I've had my 2002 4wd Ranger for several months, and under load it has what sounds like detonation at any rpm above about 2300. I've run 91 octane in it (the highest available around here), but it doesn't seem to make a difference. I've read that this engine is known for a timing chain "rattle", and I'm wondering if that could be what I'm hearing. The engine has about 150,000 miles on it, and it runs great aside from this.

Thanks,
Jay

Yes if the higher octane gas didn't mask the sound it would be the timing rattle

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I also have 150K on mine and I hear what your hearing when going up a steep grade hill. Sure sounds like detonation or knocking to me. I know if the timing is off slightly the 4.0 does not care for it. Im not sure what your hearing is timing chain rattle....

I'm not sure either. I've heard knock before in other vehicles, and that's what this sounds like. But you guys have a lot more experience with the 4.0 (I've had 3 4-cyl Rangers) than I do. I've also never known detonation not to at least improve with higher octane fuel, and this noise doesn't change at all going from 87 to 91.

I'm reading up on some of the posts here about using Seafoam to clean out the combustion chambers, and I may try that for good measure. I might also look into checking the timing based on your comment about the 4.0 not being tolerant of it being a little off.

You'd be able to tell a difference between a knock or a chain slap. A knock would be rhythmatic. A chain clap is completely random.

150k on a 02 4.0.....you're lucky. I'm 90% positive those chain guides are about gone, causing the chain to slap. I've been meaning to edit my vids of my stock engine and the timing rattles. Mine progressively got very bad...it was doing it at idle, at no load, any rpm, all the time. Typically, the rattle will start underload at above 2000rpms. Then no load above 2000rpms. Then progressively get worse and worse over time. Mine started at about 35k in my '01. It had 110k when I replaced the engine(cheaper to replace beat ass teenager owned engine than to just do chains at least in my case).

Timing is computer controlled so idk how you'll change it without a tuner. And if the chain guides are loose(or completely gone), timing may be comprimised a little bit.

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'01 XLT disguised as a '03 Fx4 Level II.

I wouldn't consider this random, but it is only above 2300 rpm. It sounds like a muffled version of one of those old toy plastic machine guns.