A buddy of mine recently put some ported heads on his modded LT1 Camaro. He got on it hard the other day and it was running great, but when he stopped for a red light, it started making a knocking noise. We're trying to determine the cause and severity of the problem. Here's what we know so far...

1. Noise is loudest near the top front left of the engine.
2. It is quieter from under the car.
3. It can be heard at idle, but is louder when the engine is revved slightly.
4. Its not a rocker.
5. Disconnecting #1 spark plug makes the knock worse.
6. Disconnecting #3 plug doesn't seem to effect the noise.
7. Disconnecting #2 plug makes it slightly worse.
8. Disconnecting single fuel injector does not effect the noise.

9. Using an engine stethoscope the knock can be heard very well from in the engine valley between the #1 cylinder's pushrods
over where the chamber should be (could still have been on the head and not on the block). It gets much quieter moving farther down in the valley. The noise can also be heard well from under the car, putting the stethoscope rod on a bottom edge of head that sticks out past the block. Listening on the front oil pan bolts gives a fainter noise.

So, is this a spun bearing or something else? Why does it get worse when the #1 cylinder isn't firing?

Any help is appreciated.
Joe

 

Sorry to hear about this.

What is the frequency of the knock?

Fast tick- associated with valve train or a slow tick (1/2 as fast)- like a rod?

My guess would be a rod knock if it gets worse when spark plug is not firing and the valve train appears to be operating correctly. Did you run it without valve covers on to see if all rockers are moving about the same?
Also, if it isn't missing while running under load, I'd suspect a rod, not the valve train.

Seems kind of weird if he didn't do any work to the bottom end, though.

Best-

S-D

 

My parts car had a knocking L98 that sounded real similiar to what you are describing.
It sounded just like someone was inside the eng with a hammer.
At idle it was intermittent, but more knock than quiet.
When eng was accelerated even a little, it became more steady and a lot louder.
It was noticeably louder on the drivers side.
Turned out to be a spun #1 rod bearing.

I would guess it's louder with the ign/fuel disabled on a cyl, due to the crank having to pull the piston down on the power stroke.

Normally the piston is pushing down on the crank for all 4 strokes except the intake stroke.
With no fire in a cyl, piston would only be pushing down on the exhaust and compression strokes.
That would mean the load on the crank would change twice as often. Every time the load changes direction, slop is taken out of the excessive rod clearance and causes the knock sound.

 

Swerve, Yes, it's a stock bottom end. He has a 211/219 cam, long tube headers, real good flowing ported heads, and misc other stuff. He was running low 13's before the heads, so he's making a bit of power and reving to 6000 RPM. It's also high milage (highway miles though), I'd guess around 130k miles. I don't know what the LT1 bottom end can usually take, but he seems to have hit the limit.

We've run it without the driver side valve cover and the rockers all look to be moving nicely and the same amount. Haven't removed the passenger side since the noise seems to come from the left side. I'd say the frequency is 1/2 the valve train rate, but I'll tell him to double check. I would describe the sound as similar to a car dieseling after its shut off, but rythmic rather than the stuttering that dieseling does.

ZZ28ZZ, I'm guessing this the #1 rod bearing also. We get the same knock behavior at idle and revving as you describe. One other symptom I forgot is bad knocking when the car is started. It rattles rather loudly.

Could the car be driven at all like this (will it gradually get worse) or does that risk a sudden failure that could cause more damage?

Joe

 

Yea, that is a drag. I wouldn't bother driving it anymore, since it will have to be fixed anyway and driving it will only increase the risk of more damage.
Sudden failure is probably not in the cards as long as you baby the motor, though it can always sieze. Probably not worth driving anymore.
Best-
S-D

 

I just got over fixing a similar problem... drive it as little as possible, it will just get worse. Trust me I know that I got so frustrated by it that I didn't want to drive it until it was fixed, anyways.

 

yeah sounds like a typical rod knock.. a high mileage 350 i got for 50 bucks had a very similiar noise... i assumed it was a spun bearing or something... after taking it apart i found that the bearing never did spin.. it was actually that worn... several of them were in fact.. crank had a decent groove on it.. not much fun.. but hey what do ya want for 50 bucks? good luck!