Engine Cooling System Monitoring
10 ENGINE COOLANT SYSTEM MONITORING10.1 General description
The engine cooling system consists of the following main parts, see Figure 117:
- Radiator
- Engine Coolant Temperature Sensor
- Thermostat
- Bypass Cooling Circuit
- Main Cooling Circuit
- Radiator Temperature Sensor (certain OBD groups only)
During warm-up of the engine, the coolant flows first inside the bypass cooling circuit. After the coolant reaches a sufficient temperature the thermostat opens the main cooling circuit to integrate the engine coolant radiator.
The engine coolant temperature sensor measures a mixed temperature between the coolant coming from the bypass and main cooling circuits.
On some concepts, a radiator temperature sensor is used to measure the temperature at the inlet (OBD group CBE-V) of the radiator.
10.2 Concepts without radiator temperature sensor
10.2.1 Monitor Functional Description
Each temperature range of the sensor is monitored by a specific monitor.
10.2.2 Engine coolant temperature irrational, P3081
Monitoring Strategy
The measured engine coolant temperature is compared to a modeled engine coolant reference temperature.
Typical Enable Conditions
- None
Malfunction Criteria
At engine start, the reference temperature model is initialized with the sensor value and simulates the temperature behavior of the engine based on mass air flow, the vehicle speed and the ambient or intake air temperature.
If the sensor temperature falls below or does not reach the modeled engine coolant temperature (minus a defined tolerance), either the engine cooling system or the temperature sensor is faulty.
10.2.3 Engine Coolant Temperature Sensor, irrational signal: measured value of the sensor is stuck above or below the thermostat control temperature, P0116
Monitoring Strategy
During driving cycles with high and low cooling performance, the change of the engine coolant temperature sensor signal is compared to a defined threshold value.
Typical Enable Conditions
- Engine coolant temperature at engine start within a defined range.
Driving condition H (high cooling performance)
- Vehicle speed within a defined range
- Mass air flow within a defined range
OR
- Deceleration fuel cut-off
AND
- Substitute temperature > threshold value
FOR
- Required time > threshold value
- Frequency = defined value
Driving condition L (low cooling performance)
- Vehicle speed within a defined range
- Mass air flow within a defined range
AND
- Substitute temperature > threshold value
FOR
- Required time > threshold value
- Frequency = defined value
Malfunction Criteria
After engine start, the lowest and the highest sensor temperature are stored continuously. If the difference between the maximum and the minimum measured temperature falls below a threshold value after the enable conditions have been fulfilled, the sensor is stuck.
10.2.4 Engine Coolant Temperature Sensor, irrational signal: measured value of the sensor stuck within thermostat control range (stuck in range), P0116
Monitoring Strategy
If a cold start is detected, the engine coolant temperature is compared to a defined threshold value.
Typical Enable Conditions
- Engine-off time > threshold value (cold start is detected)
Malfunction Criteria
If the engine coolant temperature is within the thermostat control range when a cold start has been detected, the temperature sensor is stuck in this range.
10.2.5 Cooling System Performance, P2181
General Description
In order to heat the engine to its operating temperature as quickly as possible after an engine start, the coolant flow through the radiator is disabled during the engine warm-up phase. In conventional engine cooling systems, a thermostat is used, which lets the engine coolant flow through a bypass circuit instead of through the radiator, until it has reached a defined temperature.
Monitoring Strategy
The measured engine coolant temperature is compared to a defined threshold value.
Typical Enable Conditions
- Coolant temperature at engine start within a defined range
- Ambient air temperature or intake air temperature at engine start within a defined range
- Total air mass since the last engine start > defined threshold value
- The current mass air flow is greater than a defined threshold value
- Ambient air pressure change since engine start < defined threshold value (No downhill driving detected)
- Ambient air temperature change since engine start < defined threshold value
AND
vary between engine applications:
- For each operating phase with active deceleration fuel cut-off and each operating phase where the engine load was not in a defined range, the air mass taken in by the engine is calculated. The sum of these air masses is less than a defined threshold value.
OR
- The time periods of the operating phases with active deceleration fuel cut-off and the operating phases where the engine load was not in a defined range, are totaled. The sum of these time periods is less than a defined threshold value.
Malfunction Criteria
The thermostat monitoring checks if the engine warms up to a defined engine temperature quickly enough after engine start. If the measured coolant warm-up takes longer than expected, which is indicated by a defined temperature not being reached after a defined time period, a fault in the engine cooling system is detected.
