I put a set of E3.48 spark plugs in my 2008 Fit Sport automatic with 75,000 miles on 8/30/14 when I adjusted the valves to fix a stumble. The car ran but over time I noticed my mileage decrease, from about 32-34mpg to 26-28mpg. A coil went bad at 83,500 miles in February 2016 so I replaced all the coils and checked the plugs. The huge electrode on the E3.48 plugs are discolored, kind of looks like they've got surface rust. On a couple of them there is some minor pitting on the edge where the spark flies. I replaced them with Denso Iridium IK20TT's. It runs better. I'll update this thread with the mileage after a few tanks. I bought the E3.48's at advanceautoparts.com. These were then the plugs that "fit" my Fit. They no longer fit, now the E3.56 fit. Physically the plugs are identical. I'm guessing the heat range has changed. E3 is disappointing. I'm glad I didn't long run the $200 worth of Hitachi coils on the E3's! Does anyone know if the wrong heat range on a plug can contribute to coil degradation?

 

Heat range shouldn't affect coil life directly, fouling and or too wide of a gap will overwork the coils by raising the resistance across the gap. Valve adjustment can have similar affects, if your valves are not closing properly it screws up the gap resistance as well and can heat the coils.

The stuff that looks like rust is deposits from the additive package in the fuel. Its why "reading" plugs is really not very accurate, there are so many dyes and chemicals in modern gasoline you can get coloring that has nothing to do with combustion quality.. A exhaust gas analyzer is about the only way to tell if you've got the tune correct.

 

Spoke to E3 customer Service. Explained the situation, asked for difference between E3.48 and E3.56 and was told it's the heat range. Asked if E3.56 is a newer plug, it's not. Asked why the change now on a 2008 model and tap dancing started. Although not stated quite this clearly, the bottom line is they miscalculated the correct heat range when specifying E3.48 for our Fits. Exchange under warranty was offered. I'm sticking with the Denso Iridium.

 

You'd think there would be a TSB out ,, if there is a engineering reason why the original plug is superseded. It may be a correction for the higher alcohol content coming mid year.. Every body ready for E15 instead of E10?