A heavy-duty diesel forklift was brought in because it would lose all power and bog down under load after about twenty minutes of operation. The operator could let it idle for five minutes, and the power would return. The shop had already replaced the fuel filters, cleaned the fuel tank, and replaced the fuel lift pump, assuming the engine was starving for diesel. The problem persisted.
We hooked up the laptop to the engine ECU. The diagnostic software showed no active fault codes. However, we watched the live data stream while test-driving the truck. The coolant temperature was reading 175 degrees Fahrenheit when we started. As we drove it hard, the temperature slowly climbed. The moment it hit 215 degrees, the ECU instantly derated the engine, slashing the fuel injection pulse width to protect the engine from overheating. The truck lost power.
We looked at the mechanical temperature gauge on the dash, which was fed by a traditional thermal bulb. The mechanical gauge read 185 degrees. The ECU sensor was lying. The coolant temperature sensor (a thermistor) had drifted out of calibration. Its electrical resistance curve had shifted, causing it to report a temperature 30 degrees hotter than reality. When the engine was actually at 185 degrees, the sensor told the computer it was at 215 degrees, triggering a premature thermal derate. We replaced the $15 temperature sensor, and the truck ran at full power all day. Modern diesel engines will aggressively protect themselves based on sensor data; if the sensor is a liar, the computer will strangle the engine to save it from a phantom fire.
