Denso iridium spark plugs might last >200K miles in a Gen 2

This morning I replaced the Denso SK 16R11 iridium spark plugs in my 05 Gen 2 with the same. The ones I removed were installed by a Toyota dealership in June 2010 when the car had 120K miles on it. It has 237K miles currently.

After five years and 117K miles, I was amazed (and impressed) by the minimal wear on the center iridium alloy electrodes and the platinum plated side electrodes. The gaps were still within the 1.1 mm maximum for new plugs. There is a little bit of carbon build up and tan colored combustion residues on each of them; nevertheless, you couldn't ask for better results considering the time and distance. The insulators had absolutely no discoloration above the hex nut. I had always seen yellow and/or brown discoloration on the insulators on every other car I've worked on - I've been wrenching since 1974.

I might have been able to leave them in there for another five years...................................

But, I'm glad I didn't because three of the four did not loosen as easily as ones I've removed in other cars. Each had a little bit of rust at and near the base of the threads at the cylinder head. I had to turn the 3/8" drive socket wrench with much more torque than I believe it should have required. I suppose this shouldn't have been surprising considering how long they were in place.

The #3 plug was the only one that loosened "normally". There was more oil on its threads than the others (no oil on its electrodes though - looked the same as the rest). Hmm .....this might be the cylinder that is causing the minor oil consumption issue I've been experiencing since I bought it in 2010 - or not. Anyway.

One thing for sure, this is the second and last time the spark plugs will be replaced in this engine. Unless it's still alive and kicking with 437K miles (not likely, but not impossible).

By the way, changing them out is really easy - much more so than I anticipated. In addition, I found that removing the air filter housing was worthwhile as well to get a little more room to work. Not necessary, but I found it helpful.

 

I was curious why the plugs didn't have the yellow/brown discoloration on the insulator so I seached Google.

The discoloration is "corona stain" which is caused by oil and dirt particles suspended in the air in the igniter "tube" which gradually deposit on the insulator due to static electricity.

A thousand pardons to those who already knew that