How To "Read" Your Spark Plugs


When you are increasing preformance on your engine no guage installation can tell the tale of what is going on better than the spark plugs can.  The porcelain/ceramic insulators on the plugs will pick up whatever foreign materials are floating around inside your cylinders and show them to you when you remove them.  So, when you make a change to your modified engine, always install a fresh set of plugs before you "dial it in" (test it out and make adjustments) so you can see how the engine is handling the modifications.  All the credit for this information must go to Thomas "Gus" Mahon and it is also available on one of Dempsey Bowling's Turbo Performance Upgrades page, but you have to dig around.  I have reconstructed that information here so that it is easier to find and use.

Once you have made your changes, installed new plugs, and made your initial adjustments, take your vehicle for a full-throttle run down a back road where it is safe.  You don't necessarily need to speed, just maintain full boost for a few seconds.  If you can hear the engine knocking, then back off immediately and do something to reduce it (see below).  If everything sounds fine, pull back in and remove your spark plugs, being bareful not to burn yourself on them.  Below are descriptions of what you may see and what it means:

What To Look For

Grey or White Insulators and Sharp Electrodes

If the insulators are white or grey in color with no deposits and the electrodes on the plugs are still sharp around the edges, then everything is probably OK.  Your air/fuel mixture is on target and you may have more room to add boost.  Anything else you may see indicates a problem and is described below.

Rounded Electrodes

If the electrodes on the plugs have become rounded during the last run, then your engine is probably running too lean.  The high combustion temperature is actually eroding the electrodes away.  You will need to add fuel.

Cracked Insulators

If the porcelain/ceramic insulators are cracked or chipped, then you are having very severe detonation (you should have heard a loud chattering).  You will need to reduce detonation.

"Salt and Pepper" Deposits

If you see white and/or black "speckles" on the insulators, this means you are detonating.  The white deposits are molten aluminum from your piston and black deposits are molten carbon that has formed due to high combustion temperatures and/or detonation.  You will need to add fuel and/or reduce detonation.

Dark Insulators

If the insulators are dark grey or black, then you are running too rich.  You will need to reduce fuel.

Yellow Insulators

If the insulators are yellowish in color, then your gasoline has a lot of additives in it.  If the electrodes are sharp and none of the above problems exist, then your are OK.

Orange or Brown Insulators

If the insulators are orange or brown in color, then your are probably running race fuel or you have added octane booster.  If there are no other problems, then you are OK.


Reducing Detonation

If your spark plugs tell you that you are detonating, then you can do any of the following to stop it.  If the engine is detonating badly, then your compression is probably too high.  You can use a head-saver gasket (by Fel-Pro) or a head shim to reduce compression.  Another way of reducing detonation is to retard the base spark timing and/or to run higher octane fuel.  If you are also getting signs of running too lean, then may also cause some detonation.  Richen your mixture and try again.  If it was lean and badly detonating, then you may want to try a combination of methods.

Adding More Fuel (Richening the Mixture)

If you are running too lean, here are some ways to increase fuel flow.  You can install larger fuel injectors (available through the Mopar Performance Catalog), increase fuel pressure by installing an adjustable fuel regulator (or squashing the stock one in a vice, ala Gus) and a larger fuel pump (if necessary), or install additional injectors (cold-start injectors from older European cars work good) and switch them on at high boost levels.  If your fuel system is simply weak, use the Diagnosing Your Fuel System page to pinpoint the problem.

Reducing Fuel (Leaning the Mixture)

If you are running too rich (probably due to one of the mods above), then you need to reduce the amount of fuel added.  The easiest way is to reduce fuel pressure with an adjustable fuel regulator.  Or you can put a TINY bleed on your MAP sensor (at your own risk - see Bleeding Beyond MAP Sensor Limits).
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This page is maintained by Russell W. Knize and was last updated 09/01/98. Comments? Questions? Email

Copyright © 1996-2003 Russ W. Knize