When all enable conditions are fulfilled and a defined coolant temperature is exceeded, a mass air integral is continuously calculated. Once the mass air integral exceeds a defined threshold value, a sufficient air throughput for evaluating the coolant temperature has occurred. The air throughput is a measure of the energy input into the engine, which causes the coolant to warm up.
The threshold value of the mass air integral is calculated depending on the engine coolant temperature at engine start and the ambient air temperature at engine start. Since on hybrid vehicles and vehicles with a start/stop system, the engine temperature decreases during operating phases when the internal combustion engine is inactive, the calculation of the mass air integral is interrupted when the internal combustion engine is turned off.
When the internal combustion engine is started the next time, the calculation of the mass air integral is resumed with the value that was stored at an engine temperature equal to the current engine temperature before the last internal combustion engine turn-off.
A fault in the engine cooling system is detected if the measured engine coolant temperature is less than a defined threshold value after the required air throughput was reached. This threshold value depends on the ambient air temperature at engine start or on the engine coolant temperature at engine start. The fault is only registered in the fault memory if the secondary monitoring conditions at the time of the fault detection are fulfilled.
The secondary monitoring conditions are considered fulfilled when the following criteria are fulfilled:
- The mean average vehicle speed over the driving cycle is in a defined range. The defined range for the mean vehicle speed is determined from a characteristic curve that depends on the engine start temperature.
- The mean average mass air flow over the driving cycle is in a defined range. The defined range for the mean mass air flow is determined from a characteristic map that depends on the vehicle speed averaged over the driving cycle and the lowest occurring ambient air temperature in the current driving cycle.
10.2.6 Extended cooling system performance monitoring for cooling systems with thermal management
Applicable only for test group:
General Description
In order to heat the engine to its operating temperature as quickly as possible after an engine start, the coolant flow through the radiator is disabled during the engine warm-up phase. In conventional engine cooling systems, a thermostat is used, which lets the engine coolant flow through a bypass circuit instead of through the radiator, until it has reached a defined temperature. The thermal management additionally controls the coolant pump and several valves in the cooling system in order to accelerate the engine warm-up by regulating the coolant flow.
Monitoring Strategy
After a defined engine coolant temperature (ECT) has been exceeded, the mean ECT is continuously compared to a defined threshold value.
Typical Enable Conditions
- ECT at engine start less than defined upper threshold value (threshold value depends on the ambient temperature)
- ECT at engine start greater than defined lower threshold value
- ECT greater than defined threshold value (threshold value depends on the ambient temperature)
- Engine load greater than defined threshold value
- Time since last engine start greater than defined threshold value
- Time period of each "Engine stop" phase (start/stop system) in current driving cycle less than defined threshold value
- Number of "Engine stop" phases (start/stop system) in current driving cycle less than defined threshold value
- No existing "Cooling System Performance" fault memory entry
Malfunction Criteria
Engine cooling systems with thermal management use the regulation of the coolant flow to accelerate the warm-up of the engine. This type of coolant flow control system that is used in addition to the thermostat is capable of acting redundantly to the function of the thermostat and is capable of compensating for the effects of a stuck open thermostat. If the thermal management is active, an open thermostat therefore cannot and does not need to be monitored.
The conventional thermostat monitoring checks if the engine warms up to a defined temperature quickly enough. If the measured coolant warm-up takes longer than expected, which is indicated by a defined temperature not being reached after a defined air mass throughput, a fault in the cooling system is detected. However, if the thermal management is active when the fault criterion is checked, it is possible that a fault in the engine cooling system caused by a stuck open thermostat might not be detectable.
The extended thermostat monitoring amends the conventional thermostat monitoring and continuously monitors the mean ECT, after a defined ECT has been exceeded. It detects a fault in the engine cooling system when the thermal management becomes inactive and thereby the effects of a stuck open thermostat become present.
When a defined ECT has been exceeded and all other enable conditions are fulfilled, the extended thermostat monitor continuously calculates the mean ECT from the measured ECT. If the mean ECT falls below a defined threshold value for a defined time period, a fault in the cooling system is detected.
If the conventional thermostat monitor detects a fault and registers it in the fault memory, the extended thermostat monitor and the thermal management are disabled.
If the extended thermostat monitor detects a fault in the cooling system, the monitor disables the thermal management and itself. Therefore, an existing cooling system fault can be confirmed by the conventional thermostat monitor in the next subsequent driving cycle.
10.2.7 Engine coolant temperature sensor out of range check (P0118 / P0117)
The signal of engine coolant temperature sensor is evaluated and considered to be electrically out of range if either the upper or the lower thresholds is exceeded.