where to start finding a knock

B

bk53

Senior Member
Hi,
My fresh 276 has developed a nasty knocking sound. This motor has 1000 miles on it. It sits in front of a fordomatic. I guess if I'm lucky it could be a cracked flex plate. I'll try and check out the flex plate tonight.
After that does anyone have a tried and true sequence to follow to find something like this? I can lift the intake and heads but after that... I wouldn't know a rod knock if it knocked me in the head. Am I better off just dragging it over to the local diagnostician so he can hear it?
I appreciate all the good advice. The motor ran really well since mid august.
Can't say I'm not disappointed. so it goes.
It seems to me I could give it gas and it rattled when i let off. There seemed to be a slight knock just idling. when it happens it sounds like a really loud rattle can. Oh well, wish me good luck. I don't mind doing the mechanical stuff, but i'd rather be driving.

Bill
 
sometimes you can isolate a rod or piston knock by removing one plug wire at a time with it idling when you get to the bad cylinder it will get slightly quieter.also you can pull and push on the crank to check end play should be very little movement if any.maybe a compression test
 
I've had a few Fords with engine knocks. Back when you could actually buy a car for $100-$500 and drive it for a year or three then send it to the bone yard;).

You got any buddies with lots of experience with old junk?

Deep down thud-thud-thud could be a main.

A little more sharp rap-rap-rap, maybe a rod. Maybe piston slap.

Sharp clicks, valve-train.

Check your exhaust manifolds/headers and gaskets . Usually does not sound like a knock, not to my ears.

You may be on the right track with a loose flex-plate?

I would find someone that can use a mechanics stethoscope (or a big screwdriver). That and the process described above killing a cylinder at a time should pinpoint where the noise coming from.
 
My car to this day sounds as if it has a knock. It made me sick when I first heard it, just after custom exhaust was installed versus open headers.
Well it wasn't the engine, it is my throughout bearing and goes away when I depress the clutch. I hope this is your case too. Unfortunately I do have to pull my engine but it's not the worse case it could've been. My fingers are crossed for you.
 
My opinion is you need to isolate the knock and start finding what it isn't . First , has your oile pressure changed at all ??? If yes then you can just stop looking and start pulling it apart . If not ,Then I would take the belt/belts off the engine and see if it is still there . If it is you know it is not anything the belts run .From there I would unbolt the torque converter from the flexplate and push the converter in to the trans so the flexplate can't hit it. Start it and check for you knock . If it's still there then you know it is most likely an internal engine issue . Does it have and electric fuel pump or mechanical ?? The fuel pump push rod can knock when things go wrong in that area . Start from there and report back :D
Knocks are not usually something simple but ocasionally we all get lucky .
 
A long broom handle or dowel will work. Put it to your ear and move it around on the engine, heads, pan, etc... Sound will transmit thru it. It will help isolate the area that the noise may be coming from.
 
Sorry, I just reread your post to see your attached to a fordomatic, Well maybe it is still something easily resolved.
 
When I first restored my car without doing anything to the engine that had been sitting for 31 years I had a pretty bad knock in it. Still drove it a couple of years and 10,000 miles before having it rebuilt. Possible causes found were broken main bearings and fore to aft slop in the crankshaft. 30,000 miles after that rebuild I suddenly got a bad knock at the back bottom of the engine. Within an hour a piston came apart and required another rebuild. Here's that story: http://fordbarn.com/forum/showthread.php?t=104413
 
Years ago I had a 56 Merc. with automatic trans. that had a rumbling knock at idle and slightly above and it turned out to be the flex plate (loose or cracked,
I don't remember). The garage put it on the lift and welded it. It was quite a common problem and that is how they fixed it. No knocking after that.
 
Get out your extra long screwdriver or broomstick,and listen in different places on the engine to try and isolate the area. The other thing you can listen for is the speed of the knock, is it at crankshaft rpm or camshaft rpm. This will also help to isolate it.
 
I'm taking this all in fellas. lots of good input there 32 v8. I can't thank you guys enough. If I can't isolate it by friday I'll have it flatbedded over to some professional help on friday. you folks are the best.

Bill
 
Wow, do I feel dumb. a combination of dumb things.I had a blown header gasket
and so I change it out with an imported gasket that was too thin. That and the bolts In there were a tad long. I didn't notice that it was loose and the bolts were sinched down good, bottomed out apparently. apparently the new gasket blew during my test drive.

The first blown exhaust gasket didn't sound anything like this second one. I replaced all my header gaskets and used some slightly shorter bolts, No more hammer and anvil sounds. She sounds great now and while i've learned a little I still don't know what a rod knock sounds like, thank goodness.

Thank you folks for the suggestions. sorry for being such a dumb newb.
 
good catch....and thanks for coming back with the solution, etc.....have been posted in the back about those bolts going thru into the water jacket, so glad yours didn't do that.....now have fun....lol.....Mike
 
I'm glad that you found the problem without having to pull the engine. I have had my engines out so often that I can do it myself in 3 hours. Going back in is a tad longer.
Regarding the flexplate on your flat-o-matic; I have had a few flexplates crack. I found that the torque converter must have a washer placed on the stud before the stud goes through the flexplate, thus holding the torque converter back from the flexplate. If the torque converter presses on the flexplate, it can cause the flexplate to develop a crack. If that happens, it will make a squeak or some other strange sound that will drive you nuts trying to find. I have found that if that happens, remove the starter plate and look up at the flexplate and if you see a red rust line forming near the crankshaft, you have a crack either stating or about to fail. I have never had one fail completely, but it could be catastrophic. I have been lucky. Also be sure to use Loctite on the nuts holding the torque converter to the flexplate.
 
I sure wouldn't feel dumb about the leaking header gasket. I would feel very lucky that it wasn't a rod knocking on the block trying to get out!!!!
 
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