2017 Hyundai Elantra 2.0L - ECT operational theory

[Lux]

I was a Hyundai automotive service technician until last year, so I will chime in.

A technical service bulletin is presently live (since 2016 August) for certain 2017 Hyundai Elantra models . I know, with certainty, that the the campaign affects Hyundai Elantra AD frame code. The GD frame code for Elantra GT models are not likely affected. I can't remember.

The campaign addresses erratic or unresponsive behaviour of the newly equipped electronically controlled thermostat. The ensuing symptom is excessively prolonged or delayed engine warm up, sometimes producing the diagnostic trouble code P0128. The repair is a new thermostat and housing complete assembly covered under warranty. The cost of coolant is not covered; technicians are expected to recover lost coolant from the engine during the operation and refill the cooling system with it. Common sense would oppose that... New coolant can be requested at the customer's expense.

Electronically control thermostats are employed to confer computer control over thermostat activity. By moving away from mechanical thermostats, engine emissions can be more tightly controlled by influencing combustion temperature by water temperature. Electronic control failure from the in-built thermostat heater element will regress it to mechanical control, anyway. So, there is no risk of catastrophic engine damage from runaway water temperature.

Your undulating waveform in water temperature is normal for this type of system. The engine ECU must observe changes in water temperature in accordance with cyclic duty of the thermostat heater to verify its operation. This is a self-monitoring, self-diagnosis strategy.

Comfortably speaking, until now, the ensuing overheating caused by conventional thermostat failure may not always be caught in time. So this new feature could be seen as a blessing with regards to self-preservation.