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VIDEO: Spark Plug Deposit Diagnosis

What can cause chalky deposits on plugs? This ASE G1 test prep video sponsored by Autolite can give some answers.


Studying for the ASE G1 Test? Watch these test prep videos to help get you prepared for success!

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The ASE G1 test requires you to remove and inspect spark plugs. This test prep video helps you know what to look for. CLICK HERE to learn everything you need to know about plug fouling, including chalky deposits on a spark plug’s center electrode, ground strap and ceramic areas exposed to the combustion chamber.

Coolant/Spark Plugs

Here’s another ASE G1 Test question about spark plugs

You remove a spark plug and find it has a white chalky deposit on the center electrode, ground strap and ceramic areas exposed to the combustion chamber. What is the most likely cause?

A) A rich running condition

B) A leaking head gasket

C) Leaking Piston Rings

D) Sand ingested into the engine

Spark plugs can tell you a lot about an engine’s condition. A rich running engine would cause light grey deposits. Leaking piston rings would cause black deposits. Sand ingestion would look like small beads to form on the hotter surfaces of the plug. 


But burned engine coolant leaves ashy, white deposits on the electrodes and insulator, creating hot spots that could cause pre-ignition and a misfire code to be set. The right answer is B.

When the plug is pulled, it might have a chalky appearance on the ground strap and center electrode. Modern coolants do not cause this type of buildup quickly, due to the reduction of phosphate, zinc and other additives that can contaminate the catalytic converters. In the past, the converter would become clogged and stop the engine before significant damage occurred.

Internal coolant leaks can foul a spark plug and cause a misfire. The problem could be a leaking intake manifold or a head gasket, and the fouled plug might be localized to one or two adjacent cylinders.

If the owner has been fighting a leak by topping off the coolant with the water or the wrong coolant, it can degrade the head gasket and other gaskets that come in contact with the coolant. Coolant with the wrong base chemistry can damage the engine. If a cooling system requires OAT coolant (Organic Acid Technology – usually orange, yellow, red or purple) or HOAT coolant (Hybrid Organic Acid Technology coolant – usually orange or yellow) and conventional green coolant is used, it can also degrade the head gasket. Beyond that, some import nameplates specify levels of silicates and phosphates. If the levels of these additives are not suited to the engine, even more damage can occur.


Abnormal condition of the sparkplug typically indicates some other problem – your investigation will do your customers a favor to figure out what that problem is.

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