I've searched a bit and didn't really find an answer. Almost all the information that i've found is to replace the spark plugs based on mileage. The issue is that my car is 2005 MY and it has only 35k miles and I doubt the previous owner replaced them. Is there any benefit of replacing the plugs based on years or they're fine based on mileage only?

Ghostty

 

I wouldn’t sweat those as much as fluids/filters and hoses, but you might want to pull one and assess the condition and brand/part# for future replacement.

 

If you are going to keep the car for a while, just do it. Use the opportunity for other service work. If there is any leakage around the valve covers pull them, clean, replace the gaskets. It makes changing plugs so much easier with the covers off. Check all hoses. Clean the intake system. Replace the inexpensive crank sensor. Clean inside the throttle body. All of this is easier without the valve covers.
I did all of this to my 2003 SL55 with only 14,500 miles.

 

I have a 2005 SL500 with the original spark plugs and 120k miles. I doubt you need to worry about them. I suggest changing the ABS filter in the fill cap. Mine still had the original old filter. The new filters are finer. Also don't forget about the cabin air filters in the passenger footwell. You'd be surprised how many leaves get down and in that filter. Check the level of the roof hydraulic fluid. If it is down you might have a leak in the lock cylinder that may poor down on the seats. Ugly mess, so worth a check.

 

Perhaps someone can chime in on the relation between failing coil packs and bad spark plugs? Seems an obvious relation?

 

Spark plugs deteriorate with use, not with age. With use, the spark plug center and side electrodes gradually lose metal and so the gap between them grows larger. It takes more voltage to jump a larger gap, and so this makes the coils work harder to generate that voltage.

 

Generally speaking, I would change plugs if only if they're approaching the manufacturer's recommended mileage interval. Other situations may apply though - if they're difficult to get at and you're doing other work on the car that makes them more accessible, you might want to go ahead and do it. If you're doing coil packs and/or valve cover gaskets on a V12 car, for example.

If you suspect problems with the car - using oil, not running correctly. etc. - you may need to pull some plugs to see how they look. But if the car's running fine, I wouldn't do it "just to see". You can remove and reinstall tapered seat plugs without much worry, but it's not a good idea to remove/reinstall gasket seat plugs. Those metal gaskets/washers crush on installation, and they're not intended to be used twice, so you can get a compression leak and/or end up with an over-tightened plug

 

As long as they don't deteriorate due to age - I'm set. Obviously lots of things on this car are affected by years, like engine mounts, fluids or gaskets. So I was wondering about the plugs. I already did once an ABC flush when bought it, will most likely do a 2nd time this year. Besides that I've yet to experience issues after the EIS module that the dealer had to replace. And I don't think the dealer has a schedule for cars like this that have been driven little...