What causes that noise? I've always wondered

fiveohpatrol · 06-16-2002, 01:40 AM

You know the one. The weird knocking/clunking that you usually hear in an old chevy pickup when its trying to be shut off but wont shut off?

Its also the noise used as a sound effect in movies and cartoons when a car breaks down.

what causes this? an extreme lean condition? the engine rattling back and forth in the engine bay? or what?

I'm not having this problem, just heard it again today and started wondering what mechanically is going on to cause it.

PKRWUD · 06-16-2002, 03:57 AM

It's called dieseling, and is very common in carburetted engines. The main cause is having the idle set too high. What happens is when the engine is hot, and you shut it off, the throttle plate doesn't close far enough, and the rotating assembly still manages to draw enough fuel out of the carb to keep it kinda running. The actual heat in the cylinders is what ignites the mixture, rather than a spark plug, which is why it's called what it is.

Take care,
-Chris

LX5liter · 06-16-2002, 10:06 AM

I believe you usually have to have carbon deposits/buildup on the pistons and heads to accomplish this as well. The carbon buildup is still glowing after the engine shuts off, which ignites the fuel as PKRWUD stated.

fiveohpatrol · 06-16-2002, 01:30 PM

Somehow I knew that you would be the first to reply Chris

Thats a good explanation on why it wont shut off, but where does the horrible noise come from? Just the engine rocking back and forth?

Mr 5 0 · 06-16-2002, 07:57 PM

That sound you hear are valves, crank, rods and pistons jerkily attempting to operate but failing. Valves rattle, pistons clunk...not a pretty sound, as you've noticed...and no good for the rotating mass, either. Sad.

Coupe5oh · 06-16-2002, 08:17 PM

I cant be certain, but i think if the timing is off, it could cause this, but i might be wrong, wont hurt to hit it with a light.

mustangII460 · 06-16-2002, 09:27 PM

Vacum leak can also do this.

06-16-2002, 01:40 AM	#1
fiveohpatrol I'd rather be basketweaving Join Date: Feb 2000 Location: Indianapolis Posts: 2,551	What causes that noise? I've always wondered You know the one. The weird knocking/clunking that you usually hear in an old chevy pickup when its trying to be shut off but wont shut off? Its also the noise used as a sound effect in movies and cartoons when a car breaks down. what causes this? an extreme lean condition? the engine rattling back and forth in the engine bay? or what? I'm not having this problem, just heard it again today and started wondering what mechanically is going on to cause it. __________________ NMRA O/C 9516 NA pumpgas stickshift 347 10.65@125.6, 6.73@100, 1.41 60ft

06-16-2002, 03:57 AM	#2
PKRWUD Junior Member Join Date: Mar 1999 Location: Ventura, California Posts: 8,981	It's called dieseling, and is very common in carburetted engines. The main cause is having the idle set too high. What happens is when the engine is hot, and you shut it off, the throttle plate doesn't close far enough, and the rotating assembly still manages to draw enough fuel out of the carb to keep it kinda running. The actual heat in the cylinders is what ignites the mixture, rather than a spark plug, which is why it's called what it is. Take care, -Chris __________________ Webmaster: Rice Haters Club Jim Porter Racing Peckerwoods Pit Stop Support Your Local RED & WHITE!

06-16-2002, 10:06 AM	#3
LX5liter dude5l Join Date: May 2001 Location: Grande Prairie, Alberta, Canada Posts: 603	I believe you usually have to have carbon deposits/buildup on the pistons and heads to accomplish this as well. The carbon buildup is still glowing after the engine shuts off, which ignites the fuel as PKRWUD stated. __________________ Good luck! Brad R. 1992 L.X. 5.0 coupe MODS: A few little bolt ons New Best time ...11.60 @ 128 mph @ 2000+ ft altitude. Rice Haters Club member #39 Users rides #2542

06-16-2002, 01:30 PM	#4
fiveohpatrol I'd rather be basketweaving Join Date: Feb 2000 Location: Indianapolis Posts: 2,551	Somehow I knew that you would be the first to reply Chris Thats a good explanation on why it wont shut off, but where does the horrible noise come from? Just the engine rocking back and forth? __________________ NMRA O/C 9516 NA pumpgas stickshift 347 10.65@125.6, 6.73@100, 1.41 60ft

06-16-2002, 07:57 PM	#5
Mr 5 0 Conservative Individualist Join Date: May 1997 Location: Wherever I need to be Posts: 7,487	That special sound That sound you hear are valves, crank, rods and pistons jerkily attempting to operate but failing. Valves rattle, pistons clunk...not a pretty sound, as you've noticed...and no good for the rotating mass, either. Sad.

06-16-2002, 08:17 PM	#6
Coupe5oh Registered Member Join Date: Nov 2001 Location: SanAntonio, Tx Posts: 734	I cant be certain, but i think if the timing is off, it could cause this, but i might be wrong, wont hurt to hit it with a light. __________________ Police package 5spd 90 Lx, Stock original motor, 3.27grs, BM fan, fms 10.5 clutch, D&D quadrant, and adjuster, ADS chip, 180 stat, mac cai, mac h-pipe, mac subframe connectors, ASP crank pulley, ripper shifter, 26x10.5 M/T sport pros welds. 246 hp according to analyzer Race weight: 3,120 E.t-13.57 with 26x10.50's e.t. streets. 1.88 60' mph- 99.92 mph

06-16-2002, 09:27 PM	#7
mustangII460 Factoy Five Roadster Join Date: May 2000 Location: Sevier Co,Tennessee Posts: 1,681	Vacum leak can also do this.

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