I recently purchased a 1993 Jeep Wrangler (having recently sold my 1972 Jeep CJ5) but in bringing it home, I knew that engine maintenance was probably an issue as it was unknown. The engine ran great, but the oil was on the dirtier side, so I got her home and changed all the fluids. I decided to use the recommended 10w-30, five quarts, and I added one quart of Lucas Oil Additive. At startup it purrs, then as she starts to warm I’m getting what I believe to be piston slap.
It doesn’t sound like a stuck lifter, and seems to be low in the motor. I’m thinking of changing oil again to just oil with no Lucas, but I’m in a bit of a quandary. Again the engine runs great, passes smog, but this sound is unnerving. It does have 143K on the stock 4.0.
I agree with your diagnosis, as piston slap is a common issue (TSB 09-08-94) on Jeep 4.0-liter engines from the 1990s. But it’s far from a death sentence, likely more of an annoyance that will occasionally grate at your soul. After 15-ish years of dealing with 4.6L DOHC timing chain startup rattle in my Lincoln Mark VIII, I barely hear it now. More to the point, perhaps you shouldn’t worry about piston slap on your “new” acquisition.
While the sound is far from ideal, I will remain confident your Jeep is good for well over 200,000 miles if you continue proper maintenance. And if the motor is healthy otherwise (i.e. doesn’t burn oil), perhaps you might not need that oil additive? Don’t dump perfectly good oil right now, but consider the following before the next scheduled service:
Today’s engine oils are loaded with additives which could be adequate for your needs. No doubt, a worn out 4.0L Jeep could benefit from Lucas’ thickening qualities, as this isn’t some modern engine with variable-adjusting everything and whatnot. But I’d be remiss if I didn’t suggest forgoing the additive on your next oil change. If the noise comes back (or is louder?), just get another bottle of Lucas, as you won’t hurt anything by its brief omission.
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