Poor mileage on my Tiago: How I solved an issue even the ASC couldn't

Tata owners who are getting poor FE from their Nexon, Tiago, Tigor, I suggest you observe the coolant temperature gauge on the cluster to see if the described erratic behavior is seen.

BHPian sagarpadaki recently shared this with other enthusiasts:

Soft failure of thermostat in Tiago, Tigor, Nexon

This thread is to highlight the partial failure of thermostat in the petrol and diesel variants of the Tata Tiago, Tigor, Nexon cars which results in low FE and also can lead to potential wrong diagnosis for the low FE. The issue was present in my 2018 Tiago done around 25k kms.

On Tiago, three bars on the temperature meter in the instrument cluster indicates that the engine coolant has reached the optimum operating temperature. Usually, it takes 3-5 kms of driving for the coolant to reach optimum temperature of 92-degree Celsius from a cold start. Since October 2023, I noticed that the temperature bar did not reach 3 bars but was stuck at 2 bars unless in a bumper-to-bumper traffic, in which case, it would reach three bars. Once the roads opened up and the car gained speed for a while, the temperature dropped back to two bars. During early morning cold starts, where the ambient temperature was around 18 degrees during winter, the temperature meter did not even reach two bars even after driving for 5 kms. Also, started to notice that the fuel efficiency has reduced. I was getting just under 10 in normal traffic conditions and in bumper to bumper started to get 7-8 kmpl. Even on highways, the fuel efficiency dropped by 2-3 kmpl.

I discussed this issue with my friend Ashton, who is also a bhpian. We thought the reason for the temperature bar not reaching 3 was most likely due to a faulty thermostat. And since the engine was not reaching the optimum temperature, the ECU was increasing the fueling, thereby causing the drop in fuel efficiency. Ashton gave a link to a discussion in a BMW forum, where the soft failing issue of thermostat of a 3 series was discussed. A soft failing thermostat is where, the thermostat does not open or close according to the temperature range it is supposed to work. It remains open even after the coolant temperature has reduced below the thermostat's closing temperature, thereby causing the coolant to flow into the radiator and get cooled further, thereby reducing its temperature. The thermostat does not work consistently.

Ordeal with Tata Motors service - Key motors

I gave the car to Key motors service, Kanakapura road in the last week of March, explaining the issue in detail to the Service advisor whose name started with B(Do not want to take his name here). The next day, the Service advisor called back saying everything is fine and there is no issue with the thermostat nor with the fuel efficiency . I was really annoyed and asked him to explain what was the problem that I had. He was not even able to explain the problem that I had described him the day earlier! In fact, I had written the problem elaborately in the job card. I went to the service center and took a test drive with the Senior tech with their laptop hooked up to the OBD scanner and monitored the coolant temperature. There was fluctuation in the temperature reading where the coolant temperature dropped as low as 82 degrees with AC on a hot summer day where the outside temperature was 36 degrees. The coolant temperature meter dropped to 1 bar 2-3 times during the test drive of over 10 kms. It did reach 92 degrees in B2B traffic with coolant temperature meter registering 3 bars. The senior tech reluctantly agreed that there is something wrong with the thermostat and told they will place order for the same. He also added that, there have not replaced a thermostat in any Tata vehicle till now irrespective of how many kms they had run! I called them at least 20 times over the next couple of weeks to check if the thermostat has arrived or the ETA. They just gave a canned response. I did not even get one call back from them. Pathetic diagnosis of the problem and on top of that heights of incompetency! Customer service- what is that?

DIY to the rescue

I discussed this with Ashton and decided to order the part from Boodmo and do a DIY thermostat replacement. This is the part

Placed the order and the part arrived in 5 days. I replaced the thermostat- hardly took 30 mins. Went for a test drive and voila! Problem solved . The coolant temperature reached 3 bars within 3 kms of my drive and remained there. I also hooked up a generic OBD2 scan tool and monitored the coolant temperature in the Torque app. The temperature varied between 88 to 92 degrees and dropping to 86 on open roads. 86 degrees registers 2 bars in the console. But, within a minute in B2B traffic the temperature was back to 92/93 degrees. I monitored the FE over the next tankful and the FE went back to normal levels of 12-13kmpl in city and 17+kmpl on highways.

Conclusion

Tata owners who are getting poor FE from their Nexon, Tiago, Tigor, I suggest you observe the coolant temperature gauge on the cluster to see if the described erratic behavior is seen. Tata removed the coolant temperature meter from Tiago/Tigor sometime after 2018, I think. In that case, hook up any generic OBD scanner and monitor the coolant temperature to check if the coolant is reaching the optimal temperature of 86-92 degrees and staying in that range before you consider any part replacement to address poor FE. The same thermostat is used by Tiago, Tigor and Nexon so there is a good chance that these cars can face this issue. I do not know if the issue is with a batch of thermostats or all. Also, this issue, highlights and reinforces the need to have a functional coolant temperature meter in the cluster of all cars!

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