This forum is extremely light when it comes to diesels, however makes sense when in the USA I suspect we diesel car owners are less than one percent of the auto population. Never the less there is hope.
Some folks change glow plugs at the same rate as spark plugs---once again owners of cars are not logical they are very compulsive and will change any thing because a neighbor did-----very good for business. glow plugs and transmission fluid fall in that category.
My point is that you car has a very good built in diagnostic system and you will get a fault when a glow plug goes bad. Change them all then. Additionally you have to know your engine because glow plugs have changed in size and are not interchangeable. Use the dealer to tell you the correct part number for you car. And depending on the engine you have will determine the degree of difficulty in changing them. Also by OEM replacements. There is a big difference in specs and the resultant life of the glow plug.
You can search the web, say like "MB glow plug replacement" there are loads of tips.
I've seen several replies about GP, but they might be buried in other subjects.
My diesel at 220k didn't have the problem under my ownership (since 170k). Might be my luck, might be that in CA GP work for less than 5 seconds in cold morning.
What other members report is the access, that usually require intake removal and the hardest part is stripping the soft hex on the GP with improper technique.
If you go for the job, securing spare ride might be good idea.
If you have the 606.962 engine regrettably you have to remove the intake manifold to efficiently remove all GP's, which is not difficult, but tricky on the reinstall, to align the EGR flex pipe to the EGR valve and then from underneath with a very long extension tighten the EGR valve to it's bracket.
I don't know how long you intend to keep the car, or when you changed the low pressure fuel lines, but with the intake manifold off, it would be a good time to inspect and pressure test the low pressure fuel lines and pump for leaks. The lines themselves are very cheap.
With everything exposed, I would also change out the coolant temp sensor and crank position sensor---all very cheap parts, but if you have to replace later-----------off comes the intake manifold again!!
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