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Failure analysis: Denso TT iridium spark plugs with melted & missing center electrodes

1582 Views 16 Replies 9 Participants Last post by  jamesepoop
Forum member V-C provided me with a set of Denso IKH20TT iridium plugs pulled from his '08 manual-transmission FJC with about 94,000km (58.4K miles) on them.

Shockingly, the iridium tips on the center electrodes were MISSING on two of the plugs, and several other plugs looked like they were eroding so severely that they too would have lost their iridium tips if they had continued to be run in that engine.

These are the correct TT-series plugs for a 2008 1GR-FE engine.

Initial observations:
1. All external markings and the general plug construction leads me to believe these may be genuine Denso plugs. HOWEVER, I'll forward some of the photos to Desno and ask them to verify authenticity.
2. The laser-welded iridium alloy center electrode tip was completely missing from two of the plugs, resulting in a spark gap of over 2.1mm (.083") in the worst case.
3. On the remaining plugs that still had their iridium tips, the gap was ~(TBD)mm ( TBD ").
4. All plugs still had the platinum tip present on the ground electrode.
5. No unusual deposits were seen on the ceramic insulators; there was a smattering of tiny fused-on black particles on the ground electrodes.
6. There was no evidence of overheating of the ground electrode bodies or unusual erosion of the platinum tips on the ground electrodes.
7. There was no mechanical damage to either the center or ground electrodes.
8. On most of the plugs that still had their iridium tip, there was evidence of melting of the iridium tip. Rather than a general melting of a homogenous piece of metal alloy into a large hemisphere (like the tip of a tungsten electrode in a TIG welder), the melted areas had the appearance of hundreds of tiny spheres of melted material, leading me to believe that the iridium tip may be a tiny slug of sintered powdered-metal rather than a rod of solid iridium alloy.
9. On most of the plugs that still had their iridium tip, the point at which the tip was welded to the core center electrode was greatly eroded and under-cut. It would appear that with continued use, these plugs would also have continued to lose material from the weld area and have eventually become under-cut to the point where the iridium tips would have broken off.
10. All plugs showed some residual anti-seize paste in the threads of the plug body.

Denso states that the electrode material in their iridium-tipped plugs is an alloy of 90% iridium and 10% rhodium.
The 'TT" plug nomenclature stands for 'Twin Tip': there is a 0.7mm diameter platinum alloy tip attached to the ground electrode, and a 0.4mm diameter iridium alloy tip laser-welded to the nickel-alloy center electrode core.
Denso claims that: "The patented formula has the highest concentration of Iridium of any spark plug and outperforms in the most severe engine temperatures resisting oxidation and voltage wear for over 100,000 miles."
In their installation guide, Denso stresses repeatedly that iridium plugs do not need to have their gaps adjusted, and it is crucial that no tool of any kind (gap measurement or gap adjustment) ever contacts the iridium tip.

Lots of technical insight into Denso's TT spark plug design and construction can be found in their patent filings at: US6885137B2 - Spark plug and its manufacturing method - Google Patents

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Photo 1: Denso plug model ID markings on ceramic insulator
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Photo 2: Denso iridium TT markings.
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Photo 3: Laser-engraved Denso code on plug body.
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Photo 4: Plug missing iridium tip from center electrode; platinum ground tip is intact.
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Photo 5: Another plug missing its iridium tip.
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Photo 6: Gap on one of the plugs missing its iridium tip was ~2.1mm.
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Photo 7: The plugs that still had their iridium tips intact showed various degrees of undercutting and erosion that would likely have caused them to eventually lose their iridium tips.
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Photo 8: Higher magnification revealed the iridium tips to be covered with tiny spherical globules of melted metal, looking more like a sintered or powdered-metal part rather than a solid rod of iridium alloy.
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Photo 9: A different plug had a similar appearance of the iridium tip: under-cutting and tiny spherical globules. This tip was also not accurately 'centered' on the center electrode body.
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I'm not drawing any conclusions here (yet), just presenting what I saw.
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Good write up.

According to Carfax, these plugs were put in the vehicle by a previous owner at a Toyota Dealer. So I very much doubt they are fake. Now, I suppose it’s possible that those plugs were replaced at some point before I bought it with fake plugs and that didn’t appear on Carfax.

@FJtest… I’m going to start another thread this weekend to do some diagnosis of my engine using Torque Pro real time data to determine if I have any issues (lean/rich/etc). Problem is, I don’t know what to look for hence the thread. Would appreciate your input on that when I get it posted.
It’s also worth noting that these plugs are not 100,000 interval plugs… they are 20K interval performance plugs. The I in the model number denotes the lower interval.

So they were in the engine three times longer than their specs dictate.

I have these types of plugs in my performance and turbo vehicles and check/gap them annually. Many people opt to change them bi(annually) as performance suffers once it's couple thou off spec (ex .026 to .028).

IMO they're probably not worth using on the 1GR (it's not a high output engine) unless it's a built/tuned motor, has SC or lives in high altitude or most of it's RPM life up top.
Yeah, I'm surprised these high performance short-life plugs ended up in the vehicle, especially given the work was apparently done by a Toyota Dealership. At least these days, the plugs offered for this vehicle by my dealership are either the copper core OEM plugs (90919-01235) or the SK20HR11 Iridium plugs (90919-01191) which are long life 100K Mile (at least according to Denso).
Wow is right. Everything looks melted and blasted away. How about a metallurgical test to see if the blown metal parts actually consist of platinum and iridium and not something else? The rest of the steel parts of the plug still look intact and relatively undamaged. Definitely no over-rich indications or oil burning at least. VC, do you know what year those plugs were installed, not the mileage?
According to the CarFax report I have, the plugs were replaced by a local dealer in May of 2014 at 94K KM. So the plugs were in there almost 7 years and 90K KM. When I first bought the truck in late 2021, I had a local shop do a thorough inspection and they pulled a plug and said it looked good. They must have pulled one of the few that wasn't totally fried. If I had known the extent of the damage to these plugs I would have replaced them right away. Anyway, it's remarkable that it wasn't misfiring given the state of a couple of these which are missing tips. :oops:
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