Toyota FJ Cruiser Forum banner

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

Font Material property Household hardware Gas Circle

Photo 1: Denso plug model ID markings on ceramic insulator
Handwriting Gas Tints and shades Font Electric blue

Photo 2: Denso iridium TT markings.
Gas Auto part Household hardware Font Circle

Photo 3: Laser-engraved Denso code on plug body.
Camera accessory Tints and shades Camera lens Cameras & optics Fashion accessory

Photo 4: Plug missing iridium tip from center electrode; platinum ground tip is intact.
Wood Gas Tints and shades Art Font

Photo 5: Another plug missing its iridium tip.
Gas Tints and shades Font Wood Metal

Photo 6: Gap on one of the plugs missing its iridium tip was ~2.1mm.
Light Microphone Font Material property Office supplies

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.
Wood Plant Yellow Natural foods Liquid

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.
Brown Liquid Amber Fluid Insect

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.
Brown Wood Liquid Metal Macro photography

I'm not drawing any conclusions here (yet), just presenting what I saw.
See less See more
  • Like
  • Helpful
Reactions: 5
1 - 5 of 17 Posts
I have never personally seen or heard of platinum or iridium fine-electrode plugs that look like those above, especially after only 60K miles.

I think something unusual is going on here, but I don't know yet what it is.

More typically, platinum/iridium plugs only show moderate electrode erosion at 100K miles, and not completely missing electrode tips.

Personally, I think the consistent performance over an extended period of time makes this type of plug cost-effective over the long run ... I run them in all my vehicles.

More investigation required ...
Wild speculation, or do you have some specific basis for making that determination?

And of course there are many authorized Denso distributors that provide options for purchasing genuine Denso plugs other than through a Toyota dealer.
I'm VERY familiar with that thread ... I posted to it several times. I have no doubt that counterfeit Denso products exist.

My question to you is: what specific clues did you see in the photos in the Denso spark plugs in the Failure Analysis thread that led you to conclude that those spark plugs were "counterfeit"?
  • Like
Reactions: 1
Interesting on the replacement interval for plugs with "I" as the leading character.

From the link you provided above, it's not clear if the "I" plugs have an expected life of 10,000 km or 20,000 KM. Regardless, either 10k or 20k km is a ridiculously short life ... why would anyone install these plugs in a typical street-driven vehicle??

From Denso:
Lifespan of iridium spark plugs
Not all iridium spark plugs are the long-life type, there are also some that have the same lifespan as a normal spark plug: 20,000 km (light automobiles: 10,000 km, motorcycles: 5,000 km).

20,000 km type [normal lifespan type]
Models beginning with “I”

 ・IKH20    etc.

From another Denso site, the TT-series plugs are simply described as:
  • Iridium TT Twin-Tip Technology was developed from the Original Equipment Specification Super Ignition Plug (SIP) design, fusing our patented 0.4mm diameter Iridium- Rhodium alloy Center Electrode with an OE style 0.7mm Platinum Tip Ground Electrode.
  • Combines the power and torque of a high-performance plug with the endurance of a long-life original equipment spark plug over 100,000 miles
See less See more
  • Like
Reactions: 1
1 - 5 of 17 Posts