The Color of Spark Plugs

Don’t use a socket wrench to tighten a plug. Use a torque wrench and tighten to the manufacturer’s specifications. This is a critical step if the cylinder head is made of aluminum.

Always ensure the new plugs are gapped correctly to manufacturer’s specs and torqued correctly to prevent damage to the spark plug or cylinder.

Once you’ve verified that the ignition system is operating correctly, a systematic approach to spark plug diagnosis can provide an accurate indication of what is going on inside the combustion chamber of your gasoline engine.

For decades, before marine engines had computers that provide self-diagnostics and real-time readouts, mechanics would read the spark plug to determine if the engine was operating correctly. Plugs would be inspected first to help determine if the problem was fuel-related, spark supply or other factors that affect engine performance. There are times where the spark plug itself is the culprit but, more often than not, the condition of the electrode and surrounding ceramic gives an experienced technician information on how the engine has been performing since its last service.

Step 1: Put the Spark in the Plug

In order for spark plugs to perform correctly the ignition system must be provided with the correct voltage and current. For engines that use points and condensers (breaker type) there is usually a 2-ohm resistor on the coil power supply wire that reduces the voltage from the approximately 14.5 volts output by the alternator down to 8 or 10 volts. As this resistor goes through heat cycles, the resistance increases, which in turn, drops the voltage to the ignition system causing poor spark.

Electronic ignition systems have their resistors inside the coil and run at 14.5 volts into the coil. Verify the type of system you have on your engine and then, using a multimeter, check the voltage at the positive terminal on the ignition coil with the engine running. Once you’ve verified that the voltage and ignition system is operating correctly, the condition of the spark plug electrode and porcelain will give a fairly accurate indication of what is going on inside the combustion chamber of the engine.

Step 2: Check Plug Type

Always check to ensure that the spark plug used in your engine is the correct type and heat range for your engine. Never presume that the spark plugs that are currently in the engine are the right ones unless you put them there after checking the engine manufacturer’s specification. What’s in there now may be the right specification or they might just be what were handy to the last technician who changed them.

The best way to ensure you have the correct plugs is to check your owner’s or service manual for the manufacturer’s recommendation. This also ensures that the threaded portion of the plug is the correct length and that the heat range is correct. Never switch Jelectrode plugs with surface gap plugs or vise versa unless approved for use by the manufacturer.

Step 3: Mind the Gap

In order for spark plugs to function as intended they must be gapped correctly. A plug not gapped correctly cannot perform in the desired rpm range. Plugs must also be installed with the correct torque. If a plug is not torqued into the cylinder head, the tremendous heat generated cannot be transferred to the cylinder head. This causes the plug to overheat and possibly cause major engine damage.

Step 4: Color Matching

Most spark plug manufacturers have an excellent chart in their catalogs that show color photos of the different engine conditions and how the electrode area of the spark plug is affected by these conditions. Also, most engine service manuals have a spark plug chart with illustrations that depict the different spark plug failures.

A properly firing plug operating at the correct temperature range should be dry with a light gray colored insulator. Some plug conditions to watch for are: wet or dry fouling; heavy carbon-like deposits; rustcolored deposit on the firing end, which may indicate water has entered the cylinders through the exhaust manifold; and a dark insulator with few deposits, which indicates the plug is running too cool, a condition caused by low compression or by a plug of the incorrect heat range.

At right are a few examples of spark plug deposits including some extreme examples that indicate major problems with the engine and/or other support systems.
- By Steve Auger

Steve Auger is an advisor to DIY’s Technical Helpline and a service training instructor and MerCruiser product support specialist for Mercury Marine.

Surface gap plug.	Plug Condition: carbonized and worn electrode. Problem: Engine misfires during acceleration, increased fuel consumption, poor high-speed operation. This spark plug should have been replaced a long time ago.	Plug Condition: white or gray insulator, often blistered. Problem: This plug shows overheating, possibly caused by a loose plug, lean fuel, wrong heat range or bad ignition timing.
Plug Condition: black with heavy deposits of unburned fuel and oil. Problem: Seized piston rings have caused oil to leak by the rings and oil fouls the spark plug ceramic and the electrode.	Plug Condition: dry or wet fouling. Problem: Fouling caused by too rich air-fuel, extended slow speed driving, choke abuse, electrical trouble or too cold plug heat range.	Plug Condition: molten metal attached to the plug. Problem: This spark plug shows a detonated piston and the engine in need of a costly rebuild.
Plug Condition: black with soot. Problem: A restricted air flow or rich fuel mixture is the likely culprit.	Normal firing condition. Be sure to check the gap on new plugs before installing.