By on November 4, 2009

Mount that (courtesy
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.

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25 Comments on “Piston Slap: I’m Pickin’ Up Good Vibrations Edition...”

  • avatar

    Sometimes these are steady bearing or u-joint issues as well.

    Or wheels out of balance. These types of problems can drive you nuts.

    My old 86 did this at about 50 mph and quickly went away. Perhaps not same problem as Chris here, but never did track down the problem.

  • avatar

    You still should change your fluid. 45K on the original fill is pushing the serviceability limits of the fluid/filter.

  • avatar

    Have the torque converter checked also, I had a similar problem and it was the torque converter locking and unlocking (rapidly).

  • avatar

    If any of the 6 motor mount bolts were drilled too deep, they would go into the water jacket, one explanation for leakage. I agree with he torque converter lock up. Disconect the T C wire and see if the problem is gone.

  • avatar
    John Horner

    If the motor mounts check out, I would check the drive axle u-joints and center bearing (if so equipped) carefully.

  • avatar

    I second John Horner.

    The carrier bearings and U-joints may not be bad. If the carrier is improperly shimmed or the axle pinion angle is off the misalignment could cause this.

  • avatar
    Paul Niedermeyer

    I second DPerkins. A prematurely locking converter will make it feel as you describe: lugging the engine.

  • avatar

    Sounds like a torque converter. Same thing on wife’s Taurus and friend’s Impala. They at least made it over 100,000 miles though. 50,000 isn’t very many to have a TC failing already. Hope it’s the mounts.

  • avatar

    Possibly a torque converter. Also possibly normal operation–many modern automatics are programmed to keep the RPMs down to the point of borderline lugging the engine.

  • avatar

    It could be anything that has been described above. After you rule out all the easy to diag things mentioned above, go to a forum specific to the Silverado. Usually these are the places that will lead you on the right path. As a mechanic, these are problems that can drive you crazy.

  • avatar

    I third John Horner. I would very carefully examine the driveshaft carrier bearing (supports the long driveshaft) and the rubber surrounding it. If you DIY do not forget to mark the driveshaft so that it is in phase when reassembled. Otherwise you will have a new vibration.

  • avatar

    I second the u joint. On my full size Blazer, I developed a similar symptom and feared the worst. I found a guy that drove it, said what it was, and fixed it for super cheap. I don’t remember the cost, but I’d guess well under $100. I do remember being surprised at how noisy, menacing, and jolting the feel and sound were and then shocked at how cheap it was to fix.

    In retrospect, I would probably have tried to fix it myself but I didn’t know what was wrong or how to figure it out. If you get up under the truck and grab hold of the driveshaft (the long tube from transmission to differential) and then yank it around, you might be able to see it wiggle or have too much play — signs of a u joint problem.

  • avatar

    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.

  • avatar

    And prizes, don’t forget prizes.

  • avatar

    WD-40 fixes EVERYTHING

  • avatar

    I am not sure about the 2 w d, but my 01 4 w d has a one piece drive shaft.

  • avatar

    I agree with all above – and want to share my weirdest car vibration story:

    A while back I was traveling cross country in a MB 450se, when the underdash parking brake release handle started vibrating so violently I though it would crack the dash! In panic, I thought maybe the brake cable had somehow gotten wrapped around the drive shaft, so I immediately pulled over and started crawling around under the car – and found nothing. Got back onto the highway, and around 50mph the crazy dash shaking panic inducing vibration started again!

    Several stops and starts later I happen to notice that the chrome trim on top of my windshield had come loose. Giving it little thought, I pushed it back under its clip… and when I got back on the highway the vibration was gone! It didn’t actually cross my mind the two events were related until the vibration came back a few hours later, and when I got out, I noticed the trim had pulled loose again. Pushed it back on, vibration gone. I kept this up until I could hit a parts store and buy a little adhesive to help hold it under the clip. After that the vibration never came back again…

    It was a real lesson in the power of sympathetic vibration – a 10″ piece of chrome trim on top of the roof vibrating in the wind made my parking brake handle try to tear itself out of the dash.

    And that’s why mechanics hate oddball vibration issues.

  • avatar

    Check the u-joints FIRST. Back in olden times when (nearly) all cars were rear wheel drive, vibration while coasting was a common symptom of worn/failing u-joints. Not only is it an obvious, simple and cheap repair, it’s also a safety issue. If it’s the front u-joint, near the tranny, and the shaft drops while the car is moving, well, let’s just say cars and trucks are not designed to pole-vault.

  • avatar

    If it’s the front u-joint, near the tranny, and the shaft drops while the car is moving, well, let’s just say cars and trucks are not designed to pole-vault.…

    Just submitted this to Mythbusters…I, too have heard about this before…

  • avatar

    I call BS on the pole vault. Maybe if the truck were lifted and you were going really fast, but I think the angle is too shallow. Real pole vaulters have a flexible pole. In the old days, they used rigid poles which are more like a driveshaft but vaults weren’t very high. With the mass of vehicle ramming a driveshaft into the ground, I would guess it would gouge the street before the truck tumbled ass over tea kettle.

  • avatar

    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.

  • avatar
    Jeff Puthuff

    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.

  • avatar

    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.

  • avatar

    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!

    • 0 avatar
      Jeff Puthuff

      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.

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