I have 105K miles on what I presume are the factory plugs. Manual says this is the time for replacement. Car runs fine and gas mileage is OK. I know the factory NGK Plats are great and last a long time. I've pulled them at 150K+ miles in the past on different cars and they still looked great. The VQ seems to be rough on oil but is it the same on spark plugs? How long have you guys gone on the original plugs? Should I just wait until I get a misfire or bad mileage?

 

I changed mine @ 90k and then again @ 160k. Reason for the recent low mileage interval was a curiosity check on the rear VC, and my assumptions were correct, cracked and cyl #1 was terrifying.

It does run and idles smoother when you change them, noticed that both times I've changed them. You get used to the way it idles and then when you change them, it's a nice treat.

I'd say 150k/13-14 years old .... it's time.

 

Plugs, when operating properly, won't change the idle, mpg, or performance, unless there has been a problem with a plug.
135,000 on original plugs, no problems, now 125,000 miles on replacement set, and once again no problems.

 

Don't forget your car's 12+ years old now

Change it

 

OK so if I do change them, I'll also do the PCV valve and valve cover gaskets since I'll be in the area anyway. Should I also replace the ignition coils? I know those suckers are expensive if going OEM. Maybe I can just replace the three rear ones since they're real pissers to get to and I don't want to pull the intake plenum again if one goes bad. The front ones are easy to get to if any fail.

 

Definitely change the plugs. Even though you may not notice it, the ignition system, mainly the coils, is working harder to compensate for the tired plugs.

I wouldn't replace the ignition coils just yet, but i agree that things on the backside of the motor die first, like the coils and the valve cover gaskets. My theory is that the front gets plenty of cool air ventilation whereas the rear doesn't, so years and years of that mild heat soak environment take its toll on the rear coils and gaskets.

 

I would replace them simply because its a wear item, and plus their easy to change on these cars.