[sagarpadaki]

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 -
https://boodmo.com/catalog/part-ther...assy-37465935/

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!

[Aditya]

Thread moved out from the Assembly Line. Thanks for sharing!

[Jeroen]

Yes, a low coolant’ temperature might affect your fuel efficiency. But I am surprised it was to the extend you noticed.

Far more important is the accelerated wear and tear of the cilinder wall and piston rings! 10 degrees too low an operating temperature is going to cause measurable wear very quickly.

It is caused as the lub oil will not adhere properly to the too cool cilinder liner.

I have done quite a bit of testing of this phenomena on marine diesels with very surprising results. It’s not that your engine will fall apart, but with a few degrees off you can detect additional wear very quickly!

When in doubt do as you did, check the temperature readings via ODB or with an IR thermometer.

On most cars changing out the thermostat is a very simple job. Not so on my Mini. Ask me how I know! ��

Jeroen
Jeroen

[svsantosh]

Originally Posted by sagarpadaki

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

..Went for a test drive and voila! Problem solved

Brilliant observation, shows you and your car have become Jake SULLY and Toruk MAKTO!

It is rare these days to have drivers sync with their cars at this level, am blown away Sagar.

Kudos to Ashton too stick it up (err, thumb will do) to the A.S.S and partnering to do a critical job such as Thermostat replacement.

You must open a thread, do it all over again with a camera/GoPro and record it in detail for others to benefit, please do it.

[sagarpadaki]

Originally Posted by svsantosh

It is rare these days to have drivers sync with their cars at this level,......

Thanks for your kind words Santosh.

The thermostat replacement in petrol 1.2 litre Revotron engine is very straight forward. Anyone can do it with a hose clamp plier and 10mm socket! It's that easy.

[abhijeetp]

Wow! Great observation. Thanks for sharing this in detail. Wonder what kind of servicing the ASS does if the owner is not aware of these things.

[NomadSK]

Another physical tell tale sign for the erratic behavior of the thermostat (in any car) is to check the heater inside the car, generally it will not blow hot air, but just mild warm air and you can trouble shoot it from there.

In addition, one could also test the thermostat working in the boiling water to confirm if its a kaput thermostat.

[Ashtoncastelino]

Thankyou for posting this thread Sagar, Whilst I too didn't know that a thermostat can Soft fail, it was quite a learning experience. I thought it might be a one off case with your Tiago, I recently got my collegues 2016 Tiago for a front Suspension overhaul. That Tiago as well had the EXACT same symptoms along with Reduced fuel economy.

After replacing the Thermostat valve, the issue got resolved and my friend noticed a significant bump in Distance to Empty range on the cluster. Classic case if FE is lower is to clean the throttle body, injectors and replace fuel filter but look at how this case turned out. I wonder if more Tata cars are affected by this thermostat valve issue.

Attaching photos of the work performed on my friend's Tiago

1Litre OEM Tata Coolant and Thermostat Valve


Old Thermostat Valve ( 0 Visual Defect )


In goes the new Coolant.

Regards
Ashton Castelino

[srvm]

Originally Posted by sagarpadaki

....
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.
........

Thank you so much for this post .
I have the same symptoms and low FE in my 2016 Zest (Petrol).
Now I'll have to figure out what to do.

[sagarpadaki]

Originally Posted by Jeroen

Yes, a low coolant’ temperature might affect your fuel efficiency. But I am surprised it was to the extend you noticed.

Actually, I was confused with the low fuel efficiency. I did not realise the connection between the faulty thermostat and low fuel efficiency. I realized something was wrong when I did a round trip of around 300kms to Mysore which has a nice expressway. I did a tankful of petrol before the journey. At the end of the round trip, the low fuel light came in. Usually a tank full will give a range of around 500 kms on the expressway! And at expressway speeds the coolant bar remained at one bar through out the expressway. This is when I connected 2 and 2 and realized something is seriously wrong.

[freelancer6202]

Is this thermostat housing located in upper hose or lower hose ? I too replaced thermostat in my lancer due to exact same problem. In Lancer 2003 petrol, thermostat housing is located at the upper end of lower hose.

[sagarpadaki]

Originally Posted by freelancer6202

Is this thermostat housing located in upper hose or lower hose ? I too replaced thermostat in my lancer due to exact same problem. In Lancer 2003 petrol, thermostat housing is located at the upper end of lower hose.

Thermostat is located in upper hose

[freelancer6202]

Originally Posted by sagarpadaki

Actually, I was confused with the low fuel efficiency. I did not realise the connection between the faulty thermostat and low fuel efficiency. I realized something was wrong when I did a round trip of around 300kms to Mysore which has a nice expressway. I did a tankful of petrol before the journey. At the end of the round trip, the low fuel light came in. Usually a tank full will give a range of around 500 kms on the expressway! And at expressway speeds the coolant bar remained at one bar through out the expressway. This is when I connected 2 and 2 and realized something is seriously wrong.

If thermostat fails to open then car is going to overheat after few kms. If thermostat is stuck in open position then coolant will keep on circulating and it wont reach optimum temp. for normal running and computer will inject more fuel for proper combustion and that will cause fluctuations in temp. gauge and after a certain time car will eventually overheat. A keen driver who keeps a track of fuel consumption regularly will certainly notice change in fuel efficiency and delay in temp. gauge needle reaching normal temp./fluctutions range. I always track fuel efficiency and temperature reading gauges while driving after past experiences.

To the OP : It was very interesting to go through the post. Very detailed and informative. Dealing with mechanics in these busy times is a hassle. But older cars and less technical cars are easy to fix as a DIY like my lancer but mordern cars are soo complicated with all these interconnected electronics and risk of voiding warranty is one prime reason I like sticking to my 25 year old lancer as a daily runner.

[icem@n]

I faced the same issue with my now sold 2016 Tiago. This issue came up around 48K Kms/4+ year. Raised it multiple times with TASS without any response. I gave up. Thanks for sharing!

[lemedico]

I've had a chat with Ashton a few days ago about the same fuel efficiency issues on my 2021 Tiago. Being an AMT, I make that the culprit for the chronic single digit FE. But now I must consider that this small springy bit is the real devil!

Could you confirm that the part in the boodmo link is applicable for the bs6 revotron engines? Thanks