Symptoms of Bad Outboard Spark Plugs

Outboard spark plugs generate a spark that fires up the air and fuel mixture, creating the combustion that essentially powers the engine. When you’re experiencing engine troubles, check the simple stuff first.

Synptoms of bad outboard spark plugs

One of the simplest engine ignition system parts to inspect and replace is the spark plugs. So how do you know if a spark plug has gone bad? Here are some symptoms of bad outboard spark plugs, as well as tips on how to test and replace them.

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Outboard Engine Starting Problems

One of the most notable symptoms of a failing spark plug is an engine that has trouble starting. When this happens, pull out the spark plug and see if it appears black, which is a sign of fowling from carbon residue. A dark soot or black appearance on a spark plug means the outboard is experiencing low compression or a vacuum leak. It could also mean the gap on the spark plug is too high. A fouled spark plug could be the cause of your outboard starting problems. To find out if the plug is the cause, test it first (more on that later). If it’s good, then clean off the carbon from both ends of it with a soft-wire brush and pop it back in to see if that solves the problem.   

Outboard Engine Stalling or Cutting Out

Another symptom of a bad outboard spark plug is the engine stalling, sputtering or cutting off. In this case the spark plug could be wet, which is an indication of water in in the combustion chamber. Try washing the spark plug with soap and water, drying it off and plugging it back in to see if that solves the problem. However, keep in mind that a wet spark plug is a sign of a much bigger problem that needs to be addressed as soon as possible.

Outboard Engine Losing Power/Decreased Acceleration

Loss of engine power is another symptom of a bad spark plug. An outboard losing power could lead to decreased acceleration and even total engine failure. Outboard acceleration decrease and engine power loss can occur when the spark plug is overheated. You can tell this is the case if the spark plug’s insulator has a chalky or glossy white look to it. If the spark plug is overheated, either the engine is running too hot or the spark plug is not tightened correctly. Pull out the spark plug and install it again if you think it’s not tightened properly. However, keep in mind that an overheating spark plug isn’t the cause of an overheating outboard, and you’ll need to find and fix the problem.

Testing Outboard Spark Plugs 

It's pretty easy to test outboard spark plugs, as it requires little more than a couple of wrenches and a spark plug tester. You’ll want to inspect the spark plugs for damage to the insulator tip, the boot and the insulators. Look for signs of fouling.  A normal spark plug has a dry, grayish color to it. As mentioned earlier, a wet spark plug will have a white chalky residue to it, while black soot or erosion on the plug are also signs that the plug has gone bad.

Replacing Outboard Spark Plugs 

First thing’s first: Never install automotive spark plugs on an outboard motor, only marine-grade plugs. Check the owner’s manual for your specific outboard to tell you which spark plugs to install.

Outboard spark plugs replace

The basic steps for replacing outboard spark plugs are as follows:

  • Disconnect the negative terminal from the battery
  • Remove the spark plug wires carefully off each plug by twisting and gently pulling off the boot
  • Put the engine in neutral
  • Remove the plug with a deep-well spark plug socket
  • Check the spark plug gap with a feeler gauge to make sure it’s correct
  • Apply dielectric grease to the tip of the spark plug only if the manufacturer recommends it
  • Insert the new spark plug, then hand-tighten it to prevent cross-threading the plugs.
  • Tighten the spark plug with a torque wrench per the specs in the owner’s manual
  • Reconnect and secure the spark plug boots

If your outboard’s spark plugs are fouled or damaged, keep in mind they’re super cheap and easy to replace. It’s basic routine outboard maintenance, and you should inspect them after every 100 hours of use, and replace them after every 300 hours of use even if they’re still in good shape. 




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