Bosch spark plug supersession list and helpful hints

http://www.bosch.co.nz/content/langu...449_Poster.pdf

Helpful Hints

Normal condition
Insulator nose greyish-yellow to russet brown. Engine is in order. Heat range of plug correct. Mixture setting and ignition timing are
correct, no misfiring, cold starting device functions correctly.
No deposits from fuel additives containing lead or from alloying constituents in the engine oil. No overheating.

Soot - carbon fouled
Insulator nose, electrodes and spark-plug shell covered with velvet-like, dull black soot deposits.
Cause: incorrect mixture setting (carburettor, fuel injection ): mixture too rich, air filter very dirty, automatic choke not in order or manual choke pulled too long, mainly short distance
driving, spark plug too cold, heat range code number too low
Effect: misfiring, poor cold-starting performance.
Remedy: set mixture and cold-starting device correctly,
check air filter.

Oil-fouled
Insulator nose electrodes and spark-plug shell covered with shiny soot or carbon residue. Cause: too much oil in combustion chamber. Oil level too high, badly worn piston rings, cylinders and valve guides. In two stroke gasoline engines, too much oil in mixture.
Effect: misfiring, poor starting performance.
Remedy: overhaul engine, correct fuel-oil mixture, new spark plugs.

Lead deposits
In places the insulator nose is glazed brownish yellow; this may also tend towards green.
Cause: fuel additives which contain lead. The glaze appears in the case of heavy engine loading after lengthy part-load operation.
Effect: under heavy loading deposits become electrically conductive and cause misfiring.
Remedy: new spark plugs, cleaning is pointless

Heavy lead deposits
In places the insulator nose is thickly glazed brownish-yellow, this may also tend towards green.
Cause: fuel additives which contain lead. The glaze appears in the case of heavy engine loading after lengthy part-load operation.
Effect: under heavy loading, deposits become electrically conductive and cause misfiring.
Remedy: new spark plugs, cleaning is pointless

Formation of ash
Heavy ash deposits from oil and fuel additives on the insulator nose, in the scavenging area and on the ground electrode. The structure of the ash is loose to cinder-like.
Cause: alloying constituents, in particular from oil,can deposit this ash in the combustion chamber and on the spark-plug face.
Effect: can lead to auto ignition with loss of power and engine damage.
Remedy: repair engine. New spark plugs. Possibly use other oil.

Partially melted centre electrode
Centre electrode partially melted, blistered,spongy insulator tip.
Cause: overheating due to auto-ignition, e.g. due to
over-advanced ignition timing, combustion deposits in combustion chamber, defective valves, defective ignition distributor,
inadequate fuel quality, heat range possibly too low.
Effect: misfiring, loss of power (engine damage)
Remedy: check engine, ignition and mixture formation.
New spark plugs with correct heat range.

Centre electrode melted away
Centre electrode melted away, ground electrode also severely attacked.
Cause: overheating due to auto-ignition, e.g. due to over-advanced ignition timing, combustion deposits in combustion chamber, defective
valves, defective ignition distributor, inadequate fuel quality.
Effect: misfiring, loss of power, possibly engine damage. Overheated centre electrode may result in insulator nose cracking.
Remedy: check engine, ignition and mixture formation.New spark plugs.

Partially melted electrodes
Cauliflower-like appearance of the electrodes. Possibly deposition of foreign matter.
Cause: overheating due to auto-ignition, e.g. due to over-advanced ignition timing, combustion deposits in combustion chamber, defective
valves, defective ignition distributor,inadequate fuel quality.
Effect: loss of power prior to complete failure (engine damage)
Remedy: check engine, ignition and mixture formation.New spark plugs

Heavy wear on centre electrode
Cause: recommended interval between spark plug changes not complied with.
Effect: misfiring, particularly when accelerating (ignition voltage no longer sufficient for large electrode gap) Poor starting performance
Remedy: new spark plugs.

Heavy wear on ground electrode.
Cause: aggressive fuel and oil additives. Unfavourable influence of gas turbulence in the combustion chamber, possibly caused by deposits.
Knocking. No overheating
Effect: misfiring particularly when accelerating (ignition voltage no longer sufficient for large electrode gap). Poor starting performance.
Remedy: new spark plugs.

Insulator nose breakage
Cause: mechanical damage due to being struck or dropped or due to pressure on the centre electrode when improperly handled. In
borderline cases – particularly when the spark plug has been in use for too long – the insulator nose may be cracked by deposits
between the centre electrode and the insulator nose and by corrosion of the centre electrode.
Effect: misfiring. Spark jumps across at points which are not reliably reached by the mixture.
Remedy: new spark plugs