By on April 3, 2013

TTAC commentator Robstar writes:

Hi Sajeev,

I bought a new 2005 Subaru WRX STi in March of 2005, it currently has around 51k miles on it. Rotors have been replaced once, brake pads twice. The car still has it’s original clutch! It went from being an occasional commuting car in all city traffic to an every-day 60 mile RT jaunt mostly highway.

With all of that said I don’t think I’m rough on the car as it’s rated EPA 16/22 and over the latest 5300 miles (since I started keeping track) I’m averaging 23.5mpg in mixed driving. Before I present my issue, keep something in mind:

I’m hard of hearing and have lost roughly half my hearing since birth (I was born without normal hearing). I’m also currently demoing NEW hearing aides and I’m hearing new sounds I’ve never heard before. I recently heard my infant sons white noise machine for the first time as well as the doorbell.

On to the issue…..

I noticed yesterday after my drive home and parking in my driveway (faces uphill), after turning the car off that it sounds like “popcorn popping” under the hood! I’m not sure if this sound is normal or has always been there and I never noticed. This is with the CAR OFF/STOPPED/PARKED after a 25 mile highway jaunt at fairly consistent speeds in a straight line. I’ve search NASIOC and could only find people complaining about this with the car ON, so I have no idea what my issue could be. The temp gauge in the car didn’t go off and no smoke or excess heat seemed to be coming from the hood. After opening the hood, and calling my wife outside to listen, she pointed to the sound coming from somewhere around the inter-cooler or “red pipe”.Thanks for any help you can give.

Sajeev answers:

Congrats on your invigorated sense of hearing!  I can’t even imagine the joy you must feel. More to the point, about the Subie’s problem: Pop it like it’s Hot.

I have no idea what I just said.

Which makes sense, as I don’t have a good answer for your Subie’s problem.  Perhaps you have some sort of pre-ignition backfire from a fuel pump/injector that won’t shut off when the key is out of the ignition.  Or perhaps you are just hearing your extra hot, turbocharged exhaust popping as it cools down.  I think the latter is likely.

Either way, if your fuel economy is fine and there are no trouble codes generated by the computer, I wouldn’t worry about this one.  Enjoy the new found sound of your sweet ride.  It’s just poppin…like it’s hawwwwt!

 

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.

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36 Comments on “Piston Slap: I just heard…WHAT???...”


  • avatar
    Felix Hoenikker

    I’ve heard similar noises coming from the heat shield in my Taurus when the engine compartment is at normal operating temperature and I shut off the engine. It dosen’t always do it; mostly in warm weather after driving around town.

  • avatar
    danio3834

    My guess would be air gassing out of the cooling system or bubbling in the heater core. This may be more noticeable due to the way you park causing any bubbles to rush to the rad/degas bottle. If you find thats the case, ensure it’s properly bled of all air and filled to spec.

    If it continues, you can test for combustion gases in the coolant or excessive pressure building up in the system while running and revving to help identify a possible head gasket issue. If it only occurs when you park nose up, and not anywhere else, it could be a characteristic.

    • 0 avatar
      Robstar

      Op here>

      Sajeev> Thanks for taking on my question! I’ve since had to return the hearing aides so can’t hear it anymore, but I’ve noticed this when parking the car “nose up” on an inclined driveway. I’m guessing it’s normal? A buddy who listened to it says his STi (2011) does that as well…

  • avatar
    jz78817

    things like exhaust pipes and heat shields will “pop” and creak from thermal contraction as they cool down. they’re fairly thin so they cool down much more rapidly than other things underhood.

    and what’s notable about having the original clutch at 51k miles?

    • 0 avatar
      Robstar

      STis tend to be used/abused…..I know several people that have burned out their clutch before that. My point with that was that I don’t abuse the car. As of this writing I’m about to hit 60k, still same clutch.

