Rough idle leads to stall, runs well under load, no engine response on throttle blip
 

Howdy all,

Yesterday on my way to work I was decelerating for a light and went to blip the throttle to engine brake and got a muted, rough response way out of line for what is normal. The same occurred for the next three down shifts, and when I came to a stop the idle was extremely rough. The engine stalled shortly before coming to a complete stop. I was able to get it started and could keep it running by keeping it above 2k rpm, and got it off the road. There were no CELs or TC indicators.

I got it home and used the OFT to flash it back to the stock map, took it for a ~20 minute drive and it seemed all normal. Then this morning on the way to meet a client, the same thing happened again but was far worse. She threw a CEL as well as the TC indicator. I was able to get to the meeting, but reflashed my preferred map after to see if I could get her to idle (nabbed a report of the codes first). Once again, all seems fine and I drove it straight home to be safe.

I had been running the stock 'tune' for the previous two weeks to get my car inspected. I reflashed to my preferred tune four days ago after the inspection, drove it through the weekend and topped it up with fresh premium Sunday.

Mileage: 83,991 Miles
Year/M/M: 2015 Subaru BRZ Series.Blue

Engine/Chassis Codes:
P0171 - System Too Lean Bank 1
C1201 - N/A
C1231 - Steering Angle Sensor
C1241 - Low or High Power Supply Voltage (ABS)
P1604 - Startability Malfunction
U1201 - N/A
U0401 - Invalid Data Received from ECM/PCM "A"

Mods:
OFT Stage 1 Catted Tune
FT86SF Catted UEL Header
FT86SF V1 Unresonated Catback Exhaust
HKS Drop in Air Filter

To be clear, this issue has occurred on both the stock "tune" as well as the stage 1 tune from OFT. The issue is resolved by flashing a new tune or possibly clearing the CELs (unconfirmed). No major work or mods have been done on the vehicle in 2 years.

Does anyone have experience with a similar problem? I initially chalked it up to the wrong octane in the station's tanks, but it seems that even the stock tune is unpleased with it. I'd appreciate anyone's input.

Cheers.

Could be a bad ground?

I wouldn't rule it out, but I would think a poor ground would be a more consistent issue, rather than an intermittent one that can be resolved with wiping/flashing the ECU.

I've seen some threads on rough idles/engine operation that make mention of the Direct Injection Computer mounting bolts and their purpose as grounds, but I haven't messed with it at all. I think it would be a more persistent issue.

Thanks for the reply.

Have you checked the "easy"? How old is your battery?

The battery is about 18 months old. When I checked everything over this morning before work, it had 12.4 volts before starting. I feel like with the number of stalls and starts I had in short order yesterday, if I had a battery/alternator issue the voltage would be much lower.

I drove the car to work and back today, running the data logger on the OFT but couldn't replicate the issue. I'll keep logging until either my peace of mind returns or this issue rears its ugly head again.

Thanks for the response Dzmitry.

Cheers.

You've probably already checked all of the connections and wiring.

I checked everything over in the footwell a couple of nights ago, and looked at the DI computer in the bay. Didn't find anything obvious.

I'm always a believer in checking the last thing you did before the issue manifests itself. In this case, the computer was reflashed via OFT, and I got a new tank of gas. Has anyone experienced issues like this with a bad flash or poor quality fuel, water in the fuel etc?

Thanks.

Record your fuel trims. See what their behaviour is when idling and under a bit of gas. If the numbers sway closer to 0 when giving it gas, you got yourself a good ol' vacuum leak.

People have absolutely experienced a bad tank of gas before, so it is certainly possible, but who knows.