Does fuel additives in every fill up harm the engine?

Every fill up?
That may be "over kill".

It is possible the additives placed a varnish on the plugs.
Only way to know is to inspect a spark plug.

You could first do a few full throttle acceleration tests as an "Italian Tune Up" to see if the miss fire disappears.

FWIW I usually use Techron, a full bottle to an empty tank, then re-fill the fuel tank and drive aggressively until the fuel tank is emptied, then refill and change the engine oil and filter.

Good luck let us know what you find.

 

No.

Usually one of the good things about all of the automotive snake-oils and herbal treatments don't really 'harm' anything but your wallet.
As far as your spark plugs go, YOU checking them won't hurt either but they probably do not have to be changed outside of the normal maintenance periodicity.
Paying somebody to "check the plugs" is the same thing as paying somebody to change the plugs - minus the cost of the plugs.

Me?
I'd stay out of the potions and lotions aisle for a little while.

Kilmer cracks me up.
He's got some fairly goofy ideas about hybrids (an oscilloscope?! REALLY?! gimme a break!)
BUT(!!) He's right about gas treatment.

FWIW It's not a bad idea to add a bottle of Techron to your tank semi-occasionally, and changing the oil afterwards but I personally stopped doing so a few decades ago and I haven't noticed a diff.

YMMV

 

Absolutely, positively no GOOD reason to put any "snake oil" additive into your gas or oil or coolant all the time. NONE.

A good injector cleaner like Techron maybe twice a year. That's it. Anything else is just a waste of money and as you are now suspecting, can sometimes do more harm than good.

But I doubt that is the cause of your present symptoms.
If the plugs are still the originals, now probably would be a good time to change them.
How many miles on the car ??

In some very specific situations......like maybe the system was just on the verge of shutting down the engine when you step on the gas, there will be a slight hesitation and that can't be "fixed". All 4 of my Prii have done that once in a great while.
Wide painted strips on the pavement can cause it to feel like a "stumble" when the traction control kicks in too.

 

Can't remember if plugs are due at 100 or 120k. They're easy in the c, relatively speaking.

I would not expect a small amount of additive per fill-up to hurt anything.

Given the age of your car, I would immediately suspect any performance-related problem to be hybrid battery related. Have you taken any steps to investigate that angle?

 

Just stop using it every fill up!!! It's a waste of time and money and there's no telling how much more polluted your exhaust is with additives. If you want to use this stuff one to 4 times a year that seems reasonable, but on every fill up, that's just the doings of a cray cray...

 

Yeah, I don't know that app either. Battery fan clean? c doesn't need much for maintenance, but that one sometimes gets missed.

 

Try to use Chevron as much as you can, it has Techron in it. And it works, it's proven. If you have a high mileage car put a bottle of additive in a full tank before an oil change.

 

That depends upon how far you live from a refinery.

In most places, the gas that is delivered to Chevron stations (or any other brand for that matter) does NOT come from a Chevron refinery.
If it comes out of a pipeline, you don't know who makes it. Whether or not it actually has all of the Chevron additives depends on the truck driver or the station operator. Station owners have been known to bypass the sediment filters on the pumps so I don't trust them much.

I prefer to put in my own injector cleaner. That way I KNOW it is in there.

 

+1

 

Cheap lesson!

Good Luck!

 

Nice work. I inadvertently bought a fake Champion plug from Amazon a couple of years back and that was my intro to this category of counterfeiting.

Since then I only buy plugs locally, no mail order. No trouble with the local inventory, just a little less convenient.