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Old 11-10-2010, 09:28 PM
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Question Engine rattle or knock sound on slow acceleration

2002 Excursion 7.3L makes funny loud clacking or knocking sound when accelerating slowly with a "light foot". Stepping on it a little more and sounds smooth right out. It is kind of like the deceleration engine noise when coming to a stop and auto trans is gently down-shifting and engine goes back to idle.

Is this normal engine noise? My 1997 7.3L does not do this on gentle acceleration.

Thanks!

Last edited by Lightkeeper; 11-10-2010 at 09:38 PM.
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Old 11-10-2010, 09:50 PM
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Any knock or rattle can be cause for concern, particularly under light load. In that condition, the piston has little pressure on it and the rod can "rattle" up and down on the crank. Under moderate load, the piston keeps the rod pressed against the crank and the rattle goes away.

I hate to put it this way, but I'd be very wary of any long trips until you get this identified. PMR engines (of which yours is most likely) have a tendency to suffer catastrophic failures if a rod fails.
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Old 11-11-2010, 12:10 AM
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Thanks for your comments. Unfortunately, that makes sense. I am a "rookie" though and must admit I am not sure what your mean by PMR engine. This is an engine with 68K miles on it. What is the best way to identify this "rattle" ?????
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Old 11-11-2010, 07:08 AM
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PMR = Powdered Metal Rods. This is in contrast to Forged Rods which were used in earlier trucks.

As to identification, that's tough. Based on the symptom and condition in which it occurs, I'm guessing worn bearing. This can usually be identified by small metal flakes in the oil. Another method is to take a big socket wrench on the end of the crank and rock it back and forth. You can often "feel", or in extreme cases even hear, the rod end clunking against the crank.

Again, this is an extreme guess. I have no idea what the sound is like and it could be something as simple as a collapsed lifter, cracked flexplate (flywheel), loose converter, or anything else. I'm simply going off the driving condition and description of the sound.
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Old 11-11-2010, 07:22 AM
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Does it sound anything like spark knock or 'pinging' in a gasoline engine?

Does it sound like it is coming from right at your feet?

I'm thinking that it might be the infamous #8 injector 'cackle'... although Bill has me scared now, because my truck makes a very faint sound (almost like gasoline spark knock on light acceleration...

I always just assumed it was the injector 'cackle' but I've never driven another 7.3 besides my own.
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Old 11-11-2010, 10:33 AM
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Chris, I had the same fear when I got my truck but I've just attributed it to #8 cackle. Truck has run fine since I got it, towing, racing, 4-wheeling, you name it I haven't had a problem. I think if it were a rod knock I would have a large hole in the side of my block by now. Just my $0.02
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Old 11-11-2010, 10:46 AM
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Thanks Byron, yeah mine doesnt sound like a rod knock, and its pretty quiet...

I'm going to get the HPx one of these days and see if it quiets it down.
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Old 11-11-2010, 10:47 AM
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my 02 does the same thing. Kind of like a ticking or tapping pretty good until it warms up all the way. Pretty sure its an injector. I might try that Rev X additive next oil change and see if that helps.
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Old 11-11-2010, 04:12 PM
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Wink

Thanks for all the comments. Apparently it is in fact a case of the #8 injector. Had a friend drive the Excursion and he said his 2000 did the same thing and he was told by a Ford dealer's 7.3L "expert" it was the #8 cylinder's injector. Then I drove another buddy's 2002 F250 with practically same mileage as the Ex and it was even louder.

Lost some sleep last night thinking a "catastrophic failure" was possible but feel a lot better now and am planning on a good night's sleep!
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Old 11-11-2010, 06:25 PM
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I'm really sorry about the scare, but as I said it's really difficult to diagnose based on a sound we can't hear. It was the light load part that concerned me. Glad it's nothing major.

We now return you to your regularly scheduled program.
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