2012 22.1 Ask Albert

Dear Albert,
I’m preparing to give some “TLC” to my engine and have heard mixed reports regarding spark plugs, like if they’re reusable, what the different deposits mean, et cetera. Could you please share information about spark plug care?
Eric, the seasonal mechanic by force

Dear Eric,
Paying close attention to spark plugs is vital to the engine running efficiently. Spark plugs ignite gas and air-charge in gasoline engine cylinders. Keeping the plugs clean and properly gapped will help the engine start easily, be more efficient, and have more power, assuming the engine doesn’t burn oil and has good compression.

When an engine doesn’t start well, is depleted of power, loses gas efficiency, or runs rough, its a good idea to check the spark plugs. The condition of the spark plugs may provide clues to what is malfunctioning in the engine. When removing the plugs, make sure the porcelain isn’t cracked and that the plug is firing. (See figure 1)

Is the plug firing?

There are several techniques to determine if the plug is firing. A spark plug tester will show firing ability right off, but costs about $200.00. Generally, I pull the plug out, reconnect the spark plug wire, lay the plug on the engine block or another place it will ground, crank the motor over, and look to see if the plug is firing between the electrode and the ground arm. This method is easiest with two people where one person cranks over the motor while the other evaluates how the plug fires. Working alone, it can be done by putting the plug in view and cranking over the motor.

There is another method for checking spark plug firings if the motor has a distributor with points in it (this method won’t work with a magneto or electronic ignition). First, remove the distributor cap and set it aside with all the wires attached. Second, pull the coil wire out of the distributor. Using the coil wire as the tester, put the plug in the coil wire and set it down so that the spark plug body is grounded. Next, reach in and open and close the points. The plug should spark each time point is opened. Repeat process on all of the spark plugs.

The spark plug needs replacing if it doesn’t spark or if it snaps inside with the porcelain. If the spark plug is dirty and snaps, it is cleanable and reusable. Use a small wire brush to clean off the carbon deposits.

Spark plug health indicators

(See figure 2)

  • If the engine is running clean, the porcelain will be a light grayish to brown color without any powder or chunks. This indicates a proper burn: correct fuel mixture, no oil leaks, clean air, and good compression.
  • If the plug is black and powdery and the powder rubs off easily, there is too much fuel in the mixture. Check to see if the choke is on, the main jet is too big or not adjusted right, or the air cleaner is dirty.
  • If there are little globules of dark brown or glassy spherical deposits on the porcelain, the engine is not burning correctly. These deposits are additive residue that is not burning.

Cleaning the plugs

A small wire brush works for most cleaning projects, despite the availability of spark plug cleaning tools costing about $25.00. Use the brush by steadying a hand on a table, vice, or other surface to get a good solid brush stroke on both the tong and the electrode of the spark plug. A small flat file can also be used to clean the square open end of the electrode.

I suggest applying anti-seize compound on the spark plug threads once they have been cleaned.

Re-gap the plug

Be sure to check the gap on the spark plugs once they have been cleaned. Use a round wire gauge to check the gap (not a flat file – see figure 3) and adjust by bending the side, never the center, of the electrode to the proper specification. An owner’s manual will indicate the correct gap for the engine. This information may also be on a specifications sticker directly on the valve cover or engine. I use a .035 gap as a rule of thumb. To run the engine hotter, gap the spark plug larger, which makes the coil work harder. The larger the gap, the more contact area the fuel mixture has for ignition. Some engines have two spark plugs per cylinder to ensure better firing. Newer motors with electronic ignitions usually run with a .045 or .050 gap. So again, be sure to check the owner’s manual or specifications sticker if available. If gaskets are used, place new ones on spark plugs and torque to the specifications given.

Plug installation

Improper installation of spark plugs is one of the greatest single causes of unsatisfactory performance. If installed incorrectly, spark plugs will operate at excessively high temperatures and result in reduced operating life under mild operation or complete destruction under severe operation when the intense heat cannot be dissipated rapidly enough. Avoid the following practices:

  1. installation of spark plugs with insufficient torque to fully seat the gasket;
  2. excessive torque that changes gap settings;
  3. installation of spark plugs on dirty gasket seal;
  4. installation of spark plugs to corroded spark plug hole threads.

Always remove carbon deposits in hold threads before installing spark plugs. When corrosion is present in threads, normal torque is not sufficient to compress the spark plug gasket (if used) and early failure from overheating will result. Be sure to use a new gasket (if required) and wipe seats in head clean. The gasket must be fully compressed on clean seats to complete heat transfer and provide a gas tight seal in the cylinder. Using correct torque is extremely important during installation.

I hope this helps readers effectively evaluate spark plugs and reclean them if serviceable. Remember to take the spark plugs out and inspect them and the related areas about once or twice a
year. Periodic maintenance goes a long ways towards preventing expensive rebuilds.

Back in the shop for winter maintenance and repairs,

Tags: Engine, Gap, Installation, Maintenance, Plug, Repair, Spark, Spark Plugs, Troubleshooting