Engine Temperature Fix: correcting engine temperature control in the ECU

Engine Temperature Fix corrects how the ECU regulates engine temperature. It is used when an engine with electronically controlled cooling runs hotter than the job requires, or when temperature control does not work properly. For tuners: send us the original file and get a ready-to-flash file with corrected checksums, on average in 30–60 minutes.

Why a modern engine holds 105–110 °C

A conventional thermostat has one opening point, set by its wax element. In electronically controlled systems a heating resistor sits inside the wax element. The ECU heats it with a PWM signal, so the thermostat opens earlier than it would on its own. The ECU recalculates the target temperature every second, using inputs such as engine speed, load, road speed and intake air temperature.

At part load the engine is kept hot on purpose: on the VW system that is about 95–110 °C. This way it uses less fuel and produces fewer emissions. At full load the target drops to about 85–95 °C, because the intake air then heats up less and the engine makes more power. HELLA puts the torque gain from lowering the temperature under load at roughly 2–3%.

For such an engine 105 °C at part load is normal operation, not overheating. The system runs at 1.0–1.5 bar, so the coolant only boils at 115–130 °C. The electric pump helps as well: it circulates coolant on demand rather than at crankshaft speed, which saves up to 2% fuel.

When temperature is corrected in the software

• Tuning and hard use. The lower the temperature, the less the intake air heats up. The part-load target can be brought closer to what the engine holds at full load, for example so the thermostat opens at about 90 °C instead of 110 °C.
• Cooling system changes. The ECU monitors warm-up and logs a fault if the engine does not reach the expected temperature. If the thermostat or radiator has been changed, say so in the order: the target temperatures may need adjusting for the new parts.
• Unsteady temperature. If the gauge wanders while the hardware is sound, regulation can be made steadier. Rule out faults first, as described below.

Overheating and odd readings: what to check before flashing

Software will not fix a leak, a weak pump or a clogged radiator. If the engine really overheats or the temperature jumps around, start with the hardware:

• Readings. Compare the temperature on the scan tool with the real one. The coolant sensor is an NTC thermistor: a typical one reads about 2–6 kΩ at 25 °C and about 300 Ω at 80 °C, with exact values in the manufacturer's data. Check the wiring and the 5 V supply.
• Level and air. Air pockets badly reduce circulation and can overheat the engine. Fill and bleed the system following the manufacturer's procedure.
• Thermostat. Compare the temperature before and after it, and if in doubt test it in hot water on the bench.
• Pump. The electric pump is PWM-controlled and can be checked with a scan tool. A failed pump means overheating.
• Radiator and fan. Clean fins, the temperature difference between radiator inlet and outlet, the fan switch-on point, the expansion tank cap valve.

What changes in the file

The file corrects the target temperatures the ECU uses to drive the thermostat heater, the electric pump and the valves, along with the related parameters. How far to lower the temperature and what else to adjust depends on the engine and the job, so describe in the order what you need to achieve.

Part-load fuel consumption and emissions may rise slightly, because the high temperature is there to save fuel. Checksums are recalculated, so the file is ready to flash straight away. The correction can be done together with Stage 1 in one order.

How to order the file

1. Read the original file via OBD or on the bench; pinouts are in the ECU pinout section. We need an unmodified stock file.
2. Upload it to the portal, select ENGINE TEMPERATURE FIX and describe the job: the temperature you want, what was changed in the cooling system, which faults are stored.
3. Get the finished file with recalculated checksums, on average within 30–60 minutes.
4. Flash the file, clear faults and check the temperature on the scan tool at part load and under load.

Supported ECUs are listed below. If yours is missing, send the file with a request. Keep the original: flashing the stock file restores the factory regulation.