Sparkplugs changed from Toyota to denso and reduced performance

It could be the spark plugs,

But the computer also may have to relearn the best air fuel ratio mixture to create the most efficient burn for engine combustion.

The new spark plugs will likely have a different Ignition profile from the older worn plugs.

Drive for a few hundred hundred miles first and see if your power comes back.

If the problem continues then there could potentially be something wrong with the spark plugs themselves or how the technician installed them.

Posted via the PriusChat mobile app.

 

That's the right part number- but where did you get them? Amazon and eBay are full of counterfeits that don't work right.

 

I'm not sayin' ... I'm just sayin'.

I cross referenced the number you posted and it comes up as a Denso 3484 plug. When I looked up the choices for a 2014 Prius C I came up #'s: 3485, 4701 or a 5303.

The Denso 3484 fits the following vehicles:
2 different Lexus; 5 different Avalon; 3 Camry and 2 RAV4.

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The 3 Denso plugs I listed (3485, 4701, 5303) are exact fits for all models of Prius C.

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Now yeah ... I too could be wrong, but I don't think so. I'm just sayin' that when I do work on other people's cars with the parts they bring ... I cross reference them to make sure they are correct. I always let them know before if I think they've made a wrong choice.

 

Yep, they're getting good at faking them.

Denso published this PDF to help buyers identify fakes.

Go get some real ones from an authorized Denso reseller near you. Other than maybe rockauto.com, mail order just isn't a safe source for spark plugs.

Worst case go to Toyota- they will be genuine Denso plugs in Toyota boxes, but they will cost a bit more.

 

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I hope things work out well.

 

buy 4 new plugs at a dealer and do over. don't do anymore maintenance without coming here first

 

Yep, there's a video by a Toyota Master Technician of actually "Toyota OEM" stuff you bought on Amazon that are just cheap Chinese copies...and they are junk! You'd think Amazon would try to prevent this illegal stuff but, hey, money is important to them!
I heard (from a mechanic) that O'Reilly's is good selling only the real ones and, of course, the Toyota Dealer.
Tip for EVERYBODY, don't go cheap with electronics parts in a Toyota...always go with OEM. Yes, will cost more but unless you like redoing a job, go OEM. (Some Toyota's sell on-line and are OEM but at discounted prices, like McGeorge.)

Everyone needs to subscribe to "The Car Care Nut" if they want to learn about Toyota's, especially our hybrids...he's amazing!

 

What does owners manual say, denso? Toyota doesn’t MAKE plugs, just rebadges plugs AFAIK.

 

As hard as the plugs are to replace I’d go with Densos from a reputable supplier.

Fewer unknowns and you are good to go for another 100K

 

these days, i wouldn't trust any supply line for plugs but the dealer. and cross your fingers

 

I'm not sure where those numbers come from at all. The number the OP gave, FK16HR-A8, is what I'm used to a real Denso number looking like, and it is but it isn't what Toyota lists here, and in the 2014 c owners' manual, as proper for a Prius c. The Toyota-specified Denso number is FK16R-A8, without the H.

(Edit: another there is a non-Denso site, densoproducts.com, that does show the four-digit numbers Zeppo mentioned as well as the FK16R-A8 code, and shows fitment information agreeing with what Zeppo posted.)

(The "densoproducts.com" site is shown by RDAP to be registered to Monarch Products Inc., a wholesale distributor in California that also operates a retail site "sparkplugs.com" selling multiple brands.)

F: a needle-to-needle plug, 0.55 mm iridium center electrode, 0.7 mm platinum ground needle
K: 14 mm thread, 16 mm hex
16: the heat range (relatively low in Denso's 9 to 34 range, means it sheds heat slowly, therefore is a 'hot' plug. Some other manufacturers run their heat-range numbers in the other direction.)
H: 19 mm reach (this code can be overloaded and mean 26.5 mm in some models, I don't know the secret)
There should not be an H in this position. That would be the wrong reach for a Prius c. The plug without the H shows a 19 mm reach, so in the version with the H, that H must have the 26.5 mm meaning, as shown here, 7.5 mm too long.
R: plug includes a resistor
-A: according to my decoder ring, A might be a shape code, but one that's for a "special specification".
8: 0.8 mm factory gap (maybe the c uses that small a gap? My liftback uses '11' plugs, 1.1 mm gap).

You can substitute plugs from Denso's other, 'performance', lines, like the Iridium TT that Leadfoot linked in #12. Note that the TTs ("Twin-Tip") are a needle-to-needle design just like the Toyota spec'd F plug, but with a narrower, 0.4 mm center electrode needle. That makes them easier to spark at a lower voltage, which could reduce missing in some high-demand driving conditions, and the tradeoff is that the narrower needle doesn't last as long. The Toyota 0.55 mm plug is spec'd for long service life; the Denso non-OEM replacement lines like TT are marketed as performance alternatives that you will replace more often. If your Techstream misfire monitor counts stay at zero with the 0.55 mm OEM plugs, then the narrower performance ones might not buy you much besides bragging rights.

(Another edit: on densoproducts.com, the longevity for the IK16TT is shown as 100,000 miles like the FK16R-A8, even though the electrode is thinner. Seems a little implausible, like maybe they are understating the life of the OEM-spec'd plug, which doesn't come up in the Prius c Warranty and Maintenance guide until 120,000 miles.)

(Also, the IK16TT has a wider gap, 1.0 mm compared to the Toyota-spec'd 0.8 mm. Maybe that works out ok because of the also thinner electrode reducing the needed spark voltage. But the two effects might roughly cancel so the IK16TT could have less of a 'performance' benefit than expected.)

(It looks like all TT plugs have resistors and their numbering scheme just leaves off the R.)

It might be well to check into whether the 7.5-mm-too-long plugs could have interfered with piston travel.

 

If there's a Toyota dealership near you, you can probably walk right up to the parts counter and walk out with four new spark plugs right in your hand. They will turn out to be Denso FK16R-A8 and they will not be made out of toothpaste and painted pot metal.

If there is any question about the authenticity of the ones installed now, it might be a good idea to replace them rather soon. Apparently one way that counterfeit plugs can fail is with bits of the electrode or insulator melting or breaking off and dropping into the cylinder, doing engine damage.

 

ChapmanF ... Are you suggesting that you've never cross referenced any parts or that you don't know how to cross reference?

The absolute simplest thing would be to match an old plug side-by-side with a new plug. Just looking at the pictures I presented, you should see that the plug first stated ... the Denso 3484 is not the same size as the three listed as exact fit plugs.

The Denso FK16HR-A8 is the same as the Denso 3484. That is THE WRONG PLUG.

Now going a little farther ... if you look up the Denso FK16R-A8 (with NO "H") and cross reference that ... it is the Denso 3485 ... THE CORRECT PLUG.

The other 2 plugs I listed, the Denso 4701 and the Denso 5303 are both listed as replacements for the Prius C.