Piston Slap or Rattling Rods?

I have a 1997 Jeep Grand Cherokee (ZJ) with 148,000 miles on it that I just love.  I repaired it after it was totalled, and it drives great (with the exception of the need of a new tie-rod end).

Anyway, since I’ve had it, it has always made this strange “rattling” sound when it’s started cold.  I’ve always attributed it to piston slap, which is when the bottom of the piston contacts the cylinder walls.  Piston slap is often common in performance-built engines, because the pistons they use are made of a material that has to expand a little before it seals well.

It sounds somewhat like a diesel engine.  Once it warms up, it’s quiet and it runs great.

Car makers don’t usually use materials that lead to piston slap in their engine designs, because it generates complaints from customers.

So, this has got me wondering:  is this rattling sound caused by piston slap or do I have rod bearings that are a little out-of-spec?  Are the road bearings heating up and closing the gap?  I always try to let the engine idle a little before I put load on it.

However, if it’s rod-rattle, I’d like to repair it by adding some oversize bearings before I have significant damage.  I also plan to try using a thicker oil, perhpas 20w-50.  That might provide some more protection, but it will hurt fuel economy, as well…and the Jeep gets about 16MPG average, so I don’t want to decrease it any more!

Anyway, this morning, I shot a video of the Jeep on a cold start.  Please add your comments and let me know if you think it’s rod rattle or piston slap.

About Chad Douglas

My wife and I are planning to go off-grid and move to the Texas High Desert. We intend to "lose" the rat race.
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3 Responses to Piston Slap or Rattling Rods?

  1. Anonymous says:

    I don't know anything about the sound of piston slap, but that sure doesn't sound like a rod bearing problem.
    Jim McConnell (aka bluegumball)

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  2. The Rat says:

    OK. I guess I'll try running a thicker oil. If the sound doesn't change, I can feel good about piston slap. If the sound goes away or lessens, then I'll think rod bearings still.

    Like

  3. Anonymous says:

    Its hard to hear. One way I have always figured out piston slap on Jeep 4.0s is you won't hear it for a few seconds when you first start it. Then it will clack clack clack it won't be very loud and It will sound like a diesel. Pulling plug wires one at a time sometimes will make the noise lesson or go away. Sometimes there is a slight miss at idle and when you accelerate a little it will sound kind of ratty or clacky like clack clack clack. Also the noise will seem to be coming more from underneath or the middle of the motor. 4.0s are notorious for piston slap or cracked skirts. A lot of the engines I have tore down simply were too out of spec from the factory. The reason I know this is because in lower mileage 4.0s the cylinder will still have the cross hatch hone but will be 0.005 out of spec= bad engine assy or machining/casting. … I'm willing to bet you have piston slap or a damaged/cracked piston skirt. Also the engines having bad tolerances from the factory attributes to cracked skirts. I would take the block to have it fixed/machined with new wear parts.

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