By on December 10, 2014



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 [email protected]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.

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35 Comments on “Piston Slap: Start with Spark Plugs!...”

  • avatar

    I have been wondering how bad spark plug replacement on a Lexus RX350 is. I guess I will find out in a few months time, when the time comes for a service.

    • 0 avatar

      @Beemernator –

      Lexus RX? – oh, that’s some fun. A few ways to get there – special tools, long skinny arms with eyes in your fingertips, or my favorite, the “unbolt and remove anything even remotely close to being in the way”.

      (This explains why my vehicles are often on jackstands in my garage for a couple of weeks at a time for relatively simple jobs.)

      Seriously, though, check out the 2 popular Lexus forums for a good diy writeup or two I just read (because we just hit 100k, too).

  • avatar

    If you do plugs and coils, you might as well do the wires. They break down with age as well.

    • 0 avatar

      I think the OP’s car is coil on plug. Still, good advice for those to whom it applies.

    • 0 avatar

      For that price I’m thinking that he did not replace the coils but instead replaced the “spark plug wires” which consist of a boot and a coil spring made of stainless. The oil in the plug well probably caused the boots to break down allowing the spark to “leak”.

  • avatar

    I think it was your coils, this engine is notorious for going through them and what you describe sounds like a classic case. You say you replaced the forward bank and your total cost was $95. How did you find coils so cheap and if they were that cheap expect to see the problem return, this engine doesn’t much care for after-market coils

  • avatar

    My 94 Regal and 01 Grand Marquis behaved the same way after I went well past the manufacturers spark plug replacement interval. When it was the Buick I was scared the transmission was going cuz it would buck and drop out of gear but it was all because the engine couldn’t make enough low rpm power cuz of the plugs being bad. When the Grand marquis started behaving the same way I knew it was time. Changed the plugs and it runs fine again.

    Yea so also do what I did not and follow the maintenance schedule lol.

  • avatar

    “the basics”, True dat!
    Plug wires, too, fall into that catagory. Their failure also manifests itself in a manner similar to fouled plugs. The easy diagnosis trick I use was to pop the hood of a running car in a closed garage at night (just don’t stay in there with it running very long). The sparking/arcing from the trashed wires is easy to see in the dark, if you have good sight lines to them. Fixed a GF’s accord this way.

  • avatar

    Surprised by your find and fix. In the old coot school of fixins I would have looked for a vacuum leak.

  • avatar

    My folks’ 2002 Lincoln LS is notorious for bad coils, which caused an engine shutdown at, usually, the worst possible times. Did a lot of web searching and found a mechanic who had thoroughly diagnosed this and, thankfully, did a writeup. Turns out a defective coil would cause a voltage spike on the primary wiring circuit causing the ECU to completely brain fart. Replaced all plugs and coils with quality (Napa) aftermarket parts and the problem went away (couple of the plugs were a b*tch to get to).

    On another note, started to replace the plugs on my 2008 Mustang GT with 4.6 V8 and discovered all coils and plugs are straight up and easily accessible. Ended up not replacing the plugs because at 80K miles they looked brand new and, at $10 per can’t see doing it when not necessary,

  • avatar

    How does a leaky cam cover gasket (not a valve cover – this is not an OHV engine…) cause the business end of the plug to become fouled – ? The plug is threaded into the head, and no exterior oil leak should be able to reach it.

    If the oil had made its way up into the coil, and the plug was not firing, one would expect to find it had been fouled by gas, not oil.

    Any time there has been a misfire condition of any duration, change the oil right away as soon as the repair is completed – the oil may be full of gas unless the injector for the cylinder was shut down by the ECU. And you don’t know whether or not this happened.

    Hard to believe someone could endure severe symptoms like this for 14,000 miles without scanning it for codes.

    • 0 avatar

      Sounds strange but this happened with my Mazda powered Probe. Mazda engines are known for leaking oil, especially from the cam cover. I found one plug completely submerged in oil – my socket came out drenched in oil. All the oil drained into the cylinder. The other two plugs in that bank were also wet on the outside. The misfire was caused by oil. I did not see oil on the inside like the OP but I’m willing to bet his code was for misfire.

    • 0 avatar

      Agreed. Was fascinated by the oil on the one spark plug. If its dripping like that after it had been removed, your issue to me is more that poor basterd trying it’s damndest to Tinkerbell through the light sweet crude. May be why you had that hesitation below 3000 rpm as the Duratec 2.5L is a high revver. If memory serves me from my ’98 SVT Contour, my valves kicked in at 3300 rpms. Yours being similiar, would thereby finally flood the oil soaked plug with gasoline to ignite the spark until such time its RPMs came back below the magic valve opening mark.

      Was there any blue or white smoke at the back on startup?

      • 0 avatar

        My theory is that the oil dripped onto the plugs as they were being removed from the head.

        • 0 avatar

          This is what happens. The oil also conducts the spark and makes the rubber in the spark plug boot soften up causing the spark to bypass the tip of the plug. The OP will still have to re-seal his valve covers or the problem will return.

