Hi all, first post long time lurker while trying to fix all my issues! I have a new weird issue coming from my C5Z - it's got a 416 stroker in it so wasn't sure which tech forum to post it in if that matters haha. I've just been chasing one thing after another here but this one stumped me and I don't want to start throwing money at something yet.

Last 2 days I have had a weird high pitched hesitation/stuttering at higher RPM (mostly noticeable above 2500) - this occurs when I'm parked and in neutral, with the clutch in/out while revving so I don't think it's a drivetrain related issue, but could be wrong. I also hear and feel a deep grumble/occasional "boom" from around from where the headers would be right by my feet at idle in neutral and while cruising. It happens in any gear.
Two snippets of the sound in this video:
I've put new ignition coils on, these plugs & coils probably have close to 1k miles on them (NGK TR6IX), I just fixed a vacuum leak which was just the A/C vacuum line popped off the back of the intake manifold - I've smoked tested it a few times since to make sure it's stayed on and she's all good. I was dealing with it for roughly a month before figuring it out. Coolant levels are fine, oil level is fine, clutch pedal feels normal & no issues shifting. It is hard as hell to get this thing going in 1st or reverse now. AFR reads like it was before this started.

Today, the issue wasn't there! Startup had some white smoke at idle for a bit until oil got up to 130F. I had some violent surging under 2.5k rpm. AFR still reading normal the entire time.
Feel like I have to fight it in 1st and reverse, and any decel it's like going to die if under 2.5k rpm.

Is this something mechanical? Or my plugs are just fouled already - mainly asking cause I'm not trying to start throwing money at stuff (again) and interested to know the reason behind this. None of my friends could tell me what was going on with this sound. Last time I had bad plugs the misfires were more pronounced?

Thanks in advance! Sorry if it's a dumb question.

 

Did you modify this vehicle yourself, or did you purchase it with all the aftermarket bits? Have you checked your motor mounts? Excess engine movement due to deteriorated motor mounts can cause the headers to hit the underside of the car when the engine is loaded.

 

I purchased it with all the aftermarket bits - and no I have yet to check the motor mounts, but I'll add that to the list to check when I get in the garage this weekend, thank you for the callout. When I bought it that was on my list but somehow they've slipped my mind with all the other issues I've been having lol

If they were bad, would this issue & noises come and go occasionally or would it be there constantly?

 

You indicated that you hear and feel a periodic boom and grumbling noise down where the headers travel under your feet, this is why I suggested checking the motor mounts. With regards to the other noises, it is impossible to troubleshoot remotely in my opinion. Hopefully, this is not your daily driver. I foresee that will will have to spend a lot of money and effort to get this vehicle working correctly.

 

Hi dm290, I have a rattling sound during rpm acceleration parked or driving. I thought it was the catalytic converter. My mechanic took a flashlight and showed me the harmonic balancer is wobbling slightly and gets much worse at higher rpms. This is causing the rattling and was causing the squeaky belt sound that I chased by replacing the tensioners and the water pump (that did the most benefit).

The cause is the stock harmonica balancer has a rubber inner sleeve which warps over time. The fix is an aftermarket solid pulley. I'm using 303 rubber lube until I get around to fixing that and the weak clutch.

You might consider looking at your harmonic balancer with a strong flashlight any wobble at idle is gonna be big at high rpms.

 

Yeah, I am with Gary on this one. Check the balancer, if it is stock or a stock replacement, check the inner and outer shells where they are bonded together. Years ago I had a vibration that I played hell tracking down. Pulled the diff and trans off, ran the car with just the torque tube on, ran the car with just the clutch and flywheel on, ran the car with just the flywheel... Only to find that the balancer was separating.

 

Pull and post your codes.
Do you have a scanner to read data in real time?
When you hear the noise under acceleration, do you stop accelerating? What's happening with RPM at this moment?
You did a smoke test, what about a vacuum gauge ?
Explain the sequence of events leading up to this problem?

 

If you have headers, they can be rattling against something too, it’s a real tight fit in there. 😂