When changing the plugs on my 99 Nissan Almera 1.6L I noticed that the tips of all the plugs looked a bit weird with these white deposits ( all four look like this ):
For NGK, Denso and Pulstar, the higher the number, the colder the plug. In identical spark plug types, the difference from one full heat range to the next is the ability to remove 70°C to 100°C from the combustion chamber.
Further on it states:
Running too cold a plug can only cause it to foul out, whereas running too hot a plug can cause severe engine damage.
I'm wondering if that white crud is the kind of fouling they are talking about due to too cold a plug, or is it something else? I ask because when I think of fouling I tend to think of black carbon buildup.
I found a few more examples with pictures that look very similar to mine which say it's ash fouling:
Deposits Appearance & Symptoms: The electrodes—center and ground—are covered in an ashy coating. As a result of this masking of the electrodes, your engine may experience a misfire. This build-up of combustion deposits can eventually (but not usually) fill in the space between the two electrodes.
As shown below, a buildup of combustion deposits stems primarily from the burning of oil and/or fuel addatives during normal combustion. These are normally nonconductive. However, when heavier deposits are allowed to accumulate over long mileage periods, they can "mask" the spark, resulting in a plug misfire condition.