The safe minimum idle on a modern diesel: keep the target at least over 800 RPM, set 900 as the everyday answer, and go to 1000 for a sport build. The reason is not preference — the engine-and-gearbox assembly has a resonance at about 300 RPM, and a lower idle pushes stronger ignition events closer to it.
Where this comes from
At the 5 May 2026 Q&A, Dmitriy asked: with non-standard tuning, is it often necessary to change the idle speed — and what is the correct change strategy (~13:45)? Thomas answered the strategy half with a physics argument you can feel in the driver’s seat. The quotes below reference that recording.
The resonance you have already felt
Every time you switch the engine off, there is a shudder as it stops. That shudder is the answer to the question.
“If you just switch off the engine, you always feel such a vibration when the engine is stopping. Why? There is no energy — you have switched off the engine — so why this vibration, why this noise? Because there is an increased energy accumulation because of resonance. It’s a resonance of the mass of the engine and the gearbox… a mismatch of the energy dissipation. Resonance occurs at about 300.”
— Thomas, Q&A 2026-05-05, ~16:00
On shutdown, RPM sweeps down through ~300 on its way to zero — and passes the resonance point of the engine-and-gearbox assembly. At factory idle, the running engine sits comfortably above that point.
Why lowering idle makes it worse from two sides
Drop the idle target and two things happen at once. The ECU corrects the fueling to hold a stable idle at the lower speed — so each ignition event carries more energy. And the engine now runs closer to the resonance RPM.
“If we reduce the idle to a lower value, every ignition will be more powerful — the system will correct the fueling to have more energy for every ignition. But you will be nearer to the vibration; you will just come to that vibration. Then the dual-mass flywheel or the gearbox can be hit hard. So the good strategy is to keep idle at least over 800 — and for me, in my private opinion, every engine loves 900.”
— Thomas, Q&A 2026-05-05, ~16:46
The components that absorb a low, lumpy “real diesel” idle are the dual-mass flywheel and the gearbox — parts that cost far more than the tune.
The working numbers
| Target | Verdict |
|---|---|
| Below 800 RPM | Don’t. You are moving toward the resonance with stronger ignition events. |
| 800 RPM | The minimum — “keep idle at least over 800”. |
| 900 RPM | The everyday answer — “every engine loves 900”. |
| 1000 RPM | For a sport car with non-standard tuning — “also very good”. |
One more trap from the same answer: if idle is unstable after an engine rebuild, the cause is usually not the idle target.
“After rebuilding of the engine, the main trouble with the non-stable idle — it is just improper fueling. Typically it’s too lean or too rich. But after adaptation it also improves the situation.”
— Thomas, Q&A 2026-05-05, ~17:47
Check the fueling first, let adaptation run, and only then come back to the idle family.
Related on Tuners Guild
- Finding the maps to change: Find idle maps in EDC17 and MD1 — and the 900 RPM rule — the RPM×2 search signature, the WinOLS help-line workflow, and where the maps sit per ECU family.
- The other rule from the same Q&A: EGR off — why did my DPF stop regenerating? — the +20% post-injection fix.
- The full diesel method: Chip Tuning Diesel — Practice covers idle strategy, fueling analysis, and the torque structure around it on EDC17 and MD1.
Method credit: the answer is Thomas Pirowski’s, from the 5 May 2026 live Q&A; timestamps reference that recording.
Your turn
Have you run a diesel at idle below 800 — and what did the dual-mass flywheel think of it? Or found an engine that genuinely needed a different number than 900? Post the engine, the target you set, and what happened over the next 20,000 km. Long-term outcomes are the data this thread is for.