Thread: Pressure in motor - knocks when I shut it off

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As this happened to anyone? My motor makes this wierd knock
after I shut off the motor, it does it for 15 - 30 sec after the motor
has been shut off. I have a small block chevy, with breathers on
both valve covers. One goes back to the base of the carb.
It doesn't do it all the time, but 50% of the time, when I shut off
the motor, and a few times while ideling at a light, you can hear
this knock? Any ideas? Obviously, nothing is turning or moving
when I shut it off, so I just assume its pressure in the motor that
eventually goes away. - - -If it is pressure in the motor, how can
I relieve it, so it doesn't knock anymore?
Thanks

A knock, or are you just hearing the lifters bleed off????

Sounds like a pressure knock. I didn't know that lifters make a knock sound?
Only other sound, that I've heard, that is similar, is when its cold out, and you
shut off your car, and the exhaust pipes seem to make a creeking noise. Its like that, but its coming from my motor.

Wow! I believe when the engine shuts off the engine temp will briefly increase. Would that have anything to do with it instead of pressure? Something expanding from the rise in temp?
Just throwing something out, food for thought and maybe trigger an idea from somebody else kinda thinking.

The lifters won't "knock", but hydraulic lifters can make a tapping sound when they bleed off.....

Last time I shut it off, I stuck my ear on the side of the motor, since the engine
panel was off, and you can hear a knock, knock, knock sound. Like I said above, - - obviously nothing is moving, so I figured engine pressure, or pressure in engine from radiator? But the sound is coming from the engine, and not the
radiator. Could it be my aluminum heads cooling off quicker than the cast iron block?

I have had a motor so hot that it would make thunking sounds as the water boiled in the block. Is that sort of what it sounds like?

Not to be a smart-ass here, but are you sure the engine isn't still turning over with the key off? High comp engines will "diesel" for a while with no ignition sometimes, and make one hell of a knock.

Originally Posted by R Pope

Not to be a smart-ass here, but are you sure the engine isn't still turning over with the key off? High comp engines will "diesel" for a while with no ignition sometimes, and make one hell of a knock.

I'm the same way, R Pope. Mike, when you had your ear down by the block did you glance at the belts/pulleys and see that there was no rotation? Sounds like you're describing diesleling to me.

Yes, Its shut off. Even my buddy said it sounds like pressure, built up in my motor. The knocks get less and less, over 30 - 45 seconds, and eventually stop. It starts immediately, after shutting off the engine. There's been a few times, when it doesn't knock.

I wonder if you could have a cracked exhaust manifold on the bottom side.
Weird how you say it knocks. I just can't imagine the sound of the knock.

you may have air in the block when turned off coolant is getting into hot areas????

Get a stethoscope/prybar/long extension etc and listen to the engine and see if you can pinpoint an area.
Probably them gremlins raisen the devil in there and you are just going to have to get an exorcism!

Since you are located in a warm climate area, do you run anti freeze in it??? What temp thermostat??? and what pressure radiator cap???

So, what's the bottom line? Did you ever figure it out? Don't leave us all hangin'!