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.