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#2608935 - 04/24/12 11:08 PM when an engine starts to knock
Dakota1820 Offline
formerly chevyboy14


Registered: 04/28/11
Posts: 3894
Loc: missouri usa
So I've seen it happen where a decent vehicle would be driving along and all of the sudden (usually) while getting on an on ramp or a similar situation would start rattling. Beginning of a thrown rod. But what causes an engine that seems sound and makes no noises all of the sudden to rattle. Whay happens inside to cause this. I had it happen to me once. Shifted from 2nd at high rpm to 3rd and my cavalier started to rattle. What happens to make an engine out of no where start rattling? Just curious. Thanks!
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#2608937 - 04/24/12 11:13 PM Re: when an engine starts to knock [Re: Dakota1820]
Shannow Online   content


Registered: 12/12/02
Posts: 28809
Loc: a prisoner island
Are you referring to (combustion oriented) knock ?

We live near an intersection, and hear many engines "knock" as they pull away from the give way sign...it's a long way from a thrown rod (leg out of bed in local parlance)

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#2608943 - 04/24/12 11:24 PM Re: when an engine starts to knock [Re: Dakota1820]
LeakySeals Offline


Registered: 02/18/11
Posts: 5070
Loc: MA
Oil thats too thick, poor oil flow and sludge blockages will make it rattle at high RPM's.
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#2608965 - 04/25/12 12:01 AM Re: when an engine starts to knock [Re: Dakota1820]
Dakota1820 Offline
formerly chevyboy14


Registered: 04/28/11
Posts: 3894
Loc: missouri usa
I'm talking about engines that are in good shape and then all the sudden one day when you kick it in passing gear or something and you start to hear the beginning of a thrown rod
(Rod knock) example my cavalier ran great and always had 5w30 its whole life and then one day it just started knocking until 2 weeks later when it threw the rod
I'm not talking about neglected engines or combustion knock I am asking what happens to start rod knock?
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#2608976 - 04/25/12 12:36 AM Re: when an engine starts to knock [Re: Dakota1820]
JimPghPA Online   content


Registered: 08/22/09
Posts: 2913
Loc: Pittsburgh,PA U.S.A.
It use to be that besides octane, it could be valves requiring adjusting. Sometimes it was a worn timing chain, causing the cam to be out of time with the crank.

Now days with hydraulic lifters usually the valves do not require adjusting. The timing chain could still worn to the point of having too much slop.

Some old engines with a worn timing chain were best tuned by tweaking the distributor timing by a few degree followed by a demanding road test using a lot of gas peddle, repeated until the engine ran well, if the funds and or time was not available for replacing the timing chain. Sometimes the tweaking of the distributor was enough to get an engine to run well, and sometimes it was not enough.

Now days with all of the electronics on engines and the emissions requirements, tweaking a ignition timing is not something that is common.
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#2608977 - 04/25/12 12:38 AM Re: when an engine starts to knock [Re: Dakota1820]
JimPghPA Online   content


Registered: 08/22/09
Posts: 2913
Loc: Pittsburgh,PA U.S.A.
PS cam lobes could be worn down.
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#2608986 - 04/25/12 01:00 AM Re: when an engine starts to knock [Re: Dakota1820]
genynnc Offline


Registered: 10/13/08
Posts: 379
Loc: OR
Originally Posted By: chevyboy14

I'm not talking about neglected engines or combustion knock I am asking what happens to start rod knock?


A spun rod and/or main bearing.
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#2608987 - 04/25/12 01:10 AM Re: when an engine starts to knock [Re: Dakota1820]
tommygunn Offline


Registered: 01/27/11
Posts: 3061
Loc: usa
Sounds more like bad fuel causing pinging.

If its actually rod knock, the connecting rod cap on the crankshaft is damaged or the bearing is spun/melted and the bottom of the connrod is literally knocking around as the piston reciprocates.

At any rate actual, serious rod knock is a huge concern and you have three options then.

Rebuild the engine to fix the issues, run a thick nasty oil to mask the problem until it completely kills itself, or do nothing and seize a piston.

So, pray its not actual rod knock.

Edit: Rattling, hard to say, but rod knock comes from abuse and will sound terrifically bad at all times, especially at idle. It sounds like a diesel afterwards.


Edited by tommygunn (04/25/12 01:11 AM)
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#2609004 - 04/25/12 02:14 AM Re: when an engine starts to knock [Re: genynnc]
Texan4Life Offline


Registered: 01/28/10
Posts: 2233
Loc: Texas
Originally Posted By: genynnc
Originally Posted By: chevyboy14

I'm not talking about neglected engines or combustion knock I am asking what happens to start rod knock?


A spun rod and/or main bearing.


+1


or if it is worn out it will slowly develope knock. like one day you will be like hey did notice that noise before. and then it gets worse exponentially until you got the full on brick bouncing around in the crankcase noise.
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#2609037 - 04/25/12 05:01 AM Re: when an engine starts to knock [Re: Dakota1820]
NHGUY Offline


Registered: 10/09/11
Posts: 3886
Loc: USA
Worn bearing will not retain its position in rod end,it will spin,blocking the oil hole,bearing and journal will get chewed up,increasing clearance and sending junk through the engine.That clearance will make a knock as the piston is being forced up/down,eventually engine failure will result.

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#2609065 - 04/25/12 06:14 AM Re: when an engine starts to knock [Re: Dakota1820]
bullwinkle Offline


Registered: 10/09/04
Posts: 4612
Loc: Cincinnati, OH, USA
I think the rod bearing starts to spin and blocks (at least partially) an oil passage through the bearing & crank. That's when it's time to start looking for another engine or fix it before it throws it. I've only had 1 engine toss a rod on me-an old 2 liter Pinto motor that I ran out of oil-it doesn't seem to happen often on well-maintained engines.
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#2609103 - 04/25/12 07:14 AM Re: when an engine starts to knock [Re: bullwinkle]
Scott_Tucker Offline


Registered: 12/31/09
Posts: 878
Loc: Santa Cruz, Ca.
If it is indeed rod knock then it is excessive rod bearing clearance due to wear. Rod bearings don't all of the sudden wear, it would have to have started some time before.
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#2609127 - 04/25/12 07:51 AM Re: when an engine starts to knock [Re: Dakota1820]
KrisZ Online   happy


Registered: 12/23/06
Posts: 4640
Loc: Toronto, Canada
Hmm, 2nd gear, high RPM and high engine load. Sounds like oil starvation to me or the engine wasn't as healthy as initially thought.
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#2609144 - 04/25/12 08:18 AM Re: when an engine starts to knock [Re: Dakota1820]
Dakota1820 Offline
formerly chevyboy14


Registered: 04/28/11
Posts: 3894
Loc: missouri usa
Well I can tell you for an ohv 2.2 it was prettt quiet. Lol maybe that's why it broke smile its possible to throw a rod from over revving to. But I dont understand how
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2004 Honda civic vp -m1 5w30
1998 Saturn sl2 -mobil super 5000 5w30


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#2609298 - 04/25/12 11:05 AM Re: when an engine starts to knock [Re: Dakota1820]
rslifkin Offline


Registered: 06/27/10
Posts: 2310
Loc: Stamford, CT / Rochester, NY
If you over-rev and engine, if you don't float the valves into the pistons and kill it that way, you can over-stress bearing caps, etc, and cause them to fail, leading to a thrown rod.
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1998 Jeep Grand Cherokee 5.9 Limited with Heads/Cam/Headers/Exhaust/Tune/Shift Kit/Suspension Work (185k miles, bought with 100k)
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