'83 - loud irregular knocking engine noise

'83 Caprice 305, 102,000m

Last week (on the 24th) the '83 began having a knocking noise from the engine. My son didn't mention it to me but drove it that way and on the third day (26th) it 'stalled' for him and would not start. He said when he tried to restart it the car just made a grinding sound. I retrieved the car the next day. It started right up and idled fine but there was a loud knocking sound. I drove it the 1/4 mile home and parked until today when I could take a look.

The car idles smoothly but there is a knocking sound in time with the RPMs. When you rev up the RPM a bit the sound is much more pronounced. There is an occasional irregular knock that sounds just like there was a heavy wooden ball being tossed around inside the engine - makes you really want to shut it off ASAP. The irregular knock is much louder than the rythmic sound. It is present at idle but comes more often and is louder as you increase RPM. It seems to be louder on the drivers side of the engine and low. At first I thought from his description that there was a starter problem in addition to the knock, but I have started it several times and it starts up just fine.

The only recent service was an oil change I did on the 14th. I used a lighter grade oil than before (3qts 5w20, 1qt 5w30, 1qt Rislone). I picked up the 5w20 by mistake - had been using 5w30.

Got some ideas? Broken connecting rod? Prognosis?

Thanks

I'd start at the torque convertor bolts....

I'd start at the torque convertor bolts....

I'm thinking broken flex plate.

You're "irregular sound"... I think I know exactly what you're taking about, the motor I blew did that at the end of it's life. The problem is I have yet to perform an autopsy on it to see what went wrong. The other sound sounds like a potential collapsed lifter, or bent pushrod. In worst case scenario a spun crank bearing. Check between the transmission and engine thoroughly first, to see if indeed it is a broken flex plate or loose torque converter bolts. Flex plates commonly crack and break around the center up where it bolts to the crankshaft.

You're "irregular sound"... I think I know exactly what you're taking about, the motor I blew did that at the end of it's life. The problem is I have yet to perform an autopsy on it to see what went wrong. The other sound sounds like a potential collapsed lifter, or bent pushrod. In worst case scenario a spun crank bearing. Check between the transmission and engine thoroughly first, to see if indeed it is a broken flex plate or loose torque converter bolts. Flex plates commonly crack and break around the center up where it bolts to the crankshaft.

i get a stuck valve on my 83 every now and then and it sounds VERY loud. it can be heard inside the car

If the flex plate is broken, this can easily be detected without removing anything but the torque converter inspection cover. Once you have that off, grab hold of the torque converter and see if you can rock it around. If you can turn it without the crankshaft immediately turning with it, the flex plate is broken. Likewise, if you can move the converter fore and aft, the flex plate is likely broken. If the converter moves but the flex plate doesn't, look for loose converter bolts.

The flex plate is what transmits torque from the engine into the transmission. It has the ring gear for the starter on it - many people incorrectly call this a flywheel (only cars with manual transmissions have flywheels).

The intermittent starter issue and the knock are probably related. You can narrow it down (without getting dirty) by taking a wooden dowel and with the car running, hold one end on the bellhousing and putting the other end to your ear. Move it to the cylinder head or top of the intake manifold and listen. One should be dramatically louder than the other.

Bob

Well, I did some investigating on my Caprice today to try to find out why it rattles and knocked so loudly and randomly. It had no true pattern to base myself off of for diagnostic. It knocked during engine cranking and any RMP level under about 1200 RPM moreso when the engine was cold. I looked to see if maybe it was the torque converter bolts even though the knocking didn't quite fit the normal pattern of such... well it wasn't those bolts, BUT it did turn out to be the bolts that go from the crankshaft the the flex plate. Tightened them up with some lock tite: Voila! knocking gone... engine is still shot, but now it doesn't sound so bad when I run it! So ddax... this may be the solution to your problem.

Thanks Pete. I was going to get under the car and check the flex plate today but I just could not get the time - to many other things had to be done! I'll certainly look at that and hope it's my problem as well. I appreciate you looking into your problem so you could help me more. Sounds like even a mechanical idiot like me can handle that.

I had a guy I know who used to be a mechanic listen to the car yesterday morning. He thinks the rhythmic knocking is worn rod bearings, but was a bit baffled by the random loud knocking. He also thought that I should check the flex plate but thought the sound should have a more 'metallic thunder' sound than he was hearing.

He suggested replacing the bearings without removing and turning the crankshaft (if it was not too worn) and it might work for a long time. He said it would be hard to get a regular shop to do that, as they could not offer any guarantee that it would work but might be done by an individual at much less than having to have the crankshaft turned. He can get me a bearing kit (which includes a new oil pump and oil pan seal) for $80 and is going to ask a friend of his if he'd be interested in the job and for a quote.

Thanks to all you guys for the help! You're great and real saviors for people like me!

I'll post as I find out what's going on.

Well, here’s an update on what I’ve done:

I checked the flex plate. No movement of the torque converter or flex plate. The flex plate look fine. This tranny was replaced (with rebuilt) about a year ago.

When I hold a broom handle against the bottom of the engine (oil pan) or even the exhaust pipe the knock sound is very loud. Holding it to the valve covers it’s not very loud.

I removed spark plug wires from each cylinder (one at a time, of course) and that did not change the sound any. A friend had suggested if it was a rod bearing that I could tell which cylinder it was that way (that last one on the right is fun to reach!).

You can hear the engine here:


Caprice engine knock2 (http://vimeo.com/380539)

Caprice engine knock3 (http://vimeo.com/380374)

Well, here’s an update on what I’ve done:

I checked the flex plate. No movement of the torque converter or flex plate. The flex plate look fine. This tranny was replaced (with rebuilt) about a year ago.

When I hold a broom handle against the bottom of the engine (oil pan) or even the exhaust pipe the knock sound is very loud. Holding it to the valve covers it’s not very loud.

I removed spark plug wires from each cylinder (one at a time, of course) and that did not change the sound any. A friend had suggested if it was a rod bearing that I could tell which cylinder it was that way (that last one on the right is fun to reach!).

You can hear the engine here:


Caprice engine knock2 (http://vimeo.com/380539)

Caprice engine knock3 (http://vimeo.com/380374)


Yikes. I hate to tell you this, but that is exactly what a spun rod sounds like. There's a chance that it could be a slack timing chain (this noise has been known to be loud and it has almost the same sharp rapping knock when it beats against the timing cover), but it sounds more like rebuild time.

The random loud knock you hear is likely the sound of the piston impacting the cylinder head. When a rod bearing spins, the slack in the bearing will allow the piston to move up enough to make contact with the head.

Definately a spun rod bearing, I got opinions from a couple of others around here with my laptop. Same thing happened to my '88 Buick Century years back at about 130K. I had to replace the crankshaft and all crank bearings. The engine ran great after that. Lasted me another 60k too until I sold it to a kid who proceeded to beating it to the death a year later.

Thanks all.
I'm going to look into repair options, but it looks like it may be time to get the title into the hands of a box-chevy loving mechanic who can fix it for himself!

Thanks all.
I'm going to look into repair options, but it looks like it may be time to get the title into the hands of a box-chevy loving mechanic who can fix it for himself!

awww man, its such a nice looking caprice. you could pick up a new/used engine for really nothing if it came down to replacing it.