I have a 2008 997 with 15,000 miles on it (sad I know). Service interval recommends replacing plugs - Is there any value in this at such low mileage?

 

I have a 2008 as well with 22K miles. The dealer wanted to change the plugs a few years ago ( 4 yrs ) as the schedule called for it. My care runs well and I will most likley change the plugs when the mileage limit calls for it. My car is a weekend, fair weather only toy and the ignition coils are note exposed to water, therefore, no hydrolytic degradation of the plastic housings for the coils. I would go with miles unless the car is not running well. You are not going to damage the engine.

 

I changed the plugs and ignition coils(you have to remove them anyway) on wife's '08 c4 at 30K just for peace of mind. Didn't notice any difference.

 

changed mine on my 09 when it had about 10K only because of the years. didn t want the plugs to be tough getting out with all the soft metals involved. Ran the same.

 

Coil packs tend to crack with age and more importantly with bad weather driving over time. Salty winter roads mostly. If my car was fair weather only and low mileage, I'd probably stretch the 4-year change interval to about 6 years or so, mostly so the plugs don't seize in the threads. But if the coil packs looked anything other than new, I'd do them both. The extra effort and cost to do the plugs while changing the coil packs is minor.

 

IIRC the factory calls for plug replacement based on miles *or* time.

For time the concern is left in too long the plug can seize in the head and possibly damage the head to the point the head has to be replaced.

Exposure to elements does not play a role in plug corrosion. This takes place from the plug tip up along the threaded body as compression/combustion pressure force combustion gases up in between the threads. One can obviously see this is the case when an old plug is removed. The threads furtherest away from the tip are "like new" while the threads closer to the tip are discolored and affected by corrosion.

Since the plug is the least expensive part, the plug is the sacrificial item in the plug/head interface. Better to discard a plug early than replace a head.

It is false economy to run plugs past their change by date.

 

Semi and Mac both have great points. Semi stated that if coils look questionable best to replace them because pulling the rear bumper and mufflers twice isn't cost effective if one is watching a maintenance budget..
Mac was pointing out that the problem end is the electrode end in the cylinder head. What happens as he was trying to put it carbon builds up on the threads over time. Sometimes spark plug threads extend into the cylinder this buildup can be severe over time. But also sitting and going through temperature changes(sweating) can cause corrosion at this point. When enough of this builds up it doesn't break off from the treads when removed. Instead it cuts or removes the treads as its removed causing a whole new set of problems for you and your ride. I believe that Porsche recommends the 4 yr intervals in order to minimize this buildup no matter the miles. It's a safer then sorry approach. Hope this explaination was helpful to you to better make a judgement. YMMV Good luck