Idle tip-in/throttle blip knock
 

So I've been searching the forums for a while and can't seem to find an answer to my issue.

The car will have a nasty knock sound after jabbing/blipping the throttle at Idle. Rpms will dip/struggle a bit when knocking and then make it's way up normally after that split second. Does not knock under load or when rpms are higher than idle. This is my 2nd motor and the first one did the exact same thing.

I ended up getting an OFT to see if an updated tune would fix the issue but it's still as present if not worse with the OFT stage 1 tune.

Just looking for some insight on this issue and see if anyone has figured out the exact cause and or a fix. I figure the car can't deliver fuel fast enough when the car is at idle when blipping the throttle since it has to change between closed and open loop but not sure if it's normal or if there's a fix.

Running 91 octane because it's the best we can get here but I don't think fuel quality is the issue since it doesn't knock under load. The OFT does pick it up as knock when blipping hard at idle.

Any insight would be appreciated!

This is crazy that you just posted this, because I've been experiencing what sounds like nearly the exact same thing recently.

I have a 2014 FR-S with just under 65k on it that I've had for exactly a month now. Stock tune. Purchased it used from a friend, so single and very meticulous/caring owner. Only AMSOil fluid and oil filter used,etc. Spark plugs (and maybe coil packs, too? @Kaotic Lazagna) done roughly 5k miles ago.

I recently took off the loud axleback that was on there and put on a quiet one that's roughly the same volume as stock. So, I'm not sure if this symptom has always been happening and now I'm just noticing/hearing it or what. Only other relevant mods are a dry AEM drop in filter and Perrin silicone inlet tube.

But yeah my experience mirrors what you describe. Upon stabbing the throttle for a quick rev from the resting idle of ~600rpm, there is a brief sound that I would describe as a rattle/knock, immediately before the revs rise. Sometimes it is louder than others, but it's fairly easy to replicate.

If I feather the throttle slightly to raise the RPMs and then rev, no noise. Similarly, if I rev repeatedly without letting the revs fall back down to the resting idle, no noise. It's only when revving abruptly it from that default idle state.

Also worth mentioning, because I don't know if it's relevant, and I also can't say for sure if it was happening before this event. I had a strange experience last weekend where I took the car out for about 30 mins of freeway driving, and when I got off I was waiting at a stoplight and the idle rpm was dipping and the car felt like it wanted to stall. It did this at a few more lights and eventually it dipped so low while I was waiting at one that it stalled and I had to restart it and continue home which was only a few minutes away.

Upon getting back into my driveway, I noticed the revs dipping again down to the point where the DRL LEDs would briefly shut off before the revs climbed again. I thought it might just be the idle hunt issues that some of the early factory tunes had, but when I went to start it the next day after ensuring the battery was at 12.2v it only ran for three seconds before dieing/stalling.

I tried restarting it a few times and the only way I could get it to stay running is if I held the revs around 1,500-2000rpm for around 15-20 seconds. It sounded absolutely horrible while doing this which made me think bad gas, but turned out to be likely just a bad AFR. After doing that though, it would then idle normally on it's own. However, when I revved it during this time, it sounded like it didn't want to rev and I heard some knocking. This is when I believe I first heard the "knock upon idle rev stab" symptom.

Long story short, I ended up removing the MAF and cleaning it with MAF cleaner spray, and the problem went away. No more stalling, car now starts and idled fine. Did a test drive and it pulls hard as well, felt fine.

However, now I'm wondering about that strange noise that I hear now upon quick/abrupt revs! You mentioned that your OFT was identifying the event as a knock. What data point did you use to determine that? I'd like to be able to use the realtime data display or the logging feature to see if my OFT also identifies it as a knock event. Again, my car is currently still on the stock tune.

Anyway sorry for the long post, just excited about possibly getting a better understanding of what this is and why it's happening. On my old 2013 FR-S that I owned for 7 years I never noticed this strange noise when revving.

Spark plugs only, not the coil packs. Changed them at 63,373 miles.

Try stabbing the throttle with the clutch disengaged. I'm hoping it's just gearbox rattle.

IIRC I was doing the rev stabs with the clutch pedal fully depressed, but will try it again tonight after my commute.

Ah, so even less miles on them than I thought. Thanks Laz! :thumbsup:

So I was looking at the knock correction when blipping the throttle at Idle. It would be a quick but hard correction but to be honest I haven't data logged it so it would be hard to tell for sure. I've noticed this noise when in a drive through with the clutch pressed in so it's most likely the motor. I do hear gearbox rattle too so it's not exactly the same. It's similar though.

I wonder if @humfrz or @Tcoat has any experience with this? Some say it's normal while others say it's not. Just wondering why it's happening and if there's a fix.

Cool thanks, I'll look at the knock correction line item. What was it doing? This is how mine looked at idle the other day:

Dude If your Model A is acting up Hum is your man. If you need help calibrating the 6 pack on your 440 Magnum I can help. Otherwise you have the best crowd here already.

I basically saw the knock correction bar dip real quick but didn't catch the numbers. I'm gonna see if I can get a log tomorrow or even a video of the live feed.

It might have even been the engine load bar that I saw to be honest, the noise is bad enough that it hurts to recreate it too many times in a row so I might've misread it lol. Gonna need to double check tmrw.

Either logs, or wild-ass guesses.

I hear you, it feels like you're doing something bad. I replicated it again today after driving home and was able to do so with both clutch depressed and up, didn't seem to matter.

It doesn't do it every time, but it's fairly easy to replicate. Interesting how if the revs are above the ~600rpm resting idle it doesn't do it whatsoever. So if you just feather the throttle slightly and then stab it, it revs normally and sounds fine/healthy. It only hiccups/clunks/knocks or whatever when you "surprise" it with a throttle stab when it's at rest.

I checked the DI computer bolts and all three were snug. I'll have to do some datalogging with the OFT as well and see if this is a knock event. At least if it is we'll know if it's something gasoline/fueling related.