VCCA Home Forums TECHNICAL - 1945 AND NEWER 1949-1954 216 Engine knock troubleshooting

Hello. I have a mostly stock 216 in my 1953 Chevy 3100 truck. Only significant upgrade is from Babbit to cartridge style connecting rod bearings.

I am hearing a noticeable engine knock at idle and under light acceleration. It goes away when the engine is under load, such as climbing a hill. The knock is fairly loud, similar to a spun connecting rod bearing, but not quite the same.

As a first step, I tried unplugging each spark plug wire, one at a time. The knock goes away when I unplug cyl #1-5. But, when I unplug cyl #6, the knock does NOT go away. I tried this spark plug test at a fast/choke idle and a low idle when warmed up, and got the same result.

Any ideas what this might mean? Does my spark plug test suggest an issue in the top or bottom of the engine? Something else? What would you check next?

If the noise is louder when the engiine is cold it may be caused by a loose piston. If it gets worse as the engine wams up and oil thins out it is a rod.

Hi Aub

It's VERY strange that the knock goes away with every cylinder EXCEPT #6.

An easy first thing to check would be to make sure the flywheel isn't loose on the crankshaft.

Another question:
Does the knock change when you push down the clutch pedal?

Also, did this come on gradually? Or did it appear suddenly?

PS
One more thing.
Is this the same engine you had to put a rod bearing/bearings in a few years back?

No difference in knock based on cold versus warmed up engine. But the knock is less for the first 15-30 seconds after a cold start (well before the engine warms up)

Key factors I have identified so far are:
Load on engine
Difference when I pull wires for cyl #1-5 versus #6

I plan to check compression next.

Unless folks have other suggestions, I will then drop the oil pan.

You will probably be removing the flywheel cover first anyway, so give the thing a good wiggle before you go further.

Also, as per my PS above,
Is this the same engine that spun a rod bearing several years ago?

I will check out the effect of pressing the clutch pedal. I don't think they names a difference, but it will be good to verify.

I am not sure if this was sudden or gradual. I don't recall an incident, so probably gradual? Maybe it was present, but I didn't notice until it became noticable? All I can say is the sound is noticable now.

One more thing I need to check is the spark to cyl #6. Since that plug/unplug made no difference, perhaps I'm not getting spark there?

Yes, same engine I put new cartridge style rod bearings in, back in 2020. So those are prime candidates to recheck.

I was first expecting another issue with the connecting rod bearings, but when I stared pulling spark plug wires, those results surprised me, as they don't seem consistent with my original theory.

You dont't notice if it is running on 5 cylinders?

I don't observe a difference when I pull the spark plus wire to cyl #6. The knock continues and the engine does not change how it runs.

When I pull wires to cyl #1-5, I do notice a difference. The knock disappears. The rpm decreases slightly.

Check for a stuck intake valve or broken rocker arm, etc.

It appears the spark plug on cyl #6 is bad.

Verified spark on the 6 wire.

Swapped the 6 plug with the 1 plug. Then repeat my previous test of pulling 1 wire at a time from a plug. Only cyl 1 is dead, meaning the knock remains. Pull 2-5 plug wires and the knock goes away.

So the "knock" may be due to the engine running like crap on 5 cylinders.

That's today's theory. Will get new spark plugs next.

And I checked the #6 plug resistance with a multimeter. The resistance is briefly around 10 ohms, then jumps to infinite. Should be 4-8 ohms I believe. It's definitely bad.

Posible that a piece broke off the spark plug and is caught between the top of the piston and head.
remove and inspect the plug.

Yes, I removed the #6 spark plug, and it looks fine. Nothing broken off. But the testing I described above shows that plug has failed. Hoping that's the only problem with my engine. Will update after I try a new set of plugs.

I have a classic 2 points of failure. Installed a fresh set of spark plugs and the engine run much better. One problem solved.

But the knock remains. In fact, now the knock is much more defined and clear. As if the rest of the engine has quieted down, so the sound of the knock has less background noise to fight through.

Welp, looks like my next steps are to have a look under the valve cover and drop the oil pan.

I would repeat pulling the plug wires to help isolate the specific problem child.

Good idea.

The odd one now is cyl#3. When I pull it, the knock changes to almost a double-time pattern.

In contrast, when I pull the plug to ant of the others (cyl 1,2,4,5,6), the engine RPMs drop a bit, and the engine may stumble a bit, but the knock doesn't change.

I wonder if the plug wires are marginal. Don't forget the coil to distributor high tension wire.

Swap out both the spark plug and wire with another cylinder and see if the problem goes away with six and develops in the new cylinder with the ignition components from cylinder 6. This is not OBDII, not much going on