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Spark plugs blackened

Toyota corolla with 152,000 miles, had new spark plugs at 141,000 miles. Dealer says spark plugs are black and should be replaced. To replace spark plugs after only 11,000 miles seems wrong. There must be something causing the blackening which should be addressed. Any suggestions?

How do they know they are blackened? The only way to tell is to remove them.

Blackened plugs usually means the engine is running overly rich.

Did the dealer suggest anything else besides just changing the plugs? Because if they didn’t you’re going to get the same problem again.

Is the vehicle running poorly? How’s acceleration? Any black smoke from tailpipe when you accelerate?

I’m very very skeptical of this. Sounds like a dealer has a boat payment due.

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Try using a hotter spark plug.


  1. Were the proper plugs installed?
  2. Is the Check Engine light on?
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The dealer did not suggest anything else. I haven’t noticed any performance issue but mostly do short drives in city or 30 miles on highway at 55-60 mph. I’ll take it out for a drive and pay more attention to acceleration, and look for smoke from the tail pipe.

Thanks your suggestions are helpful.


This might be something I could recommend to the dealer/mechanic. I’m not going to do the change myself.


The dealer changed the plugs so I assume they installed the proper ones.
The Check Engine light has not come on since the dealer associate mentioned the problem. The car was in for a routine oil change/ tire rotation.

It would be extremely odd for a mechanic to remove a plug just to take a look, given no other problems. If it’s running fine, mpgs are fine, oil use is fine, I’d ignore their comment.



I haven’t been watching the gas mileage closely because I’m only taking short errand runs and going 3 to 4 weeks between fill-ups. For oil changes I’ve been going on time not mileage because I’m not reaching 3,500 miles in a year. I’m going to start checking my mpg at each fill-up and checking the oil level at the same time.
Thanks, your suggestions are helpful.

A mechanic who cares would try to find out why they are blackened.

I have to agree with @texases, there is no reason that a mechanic would pull a plug just to look at it unless you had some complaint about the vehicles performance. This comes under troubleshooting, not routine maintenance.

But while we are on the subject, when was the last time you had the air filter changed?


 The car was in for the biannual State Inspection which it passed.  The engine air filter was changed at 132,000 miles.  The car was at 152,000 miles,  20,000 miles difference. The dealer associate only mentioned the blackened spark plug issue. The car is not idling roughly, acceleration is ok, no black smoke from the exhaust pipe, no check light on.

Ray, would you please login and reply . Trying to read scroll posts is a little annoying.


I have logged in for all my replies even though I am receiving emails like yours directly on my computer without being on the car talk site. This is my first time using car talk for a question. Perhaps I’m doing something wrong. On my reply to keith, my answer appears with in the reply box on the left hand side of the page but on the right side of the screen the message ran off on one line. I tried to edit it but nothing changed after I attempted the edit to keep my answer more compact. All my previous answers appeared the same in length and number of lines of type on both sides of the page.

Your spark plug probably appear dirty because of the short trips. As long as there is no drivability problem, no misfires or check engine light, there should be no reason to be concerned.

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Thanks, I feel better knowing the cause of the blackening.

Another reason would be “upsell”.
I had an incident where dealer pulled plugs to “make inspection” and to sell me services I did not order.

Since it is the only Mazda dealer close to me, I consider placing a big “no work unless explicitly ordered is permitted!” sign on the engine if I ever need to get back to them

The point I was trying to make was, if I brought my car in for an oil change and the service rep came to me to tell me my spark plugs were blackened, they would probably call the cops on me again. That whole incident was pure BS. Unless you asked them to check your plugs or asked for some specific troubleshooting for a problem, they had no cause to pull a spark plug.

I agree with @thegreendrag0n

When I was at the dealerships, the service manager told the mechanics he expects upsells on every single car, no matter how new and/or pristine they are

One time, I had just completed some driveability complaints on a 380SL . . . due to the use of aftermarket parts. German aftermarket parts, actually

The car was probably 20 or 25 years old at the time. It had never been to our dealership, but it had service stickers from a well respected Benz specialist just a few miles down the road.

Once I had verified the repairs, I gave the car the cursory once-over and determined that it didn’t need a single thing. The guys at the independent shop had obviously been doing an excellent job maintaining the car. It looked great inside and out, and was mechanically in excellent shape.

So I didn’t have any recommendations for upsell, at which point the service manager got upset and said I wasn’t doing my job.

I put the car on the hoist, removed all the tires and called him over. I politely asked him to show me what the car needed. The suspension, steering, tires, brakes, driveshaft, flex discs, exhaust, etc. were all in great shape, some of them had obviously recently been replaced. There was not one single leak on the entire car, not even any sweating . . . !

He said “You were right” and walked away, shaking his head.

The service manager’s antics probably cost me an hour or more of lost time, all told


Great work ethics, @db4690!

Unfortunately, my limited experience trying to find a go-to mechanic resulted in two categories of failures: either they would fail to address the issue from the first time (and usually leave some parts like clips missing/broken) OR they would start making some bogus upsell items car obviously did not need :frowning:

Slowly, but surely it set me on DIY route.

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