when an engine starts to knock

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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!
 
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)
 
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?
 
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.
 
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.
 
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.
 
Last edited:
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.
 
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.
 
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.
 
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.
 
Well I can tell you for an ohv 2.2 it was prettt quiet. Lol maybe that's why it broke
smile.gif
its possible to throw a rod from over revving to. But I dont understand how
 
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.
 
On a rebuilt engine I had a bearing spin, rod sheared clean off, and broke the side of the block. I had just buttoned it all up about a month prior. As soon as I heard it knock, I got off the gas, but it was too late. I heard it knock for about 10 seconds, speed was around 125MPH, and the ensuing oil light, spray on the exhaust, and fire was roughly about 15 seconds later. I put most of the fire out by popping the hood before coming to a complete stop. I carry fire extinguishers in all my vehicles to this day, but I do alot less idiotic things today as well.
 
"...speed was around 125 mph" + "On a rebuilt engine... ..just buttoned it all up..." + "...popping the hood before coming to a complete stop" (is there another kind of stop?) = a mechanical problem, for sure!

Was it a "loose nut behind the wheel?"

We'll never know...

Cheers!

p.s. Did you rebuild it again? New block? How bad did the hood twist?
 
Originally Posted By: Norm Olt
"...speed was around 125 mph" + "On a rebuilt engine... ..just buttoned it all up..." + "...popping the hood before coming to a complete stop" (is there another kind of stop?) = a mechanical problem, for sure!

Was it a "loose nut behind the wheel?"

We'll never know...

Cheers!

p.s. Did you rebuild it again? New block? How bad did the hood twist?


Yeah, one of my more shining moments for sure.

And, yeah a complete stop is stand on the brake and hope I don't become a Darwin award stop.

I did rebuild it again, but with a used block I picked up from a friend. That engine worked great and even sold the truck many years later to an older gentleman.
 
bigmike: I'm glad you were not offended by my comment, but I couldn't resist...

Cheers!

p.s. Gotta' just LOVE those Darwin Awards! They get better (worse?) every year!
 
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