      I know clutches should last way longer than 50k, but I’ve seen/read about too many STi’s/wrx’s where owners have killed them way before…just trying to point out I’m not one of those people :)

      • 0 avatar
        MR2turbo4evr

        Robstar, the pinging sound you hear after shutting off the engine is normal, as jz788817 mentioned. My cars do that as well.
        This is unrelated, but I would recommend letting the car idle for a minute or two to let the turbo cool down after a long uphill drive or after the car has been driven hard. It’ll greatly extend the life of your turbo.

        • 0 avatar
          Robstar

          My uphill drive is about 50′ into my garage.

          Is cruising at highway speeds “driving hard”?

          It takes me about 5 minutes at 30-40mph through 2 lights after I get off the highway to actually get home. Is this enough time for a cooldown? I don’t typically get above 2500rpm these last 5 minutes….

          • 0 avatar
            MR2turbo4evr

            No, highway cruising wouldn’t be considered hard driving as you’re driving at a constant speed and your turbo wouldn’t be producing any boost to speak of and therefore shouldn’t get very hot. Based on your last post, you should be fine.

        • 0 avatar
          Power6

          Subaru turbo motors have a coolant tank up on top of the motor, specifically designed so the coolant vaporizing in the hot turbo after shutdown is siphoned off to the tank making a vacuum to pull coolant flow through the turbo bearing coolant jacket until it cools down enough to not vaporize the coolant.

          Subaru states there is no need to idle the car under typical conditions for this reason.

          I just drive easy for the last couple miles home with any turbo car. There is no need to idle a turbo car unless you feel the need to run hard boost and then shut down.

    • 0 avatar
      gessvt

      My Legacy GT does this after a typical drive.

    • 0 avatar
      28-Cars-Later

      Subbies chew through clutches?

    • 0 avatar

      I remember my older cars used to do this, but I never realized that this was a specific sound common to many cars. I can only remember this happening a few times–usually just after a drive in hot weather, coming home, parking, engine off, usually only heard in a very quiet enclosed garage, and listening very closely you can hear something. Sounds like very high pitch pinging, or popping, of a thin metal part. I always thought it was the body panels just expanding or contracting as they heat up/cool off, like the wood creaking when “settling” at night in an old house.

      I can’t remember hearing it recently. Probably I’m losing the ability to hear high pitches with age, see for example: http://www.huffingtonpost.com/2008/04/23/high-pitch-only-teens-can_n_98304.html . I suppose it is possible that with a new hearing aid you are now hearing high pitch frequencies, amplified, which you otherwise would not be aware of.

  • avatar
    grzydj

    I thought this post was going to be about the dreaded strut “thunk” that happens on many WRX/STIs.

    • 0 avatar
      Power6

      The thunk is the inverted struts unique to the STI, the whole strut body has to slide in and out of the housing and the grease in there goes bad over time. Well documented but a bit of a pain to take apart the inverted strut and lube them.

  • avatar

    Is 50,000 miles for a clutch supposed to be good? I’ve never replaced a clutch, even with well over 100,000 miles.

    • 0 avatar
      APaGttH

      I’ve never gone through full set of rotors in 51K miles – let alone two sets of brake pads. That includes cars I’ve run on track and/or heavy Solo use.

    • 0 avatar
      dal20402

      Depends on the car. I bought a 1989 Taurus SHO with 68,000 miles in 1998. I later learned it was already on its fourth clutch when I bought it.

      Funny thing: that same clutch was inspected at 90,000, the mechanic told me it was at under 10% remaining, and it was just starting to slip occasionally when I traded the car in at 160,000.

  • avatar
    28-Cars-Later

    I had a 5.0 Town Car which made odd sounds like this after being driven vigorously, I would say it sounded more like pinging a small hammer on metal though vs popcorn popping (lasted a few minutes and stopped). I noticed this seems to be a Town Car (or Panther) trait as I hear the 2000+ TCs in my building’s lot do this after they park and are turned off.

