Screeching/grinding sound when applying sudden pressure to throttle

[RZNT4R]

I'm on vacation and I've got time to go to the dealer where I bought it, I'm going mainly for a window guide scratching the glass in the left door, but I mentioned something about this sound too, so they'll have a listen and a look.

Appointment is Thursday.

[edit]

Inconclusive. Window will be sorted but the noise (as expected) isn't and the mechanic said it's "not abnormal" which I don't find to be a satisfactory answer. Ugh, fine, I'll look into it myself eventually, just not now because vacation and hopefully I'll be less swamped at work when I get back and I'll have some willpower left to bother with this. -_-'

[edit]

After toyota were useless, I decided to visit a subaru dealer and have one of their techs do a test drive before I start pouring energy into this. He concurred that it wasn't normal, but it wasn't a noise he'd ever heard before on a BRZ. I guess it's a job for me then :\

[Edit]

In the meantime I went to Subaru to enlist one of their techs for a test drive. I was hoping I wouldn't be the first, but he had never heard that noise.

I just bought a nice Sony sound recorder and I'm making a contract microphone I'll be able to stick to the engine and detect the source, as well as transfer the files to PC for post-analysis.

[RZNT4R]

Finally got off my ass, and while I'm still polishing the contact mic solution, I managed to grab it with the Sony camera.

Is this your sound?

[edit]

2ch contact microphone setup

[RZNT4R]

*Finally* started to look into it yesterday.

Started by striking manifold and overpipe shields off of the list, everything solid there, no traces of contact with anything else. NEXT!

Drove with a scan tool and live data graphs while reproducing the sound, looking for any large change of state that could correlate to a noisy mechanical event. Even if that didn't find the exact source of the noise (yet), it did clarify some things:

Hypothesis 1: large changes in the VVT angles are creating a drop in the oil pressure, resulting in less tension on the chains (until the tensioner pistons hit their ratchet teeths), resulting in some chain slapping - Very unlikely. At that RPM and at the slight throttle changes where the sound is produced, OCV duty cycles don't change that much, not nearly enough to make a pressure drop/chain slap likely.

Hypothesis 2: The new PCM controlled LIN-bus alternator going from steady state charging in cruising to light load for acceleration is making the belt tensioner swivel around more than it should - Very unlikely. It seems that with a good state of charge in the battery, in that rpm range and at those throttle openings, the requested state of charge doesn't fluctuate much, and hangs around the 50% range. Alternator duty cycle does go through large changes, but mostly on engine decel, where it jumps really high to profit from the free energy.

Hypothetis 3: The DI pump is creating the noise as the injection pressure ramps up when going from steady cruise to accel - Is a possibility. It's the only large change of state I observed in the live data, ramping almost instantly from 4000~ish kpa to over 17000 kpa, and spikes on the fuel pressure graph lined up well with the noise.

Fun Fact: whatever is slapping, is doing so so hard the knock sensor is picking it up and makes the ECM pull a few degrees of timing. Proof that "it sounds normal" isn't the proper answer. The unlikeliness of an oil pressure drop and the otherwise normal operation/sound of the engine don't point to an actual knock on the reciprocating assembly. Shell V-Power is used and an octane booster has been tested.

Now for the sad part: I gotta find a way to attenuate my contact mics. I put one on the fuel pump and one of the right valve cover and BOTH were blowing out and clipping my headphone amp and digital recorder at the lowest recording volume. Mic solution: Inconclusive.

I'll flip through the fault code list to find the easiest way to create a fault that disables only the direct injection as it's fallback operation mode. If the mics are clipping, turning the HPFP "off" simply might be the easy way to do it.

[RZNT4R]

Update:

No easy way to disable the DI system without putting the car into serious limp mode. I would have though the system to be more resilient and be designed to simply fall back on port injection but no. So, with the mics blowing out, I had to use the mics that came with the car: I put a scope on the knock sensor, Bank 2, as I felt that was the one closest to the HPFP.

And this is the story of how it eliminated the HPFP from the list of possible causes. Pictured here is the Spill Valve actuation in yellow, and B2 Knock Sensor in green.



Knowing that the HPFP is on a 3 lobe cam on the left intake camshaft, those knock events would have had to line up with the Spill valve closing and be on the same beat, as the HPFP is the only thing that goes 3 beats per 2 engine revs.

This brings up a good question: What DO those knock events line up with? Well, it's a disheartening realisation when you scope them and the crank sensor.

[PaoloJR]

im sorry i tried to reread what you said but i barely got a little of information to nothing as your thesis is not that easy to comprehend.. lol

can you please summarize this is an easier way for a person who doesn't know what youre talking about?

and the noise you produced, in the youtube link, is exactly what i am experiencing

[RZNT4R]

tl;dr:

Individual sound pulses that make up "the sound" are not in synch with the high pressure fuel pump like I had presumed, they are in synch with the crank at 2 per rev, or 4 every two revs, like engine strokes, injection events, ignition events, etc.

[sato]

Revisiting this thread after a while. Any updates? I'm liking thread development.

[RZNT4R]

Sadly I'm sans news. I was trying to think of a way to efficiently differentiate in which stroke the knock events took place with just a 2ch DSO, but all of a sudden the weather turned sour all in one go, which meant work got ultra busy and next thing I knew I was on studded tires and there was a bunch of snow on the ground.

[sato]

No worries. Be safe, and I'll stay calm until weather allows for updates.

Originally Posted by RZNT4R

Sadly I'm sans news. I was trying to think of a way to efficiently differentiate in which stroke the knock events took place with just a 2ch DSO, but all of a sudden the weather turned sour all in one go, which meant work got ultra busy and next thing I knew I was on studded tires and there was a bunch of snow on the ground.

Sent from my Pixel 2 using Tapatalk

[RZNT4R]

Truthfully, since I can't really diagnose this in the shop, it'll probably have to wait for spring, standing around outside in the cold backprobing connectors and driving around with the window down with a bunch of wires between the engine and passenger seat while the tire studs drown out the sound I want to hear is probably the most not-fun thing I can think of

[RZNT4R]

Not really a new development, but I took a brand new (300km odo) BRZ tS for a spin yesterday and I was able to reproduce the same exact sound with ease.

That doesn't tell me what it is, but it does take some stress out of it.

[chipmunk]

Well.... if you're confident that it's a 2nd order noise, then it's likely firing/injection related, or as Tcoat said, it could be exhaust leak too. Nothing in valvetrain IIRC is 2nd order.

[RZNT4R]

Since scoping the knock last fall I've been thinking it may be the injectors getting loud as the pressure spikes at throttle tip-in, the noise lines up exactly with the pressure spike (looking at live data from the fuel rail pressure sensor).

When the weather warms up (still in the single digits here) I'm planning to scope the injectors one by one along with their associated knock sensor, see if the shockwave lines up with the injection event.

[sato]

Any updates? Your findings seemed interestingly enough to warrant waking thread up from the grave.