Hot Start Fix corrects the start routine in the ECU software for an engine that is hard to start when hot: it fires straight away from cold, but after a short stop the starter cranks and cranks. Most of this article is about common rail diesels, where a hot start depends on rail pressure. The file helps when the start logic is to blame or the system is borderline, but it does not cure a worn pump or worn injectors. For tuners: send us the original file and get a ready-to-flash file with corrected checksums, on average in 30–60 minutes.
How a common rail diesel starts
During cranking the high-pressure pump is driven by the engine, so it turns very slowly, and rail pressure still has to build. Until it passes a threshold, the ECU keeps the injectors shut. A Bosch patent gives about 120 bar as the usual threshold, and Delphi quotes a minimum of 117 bar for the 2.0 HDi. The second condition is an engine speed signal: the ECU has to see the crankshaft turning and work out where the pistons are.
The ECU calculates the fuel quantity for starting separately from normal running, mainly from coolant temperature and cranking speed. To the ECU, a cold start and a hot start are different modes with their own parameters.
Why a hot engine is harder to start
Some fuel from the high-pressure side always leaks back to the tank, through the pump clearances and the injector control valves. It shows most during cranking, because the pump turns slowly and pressure at low speed depends on engine speed. Hot fuel is thinner, and with a worn pump or a leaky injector the leakage grows when the engine is hot. From cold the same engine manages to build pressure; once warm, it no longer does.
The electrics add to it. BMW issued a service bulletin for the K 1200 and K 1300 motorcycles: with a hot engine the starter turned reluctantly, never reached its speed, and sometimes the engine would not start. The fix was an extra wiring set to cut the voltage drop. A slower starter means a slower pump, and pressure takes even longer to build.
What the scan tool shows
Hook up a scan tool and log a hot start: cranking speed, requested and actual rail pressure. Often there are no fault codes at all; the ECU simply never starts injecting. Here is a log posted on FiatForum by the owner of a Fiat Grande Punto 1.3 Multijet, where a few bar make the difference:
| Start | Cranking and pressure | Result |
|---|---|---|
| Morning, cold | 194 rpm, 180 bar | started |
| Hot, with a jump pack | 282 rpm, 148 bar | started |
| Hot | 260 rpm, 142 bar | no start |
In this case the ECU requested about 300 bar for starting and stored no faults. Injectors 2 and 4 returned more fuel than 1 and 3.
Rule out the hardware first
- Battery, starter and earth straps. Compare cranking speed hot and cold: if the hot engine turns noticeably slower, the cause is electrical.
- Lift pump. For the 2.0 HDi Delphi requires at least 2.5 bar at the high-pressure pump inlet while cranking. A blocked filter or a weak lift pump gives the same symptom.
- Injector back-leak. Run the return flow test on a warm engine. If one container differs from the others by more than 10%, that injector needs checking.
- Pump, regulator and pressure sensor. Delphi warns that the rail pressure sensor can read wrong without storing a fault. The pump is tested on its own against a sealed rail.
- Crank and cam sensors. Without a speed signal the ECU does not know when to inject, and Hall-effect sensors sometimes fail only when hot.
If the car became hard to start after someone else's remap, compare the file with the factory one first: that is what the Check Stock or Not service does.
What Hot Start Fix changes in the file
File services sell this option as a correction of the start routine for a hot engine and rarely go into detail. Viezu, for example, describes a delay in BMW software on 2014–2018 cars before the ECU starts fuelling, and removes it in the file. The start logic has other parameters as well: when the ECU allows injection, what rail pressure it waits for during cranking and how much fuel it delivers for a given engine temperature.
Which of these can be changed on a particular ECU, and whether it is worth it, only shows in the file and the start log. There is no ready list of ECUs for this option: send the file with a request and describe the symptoms, and we will check whether there is a solution for your ECU. The rest of the calibration stays as it was and checksums are corrected. In the same order you can add Stage 1 or DTC Off.
The file can help
- when hot, rail pressure reaches the threshold, but slowly
- a start delay is built into the factory logic
- cold starts are fine and the hardware has been checked
The file will not help
- rail pressure never reaches the threshold while cranking
- a lazy starter or a weak battery
- no speed signal, poor lift pump pressure
If a hot petrol engine will not start
On petrol engines hot-start trouble usually comes from hardware. Fuel rail pressure drops after shutdown because of a faulty pump check valve or pressure regulator. A coolant temperature sensor reads wrong, the ECU makes the mixture too rich and floods the plugs. A hot starter starts to bind. Start there, and send the file once the hardware is fine.
How to order the file
- Log a hot start: cranking speed, requested and actual rail pressure, coolant temperature.
- Read the original file via OBD or on the bench. It has to be an unmodified stock file.
- Upload the file to the portal, select HOT START FIX and attach the log and a description of the symptoms.
- Flash the finished file and test a restart after a short stop with the engine warm.
Keep the original: the factory start behaviour comes back by flashing the stock file.
Already have the file? Upload it to the portal and select HOT START FIX, the finished file arrives on average in 30–60 minutes.
Order Hot Start FixFrequently asked questions
Why does the engine start at once from cold but crank for ages when hot?
Hot fuel is thinner, so more of it leaks through the clearances of a worn pump and the injector control valves. A common rail diesel does not inject until rail pressure passes a threshold, usually about 120 bar on Bosch systems, so the starter has to crank longer.
Will the file fix worn injectors or a worn pump?
No. The file corrects the start logic and can help a borderline system, but if rail pressure never reaches the threshold while cranking, the hardware needs repair.
Which fault codes show up with poor hot starting?
Often none: the ECU simply never starts injecting. That is why a start log with cranking speed and requested and actual rail pressure matters most.
What should I send with the order?
The original file, a hot start log and a description of the symptoms: how long after shutdown the problem appears and how the car starts from cold.
What should be checked before flashing?
Battery, starter and earth straps, lift pump pressure at the high-pressure pump inlet, injector back-leak on a warm engine, and the rail pressure and crankshaft sensors.
Is there a Hot Start Fix for petrol engines?
On petrol engines hot-start trouble is more often hardware: rail pressure dropping after shutdown, a temperature sensor, a hot starter. If the hardware is fine, send the file and we will check.
Can Hot Start Fix be combined with Stage 1?
Yes, the options go into one file in one order.
My ECU is not in the list. What now?
There is no ready list for this option. Send the file with a request and a start log, and we will check whether there is a solution for your ECU.