I have a 98 GS300 that I purchased with 75k miles on it. I don't think the spark plugs have ever been changed at the 60k service, however every other recommended service has been done.

Should I go ahead and have the spark plugs changed immediately or can I wait till the 90k service? I want to try and do everything at once at the 90k service. My car idles and runs fine at the moment, but I am worried that I am doing damage to my engine.

 

You wont do any damage to your engine. Im not 100% sure if you have platinum or iridium plugs, but if you had iridium, they are 100k miles plugs. Even so,. if you car is running well, i dont see you doing any damage to your engine leaving the plugs in there. Platinum plugs are known to last this long as well. If you start to hear pinging on acceleration, then maybe change em.. otherwise, i think you're fine.