My 2009 Fit Sport M/T felt a bit rough since it has 140k miles on it. So I decided to change the spark plugs over the Labor Day weekend and found some Bosch Fine Wire Iridium Spark plugs with a $10 rebate. Good deal I figure.

After the change (gapped at .051), the engine runs MUCH smoother after a short test drive, so I'm happy. Now going on driving it more this week, I am noticing the engine has less power, even though its running smooth. Mainly in the mid-range around 3-4k RPM.

I've never read a whole lot into the science of spark plugs and figured the Bosch are the same as stock, but since feeling the lack of power, I started reading over this site about them. Seems to be a big push towards the plugs. I'm just really surprised to find that this little engine being more on the economy side and not high strung like my other cars will be so sensitive to different spark plugs.

So, should my project be taking out my new spark plugs and replacing them with another?

 

I have not had good luck with Bosch spark plugs in US or Japanese applications.

I'm sure that others have had success, but I have not. My Fords do better with Motorcraft or NGK and our Fit does great with the Denso's. We run the IK22's that are a heat range colder than the IK20's. Research both and pick one. You may be better off with the IK20's for winter use, but Jackson Racing found more power with the IK20's back in the day.

 

Denso or NGK for Honda applications. Bosch just don't do well.

The L15 is a very efficient engine and every single part will contribute to the efficiency.

 

I did a tune up for our daughter Honda Fit. Don't know if the POs ever changed the spark plugs. The Fit is currently at 190,000 miles. Went with Denso Iridium plugs. All 4 plugs when removed were loose enough that I could remove with just my hand. Prior to changing the spark plugs, we were averaging 16 mpg in city driving. Ever since we changed the spark plugs the mileage had jumped from 16 mpg to 26 mpg.

 

 

I'd double check with the 'Yota folks, but I'm betting they will say the same thing. It sounds silly, but for some reason there is a correlation between parts brands and automaker brands. Guess it's a matter of sticking with what each brand does best. Bosch is an OEM supplier to German brands. Denso, Hitachi, and NGK are suppliers to Japanese car companies. That OEM experience probably makes them more in tune with what those cars need.

 

That makes sense, I had German cars before when I was working on my own cars, so that is probably why Bosch didn't throw up a red flag for me. I just have to get all the cobwebs out of my head since I leased for the past 4 years and didn't need to think about this stuff or do any tinkering on the cars, which was nice, but the extra money in my pocket every month will be much nicer now.

 

I did once use a Bosch O2 sensor in a Civic, and it didn't do well. Had to change it back to the OE brand (but not bought from dealership) and everything was smooth afterward.

 

My wife's car (2009 Fit Sport automatic) is a daily driver, so reliability and ease of maintenance are most important to me. My OEM NGK iridium plugs lasted 140,500 miles and they looked like they would go at least another 50,000 miles. They were a real PITA to replace due to lack of easy access. (Behind the engine & under the bulkhead/dashboard.) One replacement every 6 years is enough for me. I installed the Denso 3401 plugs ($8.36 ea, Rock Auto) three years ago and the car has been running fine with no change in performance or gas mileage.