I change my oil in a 7.3 power stroke every 3000 miles no motor problems 240,000 on motor
Heater plugs in a ford focus 1.8 engine should be changed every 18 months-2 years.
Whenever it gets dirty.
every 3000 miles
That will depend on the engine. Some will be as early as 3000 miles and others as high as 10,000 miles.
5000-7500 miles
Change it according to the manufactures recommendations as listed in your owners manual.
if your car's belt has bits of fabric coming out
Every 15,000 miles.
Diesel smoke is often caused by fouled injectors, in which case the smoking can be cured by pouring a dose of good injector cleaner into the fuel tank on the next fill-up, and change the fuel filter, too. And change the engine oil and filter!
once every season
Every other oil change, drain water valve once a month.
once every season