Completely stock engine with 74,000 miles on it stumbling between idle and 3,500 RPM
 

Recently my BRZ has begun to stumble at RPMs below about 3,500. The drivetrain is completely stock and never been modified. At first I only noticed it at throttle tip-in. However, over the last few months it has become more frequent, regardless of throttle position. In fact, this morning on the highway in 6th gear at 62 mph I floored it and it seemed to stumble two or three times per second for maybe three or four seconds. I have never seen a single CEL in this car, and the car has always been maintained impeccably well. The spark plugs have 15,000 miles on them. I always use gas from good stations that see a lot of turnover. Primarily that is Shell and Conoco.
Does anyone have any idea what might be causing this? At first it wasn't really a big deal, but it slowly seems to be happening more often in a wider variety of conditions.
Thanks!

Does it only happen on sudden(ish) throttle position change?


Have you logged at all? Even if it's not throwing a CEL, good datalogs might indicate the root cause. Especially if you can log the issue happening.


Odd as it sounds, verify the battery connections are good - clean and tight.


My first real guess is MAF sensor, possibly the idle control in the throttle body in need of a cleaning. I've had issues like this with a different car using MAP and of course that's barely similar, so these are wild guesses to get things rolling.


I had issues like this occasionally (especially from a dead stop) with my OFT tune on the '14... so it can be 100% tune related. I doubt that's your issue but note it just in case it makes someone think of something.
Hope it's an easy fix.

Edit: Is your car tuned and does this only happen when it's below 50 degrees?

Logs. Just click everything.

The tune is original. It happens in all temperatures.

Sounds exactly like a bad coil pack to me. Had these symptoms on a previous car

Oooh, good question. It does only happen when there is a sudden(ish) change in the throttle position. What does that indicate?

I'll ensure the battery terminal connections are clean and tight. I'll also clean the MAF. Those are quick, easy things to do. Thank you!

It means something is slow to react, I think. My inclination is to suspect MAF more specifically than before... time will tell.



You should see it in a log, regardless. Logs are incredibly useful. As @Ultramaroon said, log everything you can. If you don't have the tools and this isn't solved easily, I'd consider that the first thing to figure out - how to get some good datalogs.

I agree this sounds like a coil pack if everything else is stock. Even one or 2 bad ones can cause this.

Sent from my ONEPLUS A5010 using Tapatalk

I'm surprise no CEL have lit up for ignition coils (if it is the coils)

I ensured the battery terminal connections were clean and tight. I also cleaned all connections and the sensor on the MAF. I'm going to drive it for a bit and see if that solved the issue. If I don't come back to this thread within a few weeks you can expect that fixed it. That said, I expect you'll be seeing me post here again soon.
Again, thanks for all the help.

My car is modified, but I encountered an issue similar to this. I would tip in at around 3500 and it would fall on its face until 4500 rpm and clear up. I changed plugs and one bad coil pack. Didn't resolve issue. I had the car smoke tested, nothing. Swapped MAFs with a friend, nothing.

In the end it ended up being two bad direct injectors so I replaced the entire set for good measure. Resolved the issue.

Despite having a factory tune if you can even log fuel trims it would give us more information than nothing at all. Most scan tools or obd readers can do at least that. When I had my fueling issue I was one point leaner than target when I was on throttle, and at idle my fuel trims were high positives, like +14 or so.

EDIT: if it's between idle and 3500 it might very well be coil packs. Regardless logging the data mentioned will help you track the answer down.

I agree with the others, it’s probably the coil packs.

Now that’s something I haven’t said since my Audi days...

Well, the problem persists. It is happening anywhere between idle and 3,500 RPMs, regardless of the amount of throttle input. I'm hoping to have a friend log it for me sometime soon. Still no CEL. The problem seems to be getting worse, but still happens only intermittently.