Lets play guess that noise again.
 

So my car went from running pretty well to pretty crap in a few weeks without much driving. Seems like there's a big air leak somewhere as around 3500 rpm the engine start sputtering a lot. Adding throttle will get her moving faster but you can tell she doesn't like it and AFR will jump around from 12-18. There's this really bad buzzing screeching noise around when the sputtering start. The engine start vibrating A LOT too.

Anyone have an idea what is causing that noise ?

Today I started to notice that same AFR jumping issue just idling at a light. Driving under 3000 RPM seems okay ..


I had thought it was the VICS solenoid so I tried unplugging it. It didn't fix the issue.

The video was taken with accessory belt removed as well.

Also while troubleshooting I found that my PCV grommet is in desperate need of replacing so I'll work on replacing that, but am doubtful its entirely the cause of my troubles.

Also did compression test yesterday and it was reading 175 / 175 / 173 / 175 so I think motor is okay still.

Zip tie harmonics?

So I did the following in the past week ..

(1) replaced the pcv grommet that was leaking oil ( and probably air )
(2) capped off the line going from manifold to the booster
(3) check for leaks by spraying carb cleaner around the intake and listened for rpm change .. no change.
(4) check for leaks by spraying water around some of the intake manifold hoses .. none found.
(5) disabled VVT in my tune to see if this coming on is causing this noise .. problem persist.
(6) checked my grounds to make sure nothing is coming loose.

Kinda at witt's end here. Seems like something is coming on and causing a big vibration and that buzzing sound at 3500rpm .. can't think of anything else to check.

Suggestions ?

At this point .. because AFR is high when that issue happen ( and IMO I eliminated air leak ), I'm wondering if it could be insufficient fueling.

Maybe clogged injectors ? Bad fuel pump ? Without a better plan, my next step in debugging would be to add a analog guage and make sure fuel pressure isn't wonky causing. I have a cheap digital fuel pressure gauge that is giving me okay numbers but I don't trust it enough. That's another lesson learned. Don't buy cheap digital gauges. Go for the nice stuff or stay analog.

Intake manifold leak? have you tried spraying carb cleaner around the manifold to see if it causes the idle to fluctuate?

Very strange sound indeed.

Yes, I did spray carb cleaner around the manifold. No fluctuation to be found.

Again, engine compression is very good. Probably due to the recently refreshed head with Supertech double. Refreshed maybe 50 autox runs and < 100 miles ago and didn't make this sound after refresh .. but it has also been to 8K RPM a few times ( on purpose ) after the head refresh. I also wonder if something could be off there as well. Or perhaps being to 8k on an OEM block messed up something.

Someone on FB suggested to me that perhaps that something inside the IM might be loose or broken .. not sure how likely that is but if I can't figure out what could be broken, then its probably time to just start swapping parts and seeing if I get lucky.

Again - look at that zip tie on the throttle cable end going nuts at 22 seconds.

Sorry thought you were just joking the first time around about the ziptie harmonics. Yes, the entire engine vibrates but it does seem to be worse around the IM which I why I unplugged the VICS solenoid there .. proplem persist.

Perhaps something is indeed rattling inside the IM ..

I think if you trim that, you'll get rid of the screech that's leading you to believe that your problem lies above 3000 rpm. Not saying there isn't something there, but it's focusing your attention.

Trimmed and tried again but unfortunately zip tie harmonics wasn't the issue. The screech remains. I was really hoping that might have been it. :rofl:

Are you sure it's not a belt slipping? Alternator, maybe? Belt dressing is a cheap thing to try.

I'm wondering if it could be the iacv..

This has to trivial. Go get a piece of tubing, stick it in yer ear, and sniff around the area. You'll get right to the source fairly quickly. Use like 3/8th tubing, not small stuff.

Rleete: only timing belt still on .. all other belts removed during testing.

Gooflophaze: yup. I'm thinking iacv maybe fuel regulator pulsing .. have to swap out those 2 parts to test .. waiting for my skunk2 tb to get here.

Mudflap: i wish it was that easy. Spent about 15 min with a mechanic's stethoscope and still can't identify exactly where the noise is coming from. Problem is not happening at idle but at 3500+ rpm so things are quite loud all around.

I only glanced at thread, so pardon if redundant. ez pz way to test if iacv: unplug electrical connector

I find that using the stethoscope is only for pressing it against the engine to listen internally. I don't believe it works very well for finding the exact sources. Try out the 3/8" home depot tube trick. I think it'll help a lot. It is way more "directive" and doesn't require you to press it against the engine.

Hmm .. okay so not IACV.

When unplugged before starting car .. car does not start.
When unplugged after car running .. idle dips to almost dying .. then catches and stays on .. rev to 3500 and same noise.

Wondering if it could be the timing belt tensioner.

I just came here to post the same thing. Sounds like an unhappy ball bearing to me.

Thnx guys. Appreciate the guesses. It keeps me from banging my head against the wall too much while troubleshooting.