So I replaced the spark plugs on my GE8 yesterday, and something's off. The acceleration isn't as "punchy" as it used to be, and my gas mileage is down by around 5-7 mpg. I also think the exhaust smells richer, but I haven't been able to check that in a controlled environment yet. At the same time, I changed the oil, upgraded to a K&N high-flow air filter, and deleted the rear seat, so there's absolutely no reason I should be losing mileage like that. In fact, I drove it around briefly between the air filter/oil change and doing the plugs/seat delete, and was averaging 1-2 mpg ABOVE what I usually expect. I did use plugs that are one heat range colder than stock, but all my research indicated that one heat range is an acceptable deviation, and the worst I might have to deal with is excessive carbon fouling down the road. Doesn't seem like it should be causing this much harm right away.

Any thoughts?

 

Additional info: Plugs were torqued to vendor-recommended amounts as specified on packaging. Gapped to 0.044" per suggestion from the DIY spark plug thread on here. I'm 100% sure I heard all the plugs to the coil packs "click" when I plugged them back in. Applied dielectric grease to every coil pack, inserted them back in the exact same holes they came from. Not experiencing any kind of obvious stutter/misfire.

The only other difference between these plugs and stock is that they don't have a projected insulator, but all the research I did on THAT suggested only that you need non-projected plugs for a forced-induction setup, otherwise you shouldn't notice any difference.

 

These are the plugs I used:
https://shop.advanceautoparts.com/p/...earchTerm=4091

 

Possibly Incorrect plugs... should be iridium or double platinum at the least.

 

Everything I've read about spark plugs says that platinum/iridium offer lower performance, but last longer.

 

My buddy thinks I have a vacuum leak, I'm gonna check on my lunch break and report back.

 

It sounds like you were careful/thorough when installing them, so I'd lean towards incorrect plugs or that the car just doesn't like those plugs. I replaced my OEM ones a few weeks back at 72k with autolite xp plugs. People poo poo Autolite and say that only Denso or NGK should be used. I've noticed no issues so far in 4,000 miles and saved $6/plug after rebate. They are iridium and rockauto listed them as compatible with my car. I don't see the plugs you used listed on rockauto as being compatible... at least with my 2013.

 

With a vacuum leak you typically would have a rough idle

 

Idle is a little bit rougher than usual, now that you mention it.

 

Been doing some more reading. I'd never manually gapped a spark plug before, and my technique was fairly sloppy. I literally just stuck the gap tool in and pried it open. I did /try/ not to press down on the center electrode, but would damage to the core cause these results? Or would that cause a more severe failure?

 

I think you have the wrong plugs. NGK's website does not list those for the fit. Ignition systems can be finicky. I'd try plugs that are listed as being compatible for the car... if you're in doubt go to the manufacturer's website and check that way.

As far as gaping plugs goes, it's more difficult with fine wire iridium and platinum plugs because they are easier to break. Most plugs are gapped from the factory and don't need to be gapped. However, it's always a good idea to check the gap before installing the plugs. With fine wire plugs you need a wire loop style gap tool. the coin style (cheap ones at advance and zone) are for copper or other non fine wire plugs. The coin style can damage fine wire plugs.

 

Apart from just not having been officially tested (which I'd sooner attribute to the Fit not being a very popular vehicle and not a high priority for them), why would you suspect they're not compatible? What's different about them? They're the same reach, same gap, and a more conductive material than stock. What is it being finicky about?

 

I found an ngk video link explaining the differences between their spark plug lines.

https://youtu.be/wFkCVNhXMF0

it would seem that the v power plugs have a different heat range and are only suitable for racing or off road use. They fowl when used in street applications.

My knowledge falls under shade tree mechanic so it could certainly be something else.

 

Yeah, I took that into account, but I only went one heat range colder and I literally just put them in last night. No way they could be fouled yet. I do sincerely appreciate your help, but do you know if anyone here has a high degree of technical expertise on ignition systems? I'd really like to know the reason certain plugs are not compatible so I can make my own decisions based on product specs, rather than having to hope someone's already tested it for me.

 

If you don't find anything else wrong, I can recommend Denso Iridiums. We've been running IK22's (one heat range colder than stock) in our GD Fit since the beginning with great results. More power and no fouling issue.

Just have to be careful when gapping. The electrode is tiny and should not be touched...ever. Need to use a gapping tool.

Those NGK's you picked are better suited to boosted or nitrous applications where you're going to change plugs every few rounds at the track. They are meant for durability under extreme combustion chamber conditions. Not for getting all the power and fuel economy possible.

 

I'll have to double-check, but I think Denso IK22's are what I pulled out. Was kinda hoping I could put in something a little bit better :\

 

There is nothing better than an Iridium plug. You went from a $7 plug to a $3 plug. I assure you that the NGK race plugs are not as good. They are merely good at providing spark under very high cylinder pressures.

NGK Iridiums are great also and there should be a cross reference to the IK22's if you prefer that brand.

 

BTW... I used to run NGK race plugs in my Mustang when I used a TON of nitrous. They were good at not blowing the tips off like some others would do.

Once I got the spark and tune under control, I was able to switch to Iridiums and make more power.

I currently run NGK Iridiums in my supercharged Mustang.

 

https://m.carsdirect.com/car-repair/...op-performance
So, is everything in this (and multiple similar articles I've read) a lie? ALL the information I've come across says copper plugs give the best performance. Titanium and iridium are allegedly used because they're harder and thus last longer, but they're less conductive. Where is your information from?

 

Also, I've seen people on this very forum report success with copper-core E3 DiamondFires. I just don't see any reason to believe my choice of conductive material is the problem here.