Really weird idle when warm, unpredictable

[Biff_Inglewood_420_69]

Hey, I noticed in a drive through the other day that my car was idling like shit. It was running fine and not sputtering or anything, but it made the noise my 8th gen made when it was low on oil (burned a quart between changes). I thought through all the mods I've done and I thought maybe my rear motor mount bolts came loose, it's that metallic clunking kind of noise. I turned it off and fired it back up, it still made the noise. I turned it off and on again, and it's gone, idling like a dream. Has never made this noise from a cold start.

So, I checked and the motor mount bolts are still snug. I checked the oil and it was fine, but if I check right after turning the car off, it looked low. Added some more and thought I fixed it, but it's back. Catch can had almost nothing in it. No engine lights.

Mods include, because I've convinced myself this is something I have done: hardface RMM, PRL DP/FP, PRL SRI, Mishimoto catch can, Hondata tune (+5psi? It's the smaller of the two, only with sport mode on, and I normally drive with it off unless I want to rip it on an on-ramp).

The car drives perfectly. There is nothing noticeable on a cold start or after a restart when it acts up. On the road it sounds perfect. No check engine light.

Please help, I am stumped and scared to drive it.

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[Phy]

Does the crap idle coincide with the cold AC turning on by chance?

 

[Biff_Inglewood_420_69]

Does the crap idle coincide with the cold AC turning on by chance?

I think I had the air con completely off when it happened yesterday, but I can't rule it out. Also, it made the noise when it was cold out a week ago and I usually have it on auto around 72F.

My next filter and oil show up on Tuesday, I'll be checking to see if that helps, but I'll definitely pay attention to anything that would trigger the compressor. That would only be air con and front defrost, right? Or is there anything else that would be used for?

I'm going to feel like the biggest idiot if that was it...

 

[Phy]

Just ruling out NVH from the RMM. Doesn't actually affect the idle, but makes it feel really rough.

 

[Biff_Inglewood_420_69]

Just ruling out NVH from the RMM. Doesn't actually affect the idle, but makes it feel really rough.

Yeah, I'll check a little later, should be pretty easy to rule out by just turning on the AC and seeing if it happens again. I will report back with my findings. Thank you for taking the time to help me brainstorm, I appreciate it!

 

[SDAlexander8]

How many miles you got? Maybe you need some new plugs

 

[Biff_Inglewood_420_69]

Just ruling out NVH from the RMM. Doesn't actually affect the idle, but makes it feel really rough.

Reporting back, it's not the AC, no noticable difference on or off. The Hardrace RMM is hardly more aggressive than stock, so it's not too surprising, but good to cross some things off the list. Thanks again!

 

[Biff_Inglewood_420_69]

How many miles you got? Maybe you need some new plugs

Hey, thanks for taking interest in helping me get to the bottom of this! It's a 2019 I've had since new, only 13,000 miles. I doubt it's plugs, but I'll try anything! I'm hoping it goes away after my next oil change on Tuesday, but if not I'll throw a set of plugs in there. It's so weird because it cold starts perfect, purrs like a kitten. It'll run perfectly for long distances or short trips, whatever. But then I'll park to pick up food or hear it bouncing off the wall at a drive through and all of a sudden it's got this almost rod knock kind of noise that varies with RPM but only at idle and goes away with a restart. Like, wtf could this possibly be? Warranty won't cover it either way with all the mods, but like it's not even repeatable, it's completely random. I thought I fixed it for a drive or two after topping off the oil... Guess I'll just rely on the 20 year old Miata until I figure this out.

 

[0percentapr]

Do any lights or warnings come on?

 

[Biff_Inglewood_420_69]

Do any lights or warnings come on?

No, no lights, temp holds steady, RPM holds steady. I can't figure it out, you'd think it would drive funny, but it's just random when I come to a stop after a drive. Sometimes it's fine, sometimes it's metallic shuddering noises. Really hoping it just goes away after my oil change tomorrow. If so I'll shorten my change interval to 5k max.

