The phenomenon of hard hot starts
The difficult hot start Hot start is a classic problem in modern diesel engines, especially on hot summer days. The engine starts without issue when cold and runs normally during the journey, but if it is briefly turned off (for example, while refueling, picking someone up, or making a quick purchase), it is very difficult to restart: the starter motor turns for a long time, white smoke is produced, and sometimes even the accelerator needs to be lightly pressed.
Why does this happen?
- With the engine hot, the fuel viscosity decreases and the rail pressure drops faster after the engine is turned off.
- The glow plugs do not activate when hot (factory protocol), and the injectors encounter a hotter, less dense cylinder.
- The ECT sensor reports to the hot engine, activating a different starting strategy.
- If any of these parameters are slightly off, hot starting becomes problematic.
It is a common phenomenon in engines with 200,000+ km, in certain families known for this problem, or in vehicles in warm climates (southern Europe, Middle East, Latin America).
Engines typically affected
- VAG 2.0 TDI BKD, BMR, CBBB (EDC16 and EDC17CP14/CP20/CP44).
- BMW M47 and N47 (EDC16C35, EDC17C06, EDC17C41).
- Mercedes OM611/612/646/647 (CDI, EDC16C2, EDC16C31).
- PSA 1.6 HDi and 2.0 HDi DW10 (Siemens SID803, Bosch EDC16).
- Ford 2.0 TDCi / 2.2 TDCi (Delphi DCM3.5, DCM6.2).
- Renault 1.5 dCi K9K, 1.9 dCi, 2.0 dCi (SID801, SID304, Delphi DCM).
- Fiat/Alfa Romeo JTD Multijet (Marelli MJD6JF, MJD8F3).
What does recalibration do?
- Rail pressure strategy during the first few seconds after starting.
- Duration and quantity of hot pre-injections.
- ECT and IAT temperature thresholds that discriminate between "cold" and "hot".
- Injection timings adjusted to the hot cylinder.
- Glow plug strategy (include brief activation if applicable).
- Management of progress during the first cycles.
- Recalculation of the checksum.
ECU compatibility
| Manufacturer | Supported ECU families |
|---|---|
| Bosch | EDC15, EDC16, EDC17 (entire range), MD1CP001-003, MG1CS |
| Continental / Siemens | SID201, SID202, SID206, SID208, SID803, SID807 |
| Delphi | DCM3.5, DCM3.7, DCM6.1, DCM6.2AP, DCM7.1 |
| Denso | Toyota D-4D, Mazda Skyactiv-D, Nissan YD25 |
| Marelli | MJD6JF, MJD8F3, 8GSF, 8GMF |
What information helps us
- Description of the specific symptom (seconds of rotation before starting, typical conditions).
- Condition of the glow plugs (many cases are resolved by changing them before the software service).
- Rail pressure when trying to start (if the workshop has measured it).
- Vehicle mileage and weather conditions of use.
Important: Hardware verification before
This service corrects the ECU's behavior, not the physical fault. If hard hot starts are caused by burnt spark plugs, leaking injectors, a worn high-pressure pump, or a clogged DRV, software intervention is insufficient—the hardware must be repaired first. Our service complements a proper mechanical repair or resolves cases where the hardware is OK but the OEM calibration has room for improvement.
If the vehicle also has temperature sensors with drift, check IAT/ECT sensor calibration. If there are DPF problems that could be contributing to the symptom, consult a doctor. DPF/FAP solution.
Technical process
- The workshop sends the original file extracted with a professional tool.
- You describe the symptom and the context.
- We apply the specific recalibration for the ECU family.
- We validate the checksum and return the file ready to flash.
Professional notice: This service is aimed at workshops and professionals in the automotive sector. It is suitable for off-road applications, agricultural and industrial machinery, racing vehicles, repair of cloned or replaced ECUs, and cases of failure where the original hardware is no longer viable. The final application to the vehicle and its subsequent use are the sole responsibility of the professional installer and the end user, in accordance with the applicable legislation in each country.
