One of the more common reasons of overheating is a stuck thermostat. One of the most common reasons for blown head gaskets is overheating. One of the most expensive vehicle repair procedures is head gasket replacement. Most thermostats, on the other hand, cost around $20 and the replacement is well within the DIY domain.

Why isn't there a sensor on the thermostat to detect opening and closing and notify the ECU, which, in the absence of an expected pattern in thermostat activity logs, relative to other related factors, would raise a red flag in the form of an engine code to alert the driver to stop driving until the thermostat is fixed?

I imaging something like that should be relatively easy to record and preventively alert the driver.

  • I'm surprised they don't use the ECU to regulate the temperature in the engine. Even so, when a thermostat works, it does the job very well. They most often last so long (years upon years), are cheap, and are usually easily replaced. This means a new, untested technology to regulate the coolant flow is not going to be one of the things to be upgraded in engines. It just doesn't make economical sense. Jul 1 '15 at 17:09
  • The ECU will typically shut down the engine at about 250*F. However, if you are low on coolant, it may have trouble reading the temperature accurately. Also, I would blame the water pump, leaking hoses, and corroded and/or leaking radiators for being much more common than a bad thermostat.
    – rpmerf
    Jul 2 '15 at 11:57
  • 1
    @rpmerf - Do you have a source for your statement "The ECU will typically shut down the engine at about 250°F"? Jul 2 '15 at 12:02

It might seem easy to integrate a sensor that detects the opening/closing of a mechanical thermostat's pintle (LVDT, for example) but here are some challenges which would need to be addressed:

  • Cost vs Benefit

    Is a "smart" thermostat really worth the extra hassle and money when its dumb counterpart has the following to offer? :

    • it's already cost-effective
    • it isn't usually failure-prone
    • you can tell if it fails stuck-closed because coolant temperature will climb
    • you don't worry too much if it fails stuck-open as there isn't any serious risk of short-term, catastrophic engine damage (engine runs cooler than it should, HC emissions will worsen, maybe clogged secondary air tracts and cats in case of serious neglect over a sustained period)
  • Operational challenges

    Remember the thermostat is deployed in a relatively hot and wet environment, which isn't really ideal for electrical systems due to material and hazard-related challenges.

    It would be difficult to justify the introduction of an early-warning sensor at the expense of reliability and part longevity given the application's criticality.

  • KISS principle

    Beyond serving as a diagnostic aide, I cannot think of any additional benefit that modern-day engine management could reap from knowing the thermostat pintle's position. One could correlate it to coolant flow rate but we don't really care about that1; its temperature is sufficient and we already have a sensor present for that purpose.

    Given this and the thermostat's reliability, design engineers would be more than happy to delegate the responsibility of coolant temp regulation to a mechanical system and take its operation for granted.

1 - some people do, but this is more of a nice-to-have than something mission-critical

  • 1
    I'm not asking for thermostat to measure temperature, just to report to the ECU every time it opens and closes so that the ECU can raise an alert if no such signal has been reported some time
    – amphibient
    Jul 1 '15 at 16:56
  • @amphibient : Updated.
    – Zaid
    Jul 2 '15 at 3:39
  • Nice update ... +1. Jul 2 '15 at 12:00

As per your question the issue is about overheating. For that we already have a coolant temperature sensor, which can alert the driver when there is over heating. For your kind information there are electrically controlled thermostats in some modern engines of Mercedes and maybe in some others too!!!

  • there can be many reasons for overheating beyond the thermostat.
    – amphibient
    Jul 1 '15 at 21:02
  • I wrote the answer based on your first line in the question :) Jul 1 '15 at 21:03
  • which starts with "one of the ..."
    – amphibient
    Jul 1 '15 at 21:04
  • Anyways coolant temperature