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Thermostat solutions from Dayco

Few influences on the engine are quite as critical, and have so many repercussions throughout the overall system, as its operating temperature. Maintaining the correct temperature in the various parts of the engine not only optimises fuel efficiency and minimises emissions, but also ensures the oil is at its most effective in lubricating and protecting the internal components, for example.

In all but the most modern combustion engines and electric-hybrid vehicles, the temperature of the coolant sent to the various parts of the engine is controlled by a thermostat/thermostats. So, although often overlooked, they are incredibly important components, which means that when they fail, original equipment (OE) quality replacements must be the solution of choice.


Despite being designed run at around 90degC, prolonged overheating will not only affect the performance of the engine and the emissions it emits, but also potentially cause damage to its internal components. Naturally, there can be several reasons to engine overheating, but the thermostat should always be considered because, although generally reliable, evidence has shown that with their growing complexity, failure rates are increasing.

In fact, AASA 2020 report data has shown that over a five-year period from 2014, thermostat failure rates have steadily increased to 5%. At the same time, the percentage of vehicles fitted with a housing design, rather than a traditional valve thermostat has grown from 55% to 80%.

Looking at the UK’s best-selling cars in 2018, which are now reaching the age to be entering independent workshops, their thermostats perfectly reflect this 50/50 split between traditional valve and more modern housing type designs.

1.            Ford Fiesta – 77,833 registrations – Valve design

2.            Volkswagen Golf – 58,994 registrations – Flange design

3.            Ford Focus – 56,619 registrations – Thermostat housing

4.            Vauxhall Corsa – 54,239 registrations – Flange design

5.            Mercedes A-Class – 53,724 registrations – Thermostat housing

6.            Nissan Qashqai – 52,532 registrations – Flange design

7.            Ford Kuga – 41,671 registrations – Thermostat housing

8.            MINI – 41,188 registrations – Thermostat housing

9.            Volkswagen Polo – 37,543 registrations – Valve design

10.         Kia Sportage – 34,502 registrations – Valve design.


Alongside a growing failure rate, other reasons for overheating problems can come from air introduced into the system during a water pump replacement, for example, not being bled out properly.

If an airlock then subsequently occurs, particularly if it is around the thermostat, it can become locked in one position, which will prevent the correct circulation of the coolant and cause the engine to overheat. It is always therefore wise for technicians to vacuum fill the cooling system following any work that is carried out to it.

Although a traditional valve type thermostat can still be checked relatively easily by testing whether it opens when in a pan of boiling water, which is a useful way of narrowing down the identification of the underlying overheating fault, due to their relatively low cost, it is strongly recommended that they are replaced even if they operate during such a water test.

However, if the thermostat is integrated inside the water outlet and cannot be removed, the boiling water test clearly isn’t possible and diagnostic tooling is required to determine whether it has failed.

Vehicle manufacturers began using integrated housing thermostats in the early 1990’s and a decade later they were installed on approximately 30% of all new vehicles. As mentioned earlier, this design has become increasing popular, with more than 80% of today’s new vehicles coming off the assembly line equipped with this style of thermostat.

Dayco’s extensive technological know-how and drive to ensure its products deliver to the highest levels of performance means that a combination of precision couplings and quality materials has enabled the company to produce thermostats that are the equal of OE products in their quality, safety, efficiency and durability.

For more information regarding OEM quality thermostats or the power transmission products in the Dayco range, please email: or visit:

About Autotechnician
Autotechnician is a magazine published nine times a year, delivering essential information to independent garage owners and technicians in the UK. Delivered both digitally and in print, autotechnician provides readers with technical, training, business advice, product and news, allowing our readers to keep up to date with information they need to run and work within a modern workshop.
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