order on the cap and on the spark plug.

Tips

  • The firing order for most Mercruiser sterndrive V8 engines is 1-8-4-3-6-5-7-2
  • Go with your "gut" feeling. If it "feels" like a plug wire didn't snap onto the spark plug correctly, chances are it didn't.
  • Take the time to know which cylinder is which. For example, GM V8 engines (standard rotation sterndrive) have the #1 cylinder on the port side bank up forward and go back in the order #1, #3, #5 and #7 (all odd numbers on the port bank) and the STB bank is #2, #4, #6 and #8. Ford engines number straight back as #1, #2, #3 and #4 so don't get the order mixed up.
  • Use a high quality aftermarket set of spark plug wires, to replace the factory originals which are just carbon core.
  • If you are unsure as to which pole on the distributor cap is #1 you can place the engine at TDC (top dead center) on the compression stroke, remove the cap and make a note as to which direction the rotor tip is pointing. That will be the #1 cylinder. This is best done by removing the #1 spark plug and slowly turning the engine in the correct direction until you start to feel compression pushing out of the spark plug hole. This means you are approaching TDC (top dead center) on the compression stroke. You should also be able to observe the scribe mark on your harmonic balancer approaching the TDC (zero degree) on your timing mark tab.
  • Read the factory shop manual.
  • Contact online help sites such as www.sterndrive-information.com and they can usually send you the firing order.
  • Read all Caution Statements.
  • Get a good understanding of your engines firing order before attempting to install the plug wires.
  • Take your time! Check and double check each plug wire as it's installed. It is VERY easy to get the wires mixed up because it can be hard to see the spark plugs in most boats.
  • Route the wires neatly. Most engines have wire brackets which keep the wires away from hot surfaces.
  • Replace your plug wires every 3 to 5 years or sooner if needed.
  • Some older inboard Mercruiser engines might have a reverse fire order so make sure to confirm your engine rotation.
  • All sterndrive engines are LH (standard)(counter-clockwise) rotation.
  • Rotation is determined by looking at the flywheel end of the engine.

Warnings

  • Avoid sparks or open flames.
  • Remove all fuel vapors from bilge.
  • Have a clean work area.
  • Have the correct tools.
  • Have a fire extinguisher close.
  • Read the owners manual and shop manual.
  • Read all precautions.
  • Work in a well ventilated area.
  • Have adequate lighting.
  • Wear safety glasses.
  • Keep clear of moving parts, belts and pulleys.
  • Damage, injury or death can result from fire or explosion of fuel or fuel vapors so use extreme caution whenever working on your engine.
  • Beware of sharp objects on your motor such as hose clamps as they can cut you.
  • Avoid getting fuel, oils or greases on your skin as some may be dangerous to your health.
  • Have a partner close buy to assist in case of an emergency.

Related Articles

References

  1. ↑ https://www.mercurymarine.com/en/asia/service-and-support/owners-resources/
  2. ↑ https://www.melling.com/wp-content/uploads/2017/03/marine-engines-standard.pdf
  3. ↑ https://www.perfprotech.com/blog/tech-articles/find-all-your-mercuiser-secifications-here-such-as-engine-fluid-capacities-horsepower-ratings-and-wiring-diagrams-5886
  4. ↑ https://www.perfprotech.com/blog/boat-and-engine-technical-tips/marine-engines/mercruiser-engines-block-id-codes-4-cylinder-marine-engines
  5. ↑ http://chicagoengines.com/marine/marine-firing-orders.aspx
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