What causes this engine knock?

[BlueSedan]

Engine makes no noises when cold. But once it warms up to operating temperature, you can hear a knock/clack sound. But when you rev the engine up, the noise stops. So it only clacks/knocks when idling warm.

What causes this? Engine is carburated and old.

Topic

[Mike]

With a gear-driven timing setup, I’m going to say that what you’re hearing is gear clash.

When things are cold, clearances are minimal. As the block heats up and expands, the shaft centers grow apart by a tiny amount, causing a bit of clearance between the gear teeth. At idle, the camshaft wants to rock back and forth a bit, because of competing valve spring pressures. Give it a bit of gas, and the knock ceases, because now the cam gear is being positively driven by the crank gear.

What is that third gear wheel to the left? The way it’s driven by the tips of the crank gear teeth looks like a good place to generate gear noise.

[BlueSedan]

The third gear is a balance shaft for v4 application. My engine is a v6 but it has the same style of setup but only two gears.

I’ll try to pinpoint the noise better..
What i’m also experiencing is that the engine sounds like it’s running on 5 cylinders once it’s warming up. But after it warms up, it runs on 6.. Every spark plug is firing.

Could the valves be sticky?

[BlueSedan]

Okay when it’s cold, it will eat a little bit of oil and run uneavenly. But after it warms up, it’ll start idling nicely, doesn’t smoke out the tailpipe and runs like it has power. But the small little knock stays at idle.

I opened the oil cap and squirted a little bit of thinner oil on the valves trough the hole and the knock sound started to stop.
So the problem originates from the passenger side valves.

It could be a blocked oil passage.
Is it something that you can free out by pushing a metal wire inside it or do i need to do an engine flush of some kind?

[BlueSedan]

Symptoms:

-First start on cold engine will run like it’s on 5 cylinders.
-Rev up the engine over 3k rpm’s and the engine starts to run normally on all 6.
-Engine starts to make a knocking/ticking noise.

Cause:
Sticky exhaust valve has left open and the piston has hit it.
The exhaust valve is now not straight and it remains to be a little bit stuck open and or not close completely.
As the engine warms up and the revs go up, the vibrations free the valve enough that it starts to make partial sealing that will make the engine run but still have that awful noise.

How to diagnose:
-Leak down test
-Borescope view
-Exhaust paper trick

Can’t be intake valve because it’s not backfiring trough the carb.

Can you drive it as is until you can afford to fix it?
Or will driving it cause further damage?

How much does it cost to do a valve job for only one single valve?

Necessary parts are:
-Headgasket
-Fluids
-One single exhaust valve

[Author]

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