This is a diesel VS gasser question. I would rather change spark plugs even though glow plugs don't need to be changed as often. Spark plugs in my 230CE are so easy to change. I changed them today in 5 minutes. I timed myself. This included checking the gaps and torquing them to spec. They're also way cheaper than glow plugs. I paid $5.60 for a set of 4 Bosch plugs. Glow plugs are a PITA. I've changed them on a 617 and 603. Maybe glow plugs are like that because you don't need to change them often but even nowadays some spark plugs are good for around 60K miles. Glow plugs also cost around $10 to $12 each.

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DJ


84 300D Turbodiesel 190K with 4 speed manual sold in 03/2012