Code P0420 was triggered a few times recently, immediately following partial misfiring... So I removed my rust-seized 70,000 mile/7 year old Bosch Platinum-IR (Iridium Fusion) spark plugs. Rust buildup on the threads insulated them from the head. Intermittent misfiring at low rpm resulted from poor spark plug grounding... P0420 code for catalyst below normal efficiency would come on right away as soon as it misfired, but not enough for P0300 P0301/P0302/P0303/P0304 misfire codes to appear.
She's running good and strong again after simply brushing off the rust from the threads. I reinstalled the used Bosch plugs with a bit of high-heat anti-seize compound on the threads... Denso and NGK are much more resistant to long-term rusting.
You're asking me why metal rusts? Like I mentioned, they were in for 7 years, and Bosch spark plugs tend to rust much faster on the threads compared to Denso or NGK.
I did not know why your spark plugs, protected by coil packs, and other such items, were able to rust. I still do not know. You said they were also seven year spark plugs, not that they had been installed for that length of time.
I'm digging the new Denso IKxxTT spark plugs (xx = insert heat range here). It's likely not rust on the threads but carbon/varnish build up creeping up the threads from the combustion chamber. Most plugs get some on them but it rarely causes grounding issues.
Like I said, my spark plug threads were severely rusted up, and seized-up tight from rust buildup. Yes, Bosch plugs are much more prone to long-term rust on the threads compared to Denso and NGK. No, it wasn't carbon or varnish. Yes, they were installed for 7 years/70,000 miles.
Weird, well no more Bosch plugs! Try out the PK TT or IK TT plugs, they seem pretty good. I replaced a set of NGK IX that had 40K on them with IK20TT (correct heat for my car) and I'll be darned if the thing doesn't idle smoother than it did with the new IX plugs and it's even seemed to cure my longer hot restart that the car has always had. These are good plugs for the 2ZZ, I'm sure they're good for the 1ZZ as well.
My Bosch plugs are fine, and will do for the rest of the life of my car. I previously used a set of NGK Iridium IX for 84,000 miles... I do like Denso Iridium Power plugs as I run in my Kawasaki. I also got some good Denso Platinum TT plugs with titanium-enhanced ground electrode for cheap from Rock Auto, for tune ups on buddies' Ram Hemi 5.7 (16 spark plugs), and Chevy Epica 2.5 L6.
One of the major byproducts of petrol combustion is water.
The steam laden combustion gases seep into the threads a bit during the power stroke.
The gases cool off, the water condenses and stays put.
Same thing that rusts out exhaust systems.
In addition, even though the spark plug well is sealed by the boot there is still air movement. The boot just keeps out dust. As the spark plug well heats up air density is decreased and some air get pushed out. As the head cools off and the spark plug wells cool air is pulled in as density increases. Then the two dissimilar metals of the head and spark plug cool at different rates and allow the moisture to seep into the threads.
As a matter of fact, there is about one gallon of water produced for every gallon of gasoline consumed in the normal combustion process... At any rate, the real culprit is Bosch's poor oxidation resistance.
No such problems with Denso and NGK, so there is no reason for you to be concerned about it.
I had no idea it was one gallon of water for every gallon of gas consumed. That's crazy to think that. You'd think since Bosch is used on higher end cars such as Mercedes and BMW, that they'd have a better resistance to rust.
At least these spark plugs are not designed like Motorcraft spark plugs for the Triton V8 engine family.
It is a bit astounding... It's also why there's a drain hole in the muffler casing before the exhaust outlet. You can sometimes see water pour out of the muffler of the car in front as it drives off from a stop.
We had a Rolla (9g I think) at the shop we had sitting around with a 3 cylinder engine in it, we'd run it in/out a little as possible each day for few months in the fall and winter while the owner got the title straightened out (was in her brothers name from out of state) to give to us. Then we tossed in a rebuilt 1ZZ and started it up. It was bumbling and idling like shit like it was flooding itself out, so my boss grabbed the throttle and gave it a hard rev after a few minutes. It shot water out of the tail pipe about 15 ft across the shop like a shotgun!
The fuel is mostly carbon and hydrogen.
Add oxygen and stir.
The carbon combines with oxygen to make carbon dioxide.
The hydrogen combines with oxygen to make water.
Everything else is a minor side show.
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