XJ XJ6 / XJ8 / XJR ( X350 & X358 ) 2003 - 2009

plug change at 100k mi

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Old Jun 11, 2016 | 01:05 PM
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Default plug change at 100k mi

Replaced my spark plugs today, technically 99,964 miles... but you wouldn't know it looking at these plugs. I've seen far worse on 50k mile cars. Just slightly brown depots, no real change in the gap, probably could have gone another 50k with no issues!

Also was expecting the worse on cylinder #8 after reading some horror stories on the forum. Couldn't have been more opposite. Didn't need anything special, used the same 16mm deep socket with a 3/8 drive and two 3-inch extensions as I used for the other 7 plugs. No issue with access, I've had a worse time changing plugs on a SBC.




 
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Old Jun 11, 2016 | 02:00 PM
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I've seen plugs with over 400,000 miles that looked similar and worked just fine.

The 100K change interval on our cars seems to be quite conservative.
 
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Old Jun 11, 2016 | 02:24 PM
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Default I feel better

Originally Posted by mhamilton
Replaced my spark plugs today, technically 99,964 miles... but you wouldn't know it looking at these plugs. I've seen far worse on 50k mile cars. Just slightly brown depots, no real change in the gap, probably could have gone another 50k with no issues!

Also was expecting the worse on cylinder #8 after reading some horror stories on the forum. Couldn't have been more opposite. Didn't need anything special, used the same 16mm deep socket with a 3/8 drive and two 3-inch extensions as I used for the other 7 plugs. No issue with access, I've had a worse time changing plugs on a SBC.




I have wanted a full day to do this on mine. It has104,000. Good to know it should be a half day procedure.
I am hoping it will run even better. Wishful thinking.
 
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Old Jun 11, 2016 | 04:00 PM
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Originally Posted by Panelhead
I have wanted a full day to do this on mine. It has104,000. Good to know it should be a half day procedure.
I am hoping it will run even better. Wishful thinking.
While some would say it doesn't matter how long you run that plug, the real truth is, while iridium lasts a good long time, the real issue is the spark gap itself. With use, as the gap widens, it takes more initial charge from the coil to overcome the gap, and 0.38 to 0.43 is really about as large as you want it to be. Ideally, around 8kV~10kV is where you want the coils to be, and as gap widens, it can take up to twice that voltage to overcome the gap, especially on tip-in, as the engine initially goes lean, and required voltages get in the 15kV~20kV range. That's all that coil can produce before you start to overheat the thing. Spark gap plays a direct role on the coil's well being.
 
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Old Jun 11, 2016 | 08:22 PM
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Originally Posted by Panelhead
I have wanted a full day to do this on mine. It has104,000. Good to know it should be a half day procedure.
I am hoping it will run even better. Wishful thinking.
Should only need 1.5-2 hours depending how leisurely a pace you work. Save yourself a lot of suffering and do them on a cold engine, too.
 
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Old Jun 11, 2016 | 10:44 PM
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Are those OEM replacements? Did you buy them from dealer?

NGK make 2 different plugs for our cars. The NGK iridium and Laser cut iridium.

A traditional IX iridium plug such as NGK ZFR6FIX-11 has an iridium Center Electrode, however the ground electrode is a traditional nickel construction. Therefore, the ground electrode will wear out first. NGK gives them a life expectancy of 40-50k miles. But they have to temper their projections as driving conditions and motor modifications differ. Typically we have found you can expect 60,000 to 80,000 miles on an unaltered motor.

NGK Laser Iridium plugs have iridium center and a Platinum ground electrodes. If installed in a regular engine, they would likely last longer than most people keep their car (barring any motor problems that can cause premature plug death). All Laser Iridium plugs are an OEM design, made for a specific engine. Some feature a special resistor or electrode design, depending on the OE requirements. In their OE application, NGK's Laser Iridium plugs typically last 80-100k miles.
 

Last edited by pkoko; Jun 11, 2016 at 10:48 PM.
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Old Jun 12, 2016 | 08:28 AM
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Originally Posted by pkoko
Are those OEM replacements? Did you buy them from dealer?

NGK make 2 different plugs for our cars. The NGK iridium and Laser cut iridium.
OE yes, but not from the dealer. NGK 7866 {#IFR5N10} Laser Iridium
More Information for NGK 7866
 
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