BMW M5 Competition (F90) ECU & TCU Tuning with Pops & Bangs and Cold Start Delete — Replacing Another Shop’s Tune, Fixing a Rough Idle

A BMW M5 Competition (F90) came to us for ECU tuning, pops & bangs, cold start delete — and, by the end of the day, a TCU (transmission) tune as well.

The owner’s car had already been tuned by another shop. Recently, though, the idle had become noticeably unstable, and he was looking for a place to redo the whole thing properly when he found mbFAST and got in touch.

“We can do it today” — the deciding factor

When the inquiry came in, my answer was simple: “We can take the car today, right now.”

The owner later told me this was exactly what won him over. Other shops had quoted him a booking a month out — and by then, the excitement that made you want to tune your car in the first place has usually cooled off. We reply fast, and whenever possible we meet, discuss and flash the car on the same day.

The moment you decide is the moment you care the most. At mbFAST we try to work at the speed of that moment.

What we did on this M5

• ECU tuning — replacing the other shop’s file with our own calibration, with a proper power increase
• Pops & bangs (burble / afterfire) setup
• Cold start delete — removing the loud high-rpm idle on a cold start
• TCU tuning — proposed during the road test and added on the spot

The F90 M5 Competition runs the S63 — a 4.4L twin-turbo V8 making 625 hp from the factory. It’s a fundamentally excellent engine, which means the quality of the calibration shows up directly in how the car feels.

We started by flashing the ECU with our own data. The unstable idle that had been bothering the owner settled down immediately after the flash, and the power was there too — his words: “even on the ECU alone, it pulls harder than before.”

Cold start delete removes the S63’s trademark high-rpm idle right after a cold start (the catalyst warm-up phase). If you’ve ever winced starting your M car early in the morning in a quiet neighborhood, you know exactly why this is one of the most requested options among M owners.

New to pops & bangs? Our cornerstone guide explains what afterfire actually is and how we calibrate it safely: Pops & Bangs / Afterfire explained.

We don’t just hand the car back — we drive it with you

What reassured the owner most was what happened after the flash: the road test, together.

I ride along in the passenger seat to check the behavior, then take the wheel myself — highway and city driving, back to back. The typical experience elsewhere is: drop the car off, get it back with a “done, all flashed.” But that way, the owner never gets to verify how the new data actually behaves on the road.

Driving together means we can compare notes in real time — “this transition could be a touch smoother” — and fine-tune until it feels right. That’s how mbFAST works.

TCU tune proposed mid-drive — added on the spot

While checking the shift behavior during the test drive, I suggested: “If we also tune the TCU — the transmission computer — this part gets even better.” The owner’s answer: “Let’s do it.” So we did, right then.

The result was exactly what we were after. Shift response sharpened, acceleration became one continuous surge, and the low-speed jerkiness disappeared. With the TCU cleanly putting the extra ECU power to the ground, the M5 got its natural character back. The owner was delighted.

On the 8-speed automatic F90, the TCU makes a real difference in everyday smoothness — arguably even more noticeable day to day than the power itself. If you own an M5 or M8 and have only done the ECU so far, the ECU + TCU combination is something you should experience.

Already tuned elsewhere? We can replace it

If your car was tuned by another shop and something feels off — or you simply want more — we can overwrite it with our own calibration. Pops & bangs and cold start delete can be included in the same flash.

Tell us your model, year and current state of tune. Consultation and quotes are free, and English is fine.

MAMORU SARAYA

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