RPM Fluctuation At Idle
 

Hello,

My friend is having rpm fluctuation at idle. After helping him do some research, it turns out this is a common problem for this platform. I have never had rpm fluctuation at idle. We started with bleeding the coolant since possibly the system had air in it. Nope still fluctuated. We tried cleaning the throttle body. Nope still fluctuated. We changed the PCV valve. Nope still fluctuated. We did the smoke test. Nope still fluctuated.

We still need to check :

fuel trims 10% give or take

possible transmission module needs an update?

Possible head gasket?


Mods :

Catback exhaust

PCV catch can

grimmspeed drop in filter

Boost -19 idle (read from p3 gauge)

AFR 14.4 - 14.7 cruising (read from p3 gauge)

maybe @steve99 can chime in?



The car has 20k miles on the odometer

2018

Manual transmission

The car is out of warranty.


I just want him happy

Stock tune?

Try cleaning the MAF, with MAF cleaner spray.

It’s a BRZ thing, you can not fix an engineering mistake.

Starting in '17, the charging system went to intermittent battery charging.
The ECU monitors use and will turn the alternator on and off depending on current need.
That will put more or less load on the engine depending if it's charging or not.

Is alternator output load dependant ???

I am not sure if this is a different issue than the typical idle fluctuation where the rpm will drop almost stall and then rise again. If this is what you are talking about it can be tuned out. There was a popular user here a few years ago that uploaded his tables for this in ROM raider. I used them, and my idle stays between 750-800 RPM no dropping/ stalling feel.

How much fluctuation are you talking about here? 50rpm, 400rpm, almost stalling?

Think about the things the affect running condition at idle, none of the things you have done have anything to do with it. Fuel trims should be around +/- 5% or so. 10% is very high. AFR at cruise doesn't tell us anything about idle. It's either vacuum leak, metering, or a load is being placed on the engine like AC or the alternator as mentioned above. It's a random thing on some cars, as long as it is not too severe it is a "feature." There are tons of threads on it.

https://www.ft86club.com/forums/showthread.php?t=144716

Posting my findings before if that may help.

https://www.ft86club.com/forums/showthread.php?t=139501

Use this thread it completely solved idle issues for me and he posted his tables. Do some testing and you can eliminate it completely.

Afr jump around at idle. Definitely hunting around more than usual. Fuel trims look good IMO. The ac is not on. The HVAC system is off. The defroster is not on. The radio is not on. All these things put a load on the alternator/battery but they're not on. Wouldnt the smoke test detect a vacuum leak? We are talking about a 75 / 100 rpm jump or so. I will have my friend send a video when he gets the chance.

My 2014 had similar issue with RPM’s sometimes making it hard to take off because RPM would drop more than in my other vehicles. I cleaned battery terminals thoroughly and used battery terminal scraper(?) to remove corrosion and grudge build up from 5 or so years, cleaned MAF and throttle body butterfly buildup and allowed computer to go through its idle relearn after unplugging battery for 30 min. 120 miles later and it fixed much of the RPM fluctuations. 3000 miles now and car is doing amazing. I have a Optima red top as well, since, the original battery would leave me stranded after 5 years of use. Give that a try, may do the trick.