Mercury Spark Plugs & Wires

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Mercury Spark Plugs & Wires How-To's

Power Packs and Spark Plugs

Power Packs and Spark Plugs

This article talk about troubleshooting and diagnosing your Power Packs and Spark Plugs. It covers Heat Ratings, Fouling, Overheating, Troubleshooting Ignition Modules, Basic Check-Up, and Exercising Safety. ... read more

When to Replace Spark Plug Wires

Spark plugs and spark plug wires are engine parts that should be regularly inspected or replaced. There are a few warning signs that may indicate that the components may need replacing immediately.

Heat Ratings

Some of the most important factors to keep an eye out for with heat rating complications are:
  • If the position setting for the insulator tip has fallen out of place.
  • If there is sign of inactive thermal conductivity with insulator components.
  • If the overall inner structure appears to be wearing down or is damaged.
  • If there is any defect with the insulator nose.


The fouling process, also known as carbon fouling, happens when the spark plug end doesn't react to the designated temperature (approximately 842 degrees Fahrenheit). If this happens, then the carbon deposits will not be able to burn/fall off of the insulator nose. In short, if this minimum temperature is not reached, the carbon deposits will begin to collect and clump together on the nose and clot, resulting in a misfire and more fouling or collecting. Furthermore, if the heat range is too low or cold, fouling may also occur, specifically when the engine is at low speed or is operating in cold environments.

If you see any signs of carbon fouling in your spark plugs, this could cause damage to the entire engine, and it will not function as it should. You should clean either the spark plug (if the case is mild enough) or replace it completely, then determine the cause of the fouling.


Overheating is one of the more common probabilities for the need of spark plug replacement. Here are some of the things to check if this appears to be a possibility:
  • If the heat range is too hot.
  • If the air to fuel ratio/mixture is very low.
  • If there are too many chamber deposits.
  • If there is a lack of lubrication or cooling.

If the engine exceeds approximately 1472 degrees Fahrenheit, then your engine is at a high risk of overheating. Spark plugs that are not clean or have been susceptible to fouling can easily begin the process of an overheating engine. If you begin to see signs of this, make sure you check the status of your spark plugs, among a few other standard procedures. Consult with a technician if necessary, especially if you are new to boating.

Troubleshooting Ignition Modules

To start troubleshooting, you will need a few key items to run the test, specifically a spark tester and starter fluid. If your engine even briefly stalls in response to the starter fluid, then you will want to check the fuel supply/system. Otherwise, it is likely that the spark is the center of the malfunction. Use the spark tester to verify whether or not this is the case.

Basic Check-Up

After you obtain a good idea of what may be causing your engine to malfunction, you might try briefly unplugging the main harness to rule out the possibility of a faulty harness or lanyard. After you unplug it, connect a jumper cable to the battery, which will give you a good, safe source of power to the ignition system as you go.

The next thing, stand back at a safe distance because doing all of this may trigger the engine into starting (be sure that you are far enough away from the belts and pulleys). If the engine does turn on, you can shut it off by removing the jumper cable.

Exercising Safety

We do urge caution in the act of troubleshooting your engine. Any act of recklessness or neglect can result in serious injury or even death, as well as damage to the engine. Gas fumes and exhaust are very dangerous aspects of the engine, so stay clear of that and wear protective gear. Make sure you are in an environment with sufficient ventilation. If you have never attempted this before or are new to boating, we highly recommend that you consult with an experienced boater or certified technician/mechanic.
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