Reading Spark Plugs
and What Are They Telling You?

Normal
Color: tan (gray/tan to near white)
Appearance: normal

Congratulations! The cylinder the plug is in is running perfect. Your mixture is spot on, or close to it (not rich or lean). It's running in the proper heat range.

Normal - Worn
Color: tan (gray/tan to near white)
Appearance: rounded center electrode
Signs: hard starting, misfire under acceleration

Congratulations again, this cylinder is running good, but the plug should have been replaced before now, lazy bum!

Rich or Carbon Fouled Condition
Color: black
Appearance: dry, powdery (black) soot
Signs: black smoke in exhaust, engine bog

Time to get to work. Rich condition can be caused by: Rich Mixture (too much fuel) Carbs: mixture screw, sticky choke and/or cable, float height needs adjustment or sticking open Fuel Injection: clogged injectors, cold start valve and circuit, fuel pressure, temp and pressure sensors, computer, etc. Dirty Air Filter Weak Spark Caused by weak battery, bad coil, bad ignition/ignitor unit Incorrect Plug Heat Range

Overheating Condition
Color: white
Appearance: chalky
Signs: engine running hot, fast center electrode wear

Very bad, can cause engine damage! Check for correct plug heat range Lean mixture (not enough fuel) Low grade fuel (low octane rating) Proper ignition timing Engine cooling issues Too much friction (leaking seals, no oil, etc.) Vacuum leaks EGR valve functionality (not all have)

Pre-Ignition Condition- The Beginning
Color: brown to black
Appearance: ground (connected) or melted center electrode
Signs: engine knocking, loss of power

Not a good thing, you're melting your plugs! This is the next step after the overheating plug above. Check for correct plug heat range Lean mixture (not enough fuel) Low grade fuel (low octane rating) Over advanced ignition timing Engine cooling issues Too much friction (leaking seals, no oil, etc.) Vacuum leaks EGR valve functionality (not all have) Knock-sensor (not all have)

Pre-Ignition Condition- The End
Color: black
Appearance: destroyed, everything is melted away
Signs: engine knocking, loss of power, possibly backfiring

Your plugs are toast buddy, you have some serious problems. The final stage of the overheating engine. Check for correct plug heat range Lean mixture (not enough fuel) Low grade fuel (low octane rating) Proper ignition timing Engine cooling issues Too much friction (leaking seals, no oil, etc.) Vacuum leaks EGR valve functionality (not all have) Hot spots inside combustion chamber (burning carbon) If you pull your plugs and see this, pull your engine too. Quite possible there is engine damage now.

Detonation Condition
Color: very light tan to white, also lower can be black
Appearance: cracked and chipped insulator
Signs: engine knocking, loss of power

This can also occur from overheating, so check the usual. It is also a bit more difficult to troubleshoot. Check for correct spark plug gap Check for correct plug heat range Lean mixture (not enough fuel) Low grade fuel (low octane rating) Proper ignition timing EGR valve functionality (not all have)

Ash Deposit Condition
Color: light brown
Appearance: crusty deposits only electrode or ground
Signs: misfiring Many times caused by fuel additives.

Check valve guides Check valve seats Don't use a fuel additive Don't quick fix this by running a hotter plug, figure out what is causing it first. A simple fuel brand change can many times fix this.

Oil Fouling Condition
Color: black
Appearance: oil soaked
Signs: loss of power

This is caused by excessive amounts of oil in the combustion chamber. Run a compression test/ leak down test Check valve guides Check valve seats Check piston rings Check cylinder walls Possible gasket leak

Spotting or Splash Deposit Condition
Color: black
Appearance: spots

This is due to "sludge" build up or faulty fuel intake Carbs: Dirty bores, jets, and air intake Fuel Injection: Dirty or faulty injector Time for the fuel system cleaner! Best thing to do is if you have carbs, remove them and perform a clean and adjust (including the float bowl area). If you have FI, remove the injector and clean in solvent, then test for functionality.

Mechanical Damage Condition
Color: tan (gray/tan to near white) it should be normal color
Appearance: ground is moved or crushed

Either due to debris, or improper spark plug length Verify the correct plug tip length Check for debris in the combustion chamber Check for extreme carbon buildup If you have carbon buildup, do a rebuild. No amount of solvent will fix it.

Excessive Carbon Condition
Color: black
Appearance: lodged piece of carbon between ground and electrode (gap).

You have lots of carbon, consider a rebuild.

Racing Tip:
"That vertical color band on the ceramic shows you where the plug is indexed. Meaning that band is aimed slightly at the exhaust valve. Optimum location places the band (shown right). If the band is anywhere but here, it means that when the plug is not at it's optimum location. There is still discussion as to whether indexing a plug is worthwhile, but on may applications looking for that last horsepower, it doesn't hurt."

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