I noticed this a few weeks ago while messing with the car. At idle in park, there seems to be a slight knock/banging from down under. Really not loud and not synchronized either. I got under the car and can hear it better, the flex plate/flywheel (whatever you want to call it) seems to be moving in sync with the sound.

Also, when cruising and letting off the gas, I can hear the sound.

Because it happens in park, it can only be the flex plate, torque converter, or engine. Right? When the car cools off, I'm going to try and move the flex plate with my hand, I think its loose.

Any other ideas? Don't think its internal, engine running great and maintaining great oil pressure. Converter seems to be doing its job also.

You guys are going to be dealing with me for a long time. As soon as I figure this one out, we're going to figure something out about these darn valve covers. Nobody around here can get them to seal for more than a month! I think a switch from perimeter bolt to center bolt may be in order.

Thanks for the help! I could probably get a video with the sound I'm hearing if that would help.

 

my 86 has "fuel line knock". It is a condition that apparently has something to do with fuel injector pulsing at low engine speed.
it does it with the car cold.
There is a GM service bulletin about putting a fuel filter in the fuel return line. It adds enough resistance to the fuel regulator to dampen out these fuel line pulses

 

Hmm...That's interesting. My fuel lines do have some play to them, I'll have to look into those. Mine happens when cold and hot though.

It actually sounds kind of like this:

http://youtube.com/watch?v=y_0xDygXowg

Love this vid! Listen to the sound as he decelerates. It really sounds similar to mine, but I haven't came down from 197mph before!

 

Pull the flywheel inspection cover, the fuel pump fuse, and the spark plugs. Get it in the air, have someone crank it, and watch the flywheel.

 

Will do, no flywheel inspection cover to pull though, just like my '87 T/A. Wonder where they went?

Where are the normal places for it to crack? I'd assume where it bolts to the converter and crank?

 

Then you're gonna' have to pull the starter too, to get a good look at the flywheel. And someone is gonna' have to put a ratchet on the crankshaft bolt, and turn it pretty good while you watch thru the starter hole. With the plugs out, it will turn pretty easy.

And if you can't see into the starter hole, you're gonna' have to feel it with a finger while it turns. Just get your buddy to turn slower.

Unless there's some way to get a runout mic on the wheel...

You might be able to hear the noise just by pullin' the plugs, and turnin' the crank by yourself. I think you'll still have to pull the starter to see or feel the wheel.

 

Sounds like your cover is gone .Usually the bolt holes crack..could be where the TC mounts or where the flexplate mounts to the crank.

 

I think the cover is just missing. I can get a pretty good look at the flexplate as it is right now. I swear I can see it wobble a little in sync with the knocking sound, but maybe I just want to see/hear that. Sound isn't noticeable with any decent amount of throttle, just when the rpms are low. I'll do what I can tomorrow to get a better look and listen at that area.

 

Ok, got under the car today. Even on ramps, that ain't no easy task, and I'm a stick!

First, I looked for cracks, didn't find any. I tried to wiggle it, but it didn't budge. Where the tc bolts to the flywheel on the driver's side looks normal.

BUT...on the pass. side, the tc bolt looks to have backed out, a lot. I can visibly see almost 100% of the threads of the bolt. Actually looks like someone cross threaded it and couldn't get it in.

I'm trying to figure out a way to put it in, but I don't have the right tools. No socket or normal wrench will fit there, but one of those adjustble wrenches should do the trick. Any ideas?

 

Probably the best thing to do is pull the tranny, and see what's there.

It could be something cheap to fix as it is, whereas if you don't get it right, it could get worse, leave you stranded somewhere with something UN-fixable. It ain't that hard to pull the tranny - ask Mr Pete there.

What tools you don't have, you can get loaners at the Advance / Zone...

 

Well, the car is at the mechanic now. Not my normal guy, took it to the guy the rest of the family uses. He has a lift, so it'll be easy for him to look at it.

See my new thread, I think I just figured all this mess out.