Piston Slap: I'm Pickin' Up Good Vibrations Edition

Sajeev Mehta
by Sajeev Mehta

Chris writes:

I have an ’04 Chevy Silverado 2WD pickup with the 4.3 6cyl and auto tranny. Between 1100 @1300 rpms I get a strong vibration that you can feel in the floor of the truck. It happens mostly while coasting or decelerating. It feels like the vibration you get driving a manual transmission in a high gear at too low RPM, therefore “lugging” the engine.

The trans fluid looks fine and is at the proper level. The truck has 45000 miles and a fluid change is probably in order as I bought the truck used with 36000. Any ideas? The truck runs and shifts well otherwise.

Sajeev replies:

I had the same problem, though mine felt like a misfire at 1100rpm. After pulling the plugs, checking the (new-ish) wires, replacing the coil packs (2) and driving myself nuts, the condition worsened to your current status. A set of engine mounts fixed it. D’oh! Once again, I kicked myself for throwing parts at a problem.

So a good way to check for a bad mount is by torque-ing the motor when parked. Open the hood, look at the engine while running and have someone gun the throttle. HARD. If the motor jumps up pretty violently, odds are your mounts are bad.

Just for kicks, I checked the Technical Services Bulletins (TSBs) for this vintage Silverado and found this: “Coolant leak at engine mount bolt, NHTSA Item Number – 10010663. “ Who knew GM engine mounts were filled with coolant?

Just kidding. Also remember that TSBs are frequently red herrings. Who knows if this problem has any relevance to 4.3L motivated trucks, or if the (three?) customers who complained about this problem aren’t related. Just kidding, again. I think.

[Send your queries to mehta@ttac.com]

Sajeev Mehta
Sajeev Mehta

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  • Sajeev Mehta Sajeev Mehta on Nov 05, 2009
    stickman : Sajeev, I think you should start some sort of follow-up article after the person addresses the problem to let the collective know the outcome. Maybe kind of like the Curbside Classics where the right guesser gets a shout out. That's a good idea. Whether or not I hear replies from the folks originally making the inquiry is another, but maybe I can have a Piston Slap update to accomplish just this.
  • Jeff Puthuff Jeff Puthuff on Nov 05, 2009

    Driveshaft pole-vault already covered by Mythbusters. Episode 27. Myth: Car going at speed could have its driveshaft fall off and cause the car to pole-vault. Busted: Implausible that the scenario would actually occur, and unlikely the car strong enough to actually vault on the driveshaft. Read about it here.

  • Gimmeamanual Gimmeamanual on Nov 05, 2009
    Just submitted this to Mythbusters…I, too have heard about this before… I think they did that episode with that Crown Vic they keep beating on. Edit: Jeff beat me. That's what I get for having 12 tabs open and not refreshing before commenting.
  • Lorenzo Lorenzo on Nov 05, 2009

    Jeez, guys, can't you recognize a joke? BTW - you can call BS, or Mythbusters, all you want, but a dropped driveshaft is still dangerous, and that was my point. I don't need to go to Mythbusters or anyone else to find out what happens when the front u-joint fails and the driveshaft hits the pavement - it actually happened to me, in a '65 Chevy sedan. I was barely underway when the driveshaft dropped and hit a raised manhole frame. I wasn't doing more than 10 mph, but it was enough to raise the right rear tire off the ground and push the rear of the car sideways about three feet. Fortunately, there were no parked cars or pedestrians in the way. Pole vaulted? No. Dangerous? you bet your ass!

    • Jeff Puthuff Jeff Puthuff on Nov 05, 2009

      I don't disagree that it's dangerous. But there's a difference between a pole-vault movement and a three foot hop. Regardless, any Mythbusters episode dealing with cars is entertaining.