Sparkplugs changed from Toyota to denso and reduced performance

No, I'm suggesting that whatever cross-reference resource you are using, which you haven't identified, appears to be in disagreement with both Toyota and Denso about what is the plug for this application. (Edit: I've spoken too soon, the 2014 Prius c owners' manual does have FK16R-A8 without the H.)

(Further edit: looking more closely at the Toyota updates document that I linked in #17, I had glanced at that too quickly and thought it agreed with the OP's given number, but it also leaves off the H, so it agrees with the owners' manual.)

So it seems that the plug without the H is the right one, and your unidentified cross-reference source does agree with the Toyota references. I'll edit post #17.

Edit again: it looks like the mystery source of the cross-reference data and the funny four-digit "Denso" numbers could be the site called "densoproducts.com", which is registered to Monarch Products, Inc., a California wholesaler of multiple spark plug brands, also operating "sparkplugs.com".

 

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OK.

 

As an upshot of all of that, I wonder if it would be wise to check whether the plugs with the H, which are 7.5 mm too long, could have been long enough to (ouch) interfere with piston travel.

 

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2014 TOYOTA PRIUS C 1.5L L4 ELECTRIC/GAS Spark Plug | RockAuto

 

Densoautoparts.com is the main USA website for Denso, and is prominently linked from denso.com (global site).

The part-finder (application guide) on densoautoparts.com shows part matches by your choice of "long number or stock number."



From what I can see, long number can be decoded to describe the features of the plug. The stock number is strictly a four digit stockkeeping system.

When I run "2014 Toyota Prius c L4/1.5L" on the site's embedded part-finder, the top result is the FK16R-A8. Other results appear to be same fit, but different materials (not iridium.)

Clicking into the result detail, the FK16R-A8 long number is cross referenced to stock number 3485.

So it looks to me like dual part numbering system really is coming from Denso.

 

I swapped mine 3k miles ago for Autolite XP3924 and noticed improved performance.

AutoZone Link
O’Reilly Link

 

Thanks for finding that. I really hadn't encountered the four-digit bits on any publication from Denso that I'd seen.

On densoautoparts.com being their main USA website, I hope so? If they link to it from denso.com that's a good sign.

Registration-wise, it's a private individual registration through GoDaddy.

Their server certificate is signed by Amazon under a domain-validation policy with no subject information other than cn=densoautoparts.com. It's also good for densott.com.

I'm happy to believe it's their site as long as there's no reason not to, but it's weird an outfit the size of Denso would be on such an anonymity kick. Doesn't make it easy to confirm it's their site at all. If they ever happen to be late with a registration renewal and Bob Thumplefrump takes over the site, there'd be scarcely any way to tell the difference!

For the main www.denso.com the cert is issued by DigiCert, under an organization-validation policy, to C=JP, ST=Aichi, L=Kariya-shi, O=DENSO CORPORATION, CN=www.denso.com, and it is good for lots of domains including www.denso.ua, www.denso.ro, www.denso-am.eu, www.denso-am.co.uk, and so on. So they could have handled densoautoparts.com the same way; they clearly know how to do it....

 

Did you happen to save before/after values of your OBD-II misfire margin or misfire counts?

 

I've been feeling some real déjà vu on the topic.. I could swear that Bosch used two numbering systems (one descriptive, the other a simple unique identifier) when I sold their products for a living 25 years ago.

I also ran the whois and got the same unsatisfying result. Weird, but it's working for them...

 

No, I didn’t gather that data, because I wasn’t having any issues with the old plugs. But at nearly 7 years and 70k miles, I could definitely feel a difference with the new ones.

 

The ECM tracks a sensitive measure of firing performance called "misfire margin" (higher positive values good). It seems likely that any "feel"-able difference in spark plug performance would show up there, other things equal. That could offer a nice reproducible way of reporting such things.

 

It is all good as long as you are sure you bought FK16R-A8, as you have said just now, and not FK16HR-A8, as you said back in post #3:

If post #3 was a mistake and you really bought the plugs with no H, then good.

 

How would you feel about taking one out and taking a photo?

 

Ugh, if you have to ask so a "generalized question?" That is really, really not good! Changing spark plug's on a "Prius," is really an annoying PIA! You have to disassemble a lot of "annoying silly stupid stuff," just to get to them!

And if you have never changed any Spark Plug's in your life? Then you will need an inch pound Torque Wrench to reinstall them, without destroying the head! because if you "over torque," the Spark Plug's ... you have entered a new level of Hell!

I use a "Stubby 3/8 inch," drive for spark plug's myself. because over the year's, I know what "feel's," right. But if you have no clue? You are pone to "over torqueing!" IE cranking the hell out of them to "make sure," they don't come out? But you only need to worry about that, once you actually get to the Spark Plug's!

But it's a "Prius," and you have to get to the "Spark Plug's, first! I don't know what engine is in a Prius C myself? But if it is the 1.8? Then changing the Spark Plug's, would look like this. :

It's not that big a deal "if," you know what your doing? But a "Prius," is not really a good choice of vehicle to start with for your first "Spark Plug change?"

 

Thanks, it’s good to know some metrics to look at if I ever need to do some troubleshooting down the road.

As to backing up these claims on this forum, people are free to take my anecdotal reports with a grain of salt. I’m not a mechanic, just a Prius owner with 25k miles of driving in it over the past 15 months. The new plugs were my last tune up after changing the ignition coils, solenoid, engine air filter, and cleaning the the throttle body and MAF sensor, and it’s the tune up where improvements were most noticeable to me.

It’s a pretty easy to swap the plugs on the Prius C. There’s no wiper cowl or motor to get out of the way, just the ignition coils.