Using spark plugs to diagnose vehicle faults

December 05, 2017
Using spark plugs to diagnose vehicle faults
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Ignition specialist BERU has issued some useful information on how knowing tell-tell signs of spark plug problems, can also help to diagnose problems elsewhere in fuel delivery and ignition systems. 
 
Federal-Mogul Regional Marketing Manager, Jonathan Allen, says, "Worn, incorrectly gapped or poorly fitted spark plugs can give running issues, symptomatic of much more expensive problems. Without an efficient spark, any internal combustion engine will be struggling, and we can learn so much from the condition of a vehicle’s plugs.” 
 
Jonathan says a nice greyish white or yellow coloured insulator base is a sign that the engine is in good health with appropriate ignition settings. A sooty deposit, however, is a sign that an engine is running too rich and gives a clear indicator of things to be checked such as mixture settings, defective cold start systems, temperature or lambda sensors, the plug’s thermal rating (which may be too high) or that car is only used for numerous short journeys. BERU advises that garages always ask the customer how they normally use the vehicle before jumping to conclusions. 
 
Plug condition can also be an indicator of potential engine trouble before it develops. An oily residue, for example, is a sign that excess oil is present during ignition, possibly caused by the oil level being too high, or a symptom of worn piston rings, cylinders or valve guides. 
 
Melting of either the ground or central electrode can be an indicator that the spark plug is too hot, the air fuel mixture is excessively lean, incorrect ignition advance, a defective exhaust re-circulation system, faulty detonation sensor or a cross induction in the spark plug leads. In extreme cases, it can result in engine damage. 
 
The presence of black burn marks or carbon tracks running vertically down the spark plug insulator toward the shell, are caused by current discharge from the top terminal down the outside of the insulator to earth. This is due to a poor fitting or worn spark plug boot and will cause engine misfires, it can be remedied by replacing the affected spark plugs.
 
A broken insulator tip, brittle spark plug connector, stains, fouling and wear can all provide vital clues as to the condition and running characteristics of the engine. 
 
BERU provides helpful information on spark plug and glow plug checks and workshop tips on its website, beru.federalmogul.com/en-gb.
 
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