      • 0 avatar

        No sir, no smoke on start up. My guess was that oil was interfering with the plug spark and causing the misfire. I thought it might’ve been a vacuum leak but after going through the forums and noting most Mazda6 V6 drivers were experiencing the same problem around the same mileage, I figured let me change the plugs first.


    • 0 avatar

      Wondering this too. Could a slightly leaking valve stem seal pull a trace of oil into the combustion chamber on the intake stroke and foul the plug?

    • 0 avatar

      There’s apparently an o-ring that comes with the cam cover gasket kit to keep oil from going down that hole, through the spark plug threads, etc.

      No other explanation for having those six o-rings in the kit!

      • 0 avatar

        Through the spark plug threads? Um, no.

        The spark plugs bottom out on the angled plug shoulder just above the threads. There is no way for oil to get past that shoulder – that is what makes it a sealed combustion chamber, capable of holding engine compression, when the valves are closed.

        If oil could get past there, wouldn’t engine compression be able to get past going the other way? And wouldn’t it be apparent that there was blowby going past the plug? Oil would leak in while the car sat, and then would be burned up by exhaust gases when you started it up – there would be oil smoke and half-burned oil everywhere.

        Even without an oil leak in the area to get into the plug well, a loose spark plug is something that announces itself pretty plainly – it’s not going to make it past several service department visits unnoticed.

        C’mon – this is Spark-Ignition Internal Combustion Engine 101, buddy.

        The o-rings in the kit are there to prevent oil from fouling out the coil, not to keep it from getting past the plug threads.

        gtemnykh is right – the oil was picked up by the plug on its way out as it was being removed.

        The magic 3k rpm threshold was probably a combination of a weak coil due to oil fouling, marginal plug due to resultant gas fouling, and enough of a change in spark advance at that engine speed to enable the plug to produce a spark.

        • 0 avatar

          I’m w/ this guy! that’s the best answer i’ve heard yet… nicely diagnosed!! will have to commit that to memory banks… intermittent coil issues are so tricky

        • 0 avatar

          I just changed the plugs in a Mercury Milan with the 4 cylinder engine. The plugs looked just like this when I removed them. On the 4 cyl the plugs sit down in a well in the valve cover. The valve cover gasket surrounds the spark plug hole. Apparently the valve cover has been leaking a while as evidenced by a burning oil smell and stains on the exhaust manifold shield.

          When the plugs were unscrewed, the oil that was pooled around them ran down in the hole and got the electrode tip wet with oil. It also let a considerable amount of oil down on top of the piston. At start-up, the exhaust pipe looked like a mosquito fogger for a while.

          My next chore is to change that valve cover gasket.

        • 0 avatar

          Nailed it. Had similar driveability issues with a PT Cruiser (don’t hate me – I traded it on an MX-5) which has the plugs located vertically on the I-4 head. When the valve cover gasket failed a spark plug well filled with oil, fouling the spark plug connection to the wire. Would run like crap at low RPMs but smooth out at high revs. My guess is that at low RPMs you need all four cylinders firing to get smooth operation but at higher revs the missing cylinder is less of an issue.

        • 0 avatar

          I did say I was fascinated as to how so much oil managed to find its way onto the threads. If that much got on it on removal from the head, wouldn’t there be the oil pooling and burning that you had mentioned? I suppose I’m not following where this oil originated from if it wasn’t an obvious find by the service crews.

    • 0 avatar

      Camshaft-and-valve covers are still called valve covers by most people, and most manufacturers and retailers of automotive parts. I’ve never even heard of the term “cam cover”.

      The problem was simply that the spark plug tube was full of oil due to leaky spark plug tube seals.

  • avatar
    SCE to AUX

    Since the rear plugs on a V6 are so hard to reach, I always change the coils at the same time. Not worth doing the job twice.

    • 0 avatar

      +1. I did this job a few months back on my Mazda6, and had no interest in pulling the intake manifold a second time (which is necessary to access the rear bank), so I put coils in.

    • 0 avatar

      +1 as well.

      I should’ve change the head gaskets as well but after 8 hours and 4 beers I wasn’t having it. Besides, I didn’t have the part anyway so I’m saving that repair for another weekend.

  • avatar
    heavy handle

    Check for TSBs. I remember that this was a thing on V6 Mazdas almost 10 years ago. Doing a proper fix involved more than just the valve cover gaskets. I think that there are separate spark plug well oil seals, and warranty time was way less than actual time, so nobody wanted to work on them.
    You are very lucky if you are only experiencing this on the front bank.

  • avatar

    Surprised you weren’t getting any codes for the symptoms you were describing.

    If you were getting such bad misfires that the car was nearly stalling out, the OBD should have lit up.

    • 0 avatar

      The misfires never appeared during regular idling. At 70mph in 5th gear (5 speed manual) I was doing about 3000rpm. If I stomped on the go pedal, the engine would start sputtering and misfiring until the revs climbed pass say 3100-3200….or I downshifted to 4th to bring the revs up.

  • avatar

    Had a similar problem with my 2004 mazda 6 at 250,000 kms. Huge problems at idle. Turns out it was the pcv hose, it seems to be a high mileage common problem on the four bangers.

  • avatar

    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.

  • avatar

    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!

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