 

[0percentapr]

No, no lights, temp holds steady, RPM holds steady. I can't figure it out, you'd think it would drive funny, but it's just random when I come to a stop after a drive. Sometimes it's fine, sometimes it's metallic shuddering noises. Really hoping it just goes away after my oil change tomorrow. If so I'll shorten my change interval to 5k max.

Does it happen everytime you stop like at a stoplight or stop completely and turn the car off?

 

[Biff_Inglewood_420_69]

Does it happen everytime you stop like at a stoplight or stop completely and turn the car off?

It might happen every time, but I only really notice it when I'm next to something that will reflect the sound like a brick wall. But like I said, it's not every time I end up next to a brick wall. I think I have noticed it at a stop light one time, but as soon as I get moving again there's nothing out of the ordinary noise or driving dynamics wise. It's hard to remember clearly but I'm pretty sure it's just the noise, I don't think I'm feeling any extra vibrations in the cabin.

 

[PowerPerLiter]

It might happen every time, but I only really notice it when I'm next to something that will reflect the sound like a brick wall. But like I said, it's not every time I end up next to a brick wall. I think I have noticed it at a stop light one time, but as soon as I get moving again there's nothing out of the ordinary noise or driving dynamics wise. It's hard to remember clearly but I'm pretty sure it's just the noise, I don't think I'm feeling any extra vibrations in the cabin.

If it is what I'm thinking it sounds like I have posted on this before asking essentially the same thing. Seems some people are more perceptive to noises (I am ocd about them).

My 2018 and my 2020 Si' have both done this completely random light "knocking" sound from what seems to be the firewall under the dash....it was very concerning at first but I attributed it to the dual mass flywheel...they go through hell on a lower idling small displacement 4 cylinder. Look up dual mass flywheel knock on google and do some poking around about it. Videos mostly don't help because its a lot of completely failed units which make horrendous noise compared to what I think your describing. I do not think it is of concern as I have yet to have an issue from it on either car with 60k combined mileage.

Again just speculating here.....

 

[Biff_Inglewood_420_69]

If it is what I'm thinking it sounds like I have posted on this before asking essentially the same thing. Seems some people are more perceptive to noises (I am ocd about them).

My 2018 and my 2020 Si' have both done this completely random light "knocking" sound from what seems to be the firewall under the dash....it was very concerning at first but I attributed it to the dual mass flywheel...they go through hell on a lower idling small displacement 4 cylinder. Look up dual mass flywheel knock on google and do some poking around about it. Videos mostly don't help because its a lot of completely failed units which make horrendous noise compared to what I think your describing. I do not think it is of concern as I have yet to have an issue from it on either car with 60k combined mileage.

Again just speculating here.....

I'm so OCD with noises. I'm fine with them as long as I know what they are and that it's not my car falling apart, which is difficult with a 2 decade old Miata... For this particular one, though, I believe you hit the nail on the head, that's exactly what it sounds like. Next time I hear it I'll see if it goes away when I engage the clutch and report back. I knew I should have bought that ACT single mass kit when it was on sale in the $800s... Not a job I'm looking forward to, but it sounds like I've got some miles left in this one.

 

[Biff_Inglewood_420_69]

So, upon further review, the noise is unaffected by the clutch. I also spent a bit of time and the sound is coming directly from the firewall and heard only in the cabin, not outside. I am not sure why I thought I heard it reverberating off of walls. Here is a video I made where it is audible. The noise is affected by brake pedal engagement.



Based on this, and the fact that the oil catch can I have is installed on the firewall... I'm thinking it has something to do with the catch can, and it seems I'm not the first to have issues. https://www.civicx.com/forum/threads/mishimoto-1-5t-oil-catch-can-concerns.42050/

The brake pedal changing the tone makes sense since it will use vacuum for the booster and change the pressures inside the engine, thus affecting how much air is sent through the catch can.

I emailed Mishimoto and they suggested a warranty, but they did not confirm that they have a fix. In the linked thread above, it also sounds like they do not have a fix. I would like to just return to stock to at least confirm my suspicion but I have no clue where that breather hose ended up... Anyone have a part number for the stock piece?

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