Piston Slap: Start With Spark Plugs!

Sajeev Mehta
by Sajeev Mehta
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piston slap start with spark plugs

Done Duratec’d Out? (photo courtesy: Supaman)

TTAC Commentator Supaman writes:

Hi Sajeev,

Haven’t visited TTAC in a while but I’m back! My Mazda 6 has now crossed the 101k mileage marker and all your nuggets of wisdom have helped keep this classic functional and beautiful. However, there’s another problem rearing its head that has plagued even the Mazda forums I’ve visited for a resolution.

It involves the car’s driveability at anything below 3000 rpm. Doesn’t matter whether the engine is just warming up or at operational temperature, the car will hesitate (sometimes violently) in a stuttering/bucking fashion when accelerating from a stop through the gears until I crest that magic 3000 mark on the tach. Out on the highway, passing in top gear is almost impossible because of the engine’s hesitation if revs fall below that number, requiring a downshift to 4th to keep them up. At first I thought my manual skills were to question but then I never started experiencing this issue until around the 87,000 mile mark. It has since gotten worse. Sometimes the bucking is enough to trigger the CEL but then it always goes away after a while.

Browsing the forums I’ve tried everything from cleaning the MAF sensor, fuel injector cleaner, throttle body cleaning as well as replacing the O2 sensor. I’ve narrowed it down to either spark plugs (some forum members replaced theirs but the problem came back hours later), the coils or possibly a vacuum leak. One post I read indicated a potential cylinder misfire. Even Mazda dealers were confounded.

Sajeev answers:

What codes did you get with a scanner?

Supaman replies:

Haven’t scanned the codes because more often than not the CEL light doesn’t trigger.

Sajeev answers:

There’s a (remote?) chance that an intermittent code isn’t triggering the light. So scan now.

Supaman replies:

Hey Sajeev,

Took matters into my own hands and decided to throw some new parts at the Mazda. I bought new spark plugs and ignition coils and decided to dedicate a Saturday to replacing them. What was supposed to have been a three-hour job turned into eight hours of wrenching, ughing, cursing, awkward body positioning, pulling and beer (yes, beer lol).

Anyway, inspecting the forward bank of spark plugs, I found oil coating the middle plug (see pic). The forward left plug was clean (save for carbon deposits I suppose) while the forward right plug had just a hint of oil on it. This (along with the many forums I looked up) tells me my valve cover gasket is bad. I was able to use a clean cloth and mop up as much oil as I could before placing new plugs and coils into the forward bank. Apparently, the leak was so bad it made it up to the coil itself at some point.

The rear bank of plugs were clean, except for signs of normal wear. After buttoning everything up and taking it for a test drive, the hesitation and stuttering are gone and she drives as great as she did 20,000 miles ago. While at it, I cleaned the throttle body, replaced the intake port gaskets, and cleaned the EGR valve and MAF sensor. Of course I know this is a temporary fix and I’ll have to replace the valve cover gasket, hopefully sooner rather than later. Just wanted to give you a heads up and to anyone that works on their car, believe me, it’s a money saver.

Total cost in parts? $95. Total labor? 8 hours on a Saturday and a six-pack of stout. Thanks again!

Sajeev concludes:

I did a similar job to a Duratec Sable with well over 200,000 miles. While the plugs were toast and the EGR was coked up to near complete blockage, the motor still ran reasonably well. A good cleaning and new plugs were all it needed: odds are your coils were fine, just like mine were.

Working on wrong-wheel drive cars with bulky DOHC V6 engines is no fun, but the basics? The basics gotta be done.

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.

Sajeev Mehta
Sajeev Mehta

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2 of 35 comments
  • Speedlaw Speedlaw on Dec 11, 2014

    I've learned to replace plugs at the 60k interval, no matter what the book says. I had bad coils in my MDX, dealer said "if it doesn't toss a check engine light it doesn't exist". I replaced the coils and plugs, and "poof'....perfect idle. Two trips to the dealer with "all is working properly", to boot. Thanks guys. (really, you fix five cars and can't get this right ?) Plugs are a lot like o2 sensors. Over time they degrade, but don't activate a check engine...and usually so slowly you don't notice day to day. Only when you replace them you discover the fifteen horses hiding in the corner of the barn.

  • SupaMan SupaMan on Dec 15, 2014

    The car is nearly 110k and the engine is still purring smoothly. I'm wondering whether or not I should even bother changing the other gaskets like I said I would but I guess prevention is better than cure. Thanks again Sajeev!

  • Ernesto Perez There's a line in the movie Armageddon where Bruce Willis says " is this the best idea NASA came up with?". Don't quote me. I'm asking is this the best idea NY came up with? What's next? Charging pedestrians to walk in certain parts of the city? Every year the price for everything gets more expensive and most of the services we pay for gets worse. Obviously more money is not the solution. What we need are better ideas, strategies and inventions. You want to charge drivers in the city - then put tolls on the free bridges like the Brooklyn, Manhattan and Williamsburg bridges. There's always a better way or product. It's just the idiots on top think they know best.
  • Carsofchaos The bike lanes aren't even close to carrying "more than the car lanes replaced". You clearly don't drive in Midtown Manhattan on a daily like I do.
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  • Tassos A "small car", TIM????????????This is the GLE. Have you even ever SEEN the huge thing at a dealer's??? NOT even the GLC,and Merc has TWO classes even SMALLER than the C (The A and the B, you guessed it? You must be a GENIUS!).THe E is a "MIDSIZED" crossover, NOT A SMALL ONE BY ANY STRETCH OF THE IMAGINATION, oh CLUELESS one.I AM SICK AND TIRED OF THE NONSENSE you post here every god damned day.And I BET you will never even CORRECT your NONSENSE, much less APOLOGIZE for your cluelessness and unprofessionalism.