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
"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
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
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.
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.
If the insulators are dark grey or black, then you are running too rich.
You will need to reduce fuel.
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.
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
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).
This page is maintained by Russell W. Knize and was last updated 09/01/98.
Comments? Questions? Email
Copyright © 1996-2003 Russ W. Knize