I have taken a new video of the engine at a normal idle at first & then had the throttle up to about 1,000 RPM & had them hold it there for a few more.
https://youtu.be/zrDJCaha28M
OK, that works for me......laptop has fabulous speakers.
You are hearing a worn cam chain.....I'll go into detail in a minute......it will cause no harm, btw.
I would stop by the dealer that successfully repaired the car and have the mechanic give it a listen and get some printed documentation of the concern since you indicate a warranty extension of some sort has been granted.
OK, I will do my best to simplify what's going on here, but first, I need you to visualize a bicycle chain.
This is called a roller chain and it is a roller chain that is used to drive the camshafts from the crankshaft.
The ratio is two to one.....meaning every two turns of the crankshaft results in one turn of each camshaft......so, like a bicycle, there is a ratio change.......doesn't matter much to my response other than to give an idea whats going on in there.
We use a term to describe chain wear.....it is called 'stretch'. It doesn't actually stretch though......what occurs is wear takes place at each link attaching pin and wear also occurs at each roller that the attaching pin goes through.
So, the wear points are were each link rides and is secured by the staked pin and each roller, having a bronze bearing within.
Also, wear occurs at each sprocket tooth.
Chains do not wear 'stretch' evenly......there will be an area of travel from gear to gear where the chain has minimal free play and as it continues down its path there will be more free play.......
Back to a bicycle (or motorcycle for our two wheeled members)......a specification for free play is provided by the manufacturer. To correctly perform a adjustment the drive wheel is turned by hand while feeling the free play at the longest area of the chain between the sprockets.....as you continue to turn the wheel you will feel the chain tighten and as you continue to turn the wheel you will feel the chain loosen. The free play adjustment is made when the chain is at its tight spot and this is to prevent the chain from over loading the engine or pedal output bearings as well as preventing either machines rear wheel bearing from being overloaded.
If we were to adjust the chain at its loose spot, everytime the tight area is running across a sprocket, the sprocket itself as well as corresponding bearings experience a tremendous, momentary load.
Back to the Cruze: the car has a hydraulic chain tensioner driven by engine oil pressure, and two nylon chain guides. The tensioner has a one way sprag.....a shaft with little gear teeth that has a spring steel pawl.
The gear teeth are there so the tensioner won't retract at engine shutdown.......they are far enough apart though, to not allow the tensioner to 'lock' and create a too tight situation.
If the engine were to operate with the chain rotating tight and then too tight the nylon guides would be destroyed.
Whew, fingers getting sore.
This chain, if it could be taken apart at one of the pins, is about 7 or 8 feet long......essentially twice the height and distance between cam gears added together.
This means the chain will pass the same point around every fourth or fifth trip around the sprockets which lines up nicely with the timing of your sound.......and slowly speeds up as RPM rises.
You are hearing the chain slap 'lightly' as the more worn section passes the upper guide between the cams.......I have heard this sound on other 1.4's at the dealer I hang out at........they have never seen a actual failure that was a result of 'chain stretch'.
I don't know the pin count of this chain but it likely is in the vicinity of 130 to 150.......so 260 to 300 wear points........btw, we are talking thousandths here......not a quarter inch or anything that dramatic.
Hows that for the longest winded method of saying don't frett over it?
Rob
