Piston Slap: I Just Heard…WHAT???

Sajeev Mehta
by Sajeev Mehta
piston slap i just heardwhat

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.

Join the conversation
10 of 36 comments
  • Onyxtape Onyxtape on Apr 03, 2013

    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.

  • APaGttH APaGttH on Apr 03, 2013

    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

    • See 7 previous
    • Corntrollio Corntrollio on Apr 04, 2013

      @Power6 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?

  • Statikboy Those tires are the Wrong Size.
  • Mustangfast I had an 06 V6 and loved that car. 230k trouble free miles until I sold it. I remember they were criticized for being too small vs competitors but as a single guy it was the right size for me. I recall the 2.3 didn’t have a reputation for reliability, unlike the V6 and I4. I think it likely didn’t take off due to the manual-only spec, price tag, and power vs the V6 engine and the way it delivered that power. It was always fun to see the difference between these and normal ones, since these were made in Japan whereas all others were flat rock
  • VoGhost Earth is healing.
  • ToolGuy "Having our 4th baby and decided a camper van is a better use of our resources than my tuner."Seller is in the midst of some interesting life choices.Bonus: Here are the individuals responsible for doing the work on this vehicle.
  • MaintenanceCosts Previous owner playing engineer by randomly substituting a bunch of components, then finding out. No thanks.