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Engine rattle finally solved

Agreen

NAXJA Forum User
Location
Charleston
I figured I would share this here so hopefully it may help someone some day.

Let's get some facts straight about knocking/rattling noises!

- A rod knocking will get louder and faster as the engine speed increases. It does not go away, and is destructive. It is rhythmic at engine speed.

- Valve tap is faster than rod noise. Twice as fast, actually. It is sometimes described as clatter. It may get faster as the engine speeds up, but usually goes away with revving. It is not necessarily destructive, but should be fixed as soon as possible. 4.0L engines are just notoriously bad about valve tap.

- Detonation. This one sounds like pennies in a coffee can. It's usually heard during heavy acceleration. It's due to a lean condition, and is extremely destructive, usually leaves holes in pistons!

There may be a few more, but those are the biggest offenders of engine noise. However, I had one on my 98 XJ that drove me crazy for several months until I finally figured it out.

Understand that I WILL NOT replace parts unless I've diagnosed and confirmed that it's bad.

This noise happened only in 1st-3rd gears (automatic) and only during acceleration. It would go away at about 3500 rpm, then return when the transmission shifted. It sounded more like a rattle, like detonation during acceleration, but the long and short term fuel trims would always indicate normal. I knew it wasn't detonation though, as I've tuned several cars and could always feel the lean surges along with hearing the detonation. Here recently, it started making a strange squeaking noise as well!

This also started right after changing the trans filter. While I was down there, I noted how bad the transmission mount looked and decided to replace it. The engine mounts looked bad, but I didn't have time to dive into that one yet. Now, I had only just bought the Jeep, and one of the reasons I was changing the filter is that 1) it shifted way too late and 2) the fluid was way too high anyway. Like 3 quarts too high. Upon the post-work test drive I noticed that the shift points were right on target, but this strange sound suddenly appeared. For months I kept trying to think of what inside the transmission could be making that noise! My most likely candidate was the torque converter. I thought that maybe the high fluid level was done by the previous owner to mask the sound it was making? Anyway, I was reluctant to rip the torque converter out and change it based on a hunch.

I kept thinking about it daily, but it never quite added up. Why wouldn't the torque converter be screaming (clicking, rattling, chattering) at idle too? I kept thinking that the noise would 100% go away during lockup, and it must be a bent or broken fin. So yesterday I crawled under it and noticed that the driver's side motor mount was utterly destroyed. I mean it was obliterated. That's when it hit me. Could it be the metal-on-metal clatter that I hear? I could physically see that they needed replacing anyway, and for a grand total of like $30, the Jeep now drives smoother and is way more quiet.

Also, I'm convinced that the noise was being picked up as knock. The power feels almost double now, which makes sense if the ecu was pulling all my timing for a false knock!

I hope this helps someone in the future.

TL/DR:
Motor mounts can cause a weird rattle on acceleration, making it sound like your engine or transmission is about to explode.

-Terrence
 
Last edited:
Interesting, thanks for sharing.
 
Transmission%20Mount%20Bracket_zpszyqtjew5.jpg
 
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