Question: how do i know if i need to change my glow plugs?
Answer: Hard starting. A hard starting engine is the most common indicator of damaged glow plugs. ... 
Engine misfiring. The combustion process in diesel engines requires a high rate of compression, fuel injected at the correct time and in the correct amount. ... 
Rough idling. ... 
Decreased fuel efficiency. ... 
White smoke. ... 
Black smoke.