    • 0 avatar
      JMII

      Pretty much every vehicle I’ve own makes a similar sound when cooling down as described by 28-cars: pinging with a tiny hammer. The sound comes from the engine, exhaust (muffler) and even the brakes. To me it sounds almost like the clicking of a light switch being turned on and off rapidly. In a parking garage it might echo and thus seem louder. Occasionally I’ve heard a hiss like a gas (water vapor?) escaping like danio3834 describes. All seemed like normal “cool down” sounds to me related to metal expansion. I had a VW Passat 1.8T survive 100K miles and always let it sit for a few minutes to allow the turbo to cool a bit before shutting the engine off.

      • 0 avatar
        corntrollio

        If the pinging is metallic, that almost sounds more like water. That’s the sound I remember hearing on radiators connected to steam pipes used for indoor heating.

  • avatar
    vagvoba

    Most of my cars, current or past, did the same especially in the winter when the engine cools down quickly in the cold. You usually wouldn’t hear it on a noisy street but in a garage it’s noticeable.

  • avatar
    Mrb00st

    I’d bet it’s just the block pinging as it cools off. Most cars do this, if and you’re just now hearing it, I’d say it’s that it’s been happening forever and you haven’t been able to hear it before!

    CONGRATS ON THE CAR. Living the dream. Enjoy it!!

  • avatar
    onyxtape

    My 4-cyl Outback does that. The last time I asked about it, someone said it was the oil dripping back down or something like that.

    I don’t recall any of my other cars (past or present) doing that, so that’s why I went online and asked.

  • avatar
    APaGttH

    SNARK MODE ON

    Two sets of brake pads and one set of rotors in 51K miles. Shock that the clutch is still the original.

    Yup, that sure is legendary Subaru quality – nothing like setting the bar low. You got better rotor life out of a ye’ old GM A-Body, and those rotors were apparently made of cardboard.

    SNARK MODE OFF

    • 0 avatar
      Robstar

      Agreed.

      Didn’t like the brake pads or rotors. OEM replacements, without labor were $2500. $2500!

      From an online discount shop I saw $1900-$2100 depending where I looked…

      Let’s just say I didn’t buy OEM parts….and my current brake pads & rotors are not only quieter but less dusty as well.

      • 0 avatar
        corntrollio

        What?

        http://www.subaruwrxparts.com/shop/pc/viewCategories.asp?idCategory=57

        I see a little under $700 for OEM rotors and under $200 for pads. Were you talking about upgraded ones?

        • 0 avatar
          Robstar

          Adding OEM Front & Rear rotors + Pads for an 05 STi comes out to $1700. $81 for the cheapest shipping. Still ~ $1780

          Add $100 for tax and you are at about $1900…

          WAYYY too much for a $33k car new.

          • 0 avatar
            corntrollio

            “Rotors have been replaced once, brake pads twice.”

            Maybe I naively assumed that meant fronts only. In most cars I’ve had the from-the-factory rear rotors/pads lasted longer than 51K.

            I wonder why that is. I can think of other cars with factory-Brembos that have cheaper rotors than that.

      • 0 avatar
        APaGttH

        Holy over priced Batman! I could put Brembo’s front and rear on my G8 rummaging through the GM parts bin and upgrading the rotors, even going DBA for less Cheddar than that.

    • 0 avatar
      28-Cars-Later

      Holy crap I didn’t know I could toggle snark mode!

    • 0 avatar
      Power6

      You do have to be objective here…these are Brembo brakes chosen for winning magazine comparos against the Evo, look at the slick stock tires they put on the car those don’t last long either.

      Whatever benchmark you are comparing too for stock pad and rotor life won’t run all day long around any track in the US like the STI will. It puzzles me some don’t understand there are different tools for different jobs…

      • 0 avatar
        corntrollio

        As I said earlier, I can think of other cars with factory-Brembos that have far cheaper rotors than that, so it has nothing to do with being Brembos.

        However, the 60-0 for the STI wasn’t that impressive in this comparo:
        http://carsort.com/compare/Subaru-Impreza-vs-Ford-Mustang

        “Whatever benchmark you are comparing too for stock pad and rotor life won’t run all day long around any track in the US like the STI will.”

        You use stock pads for the track?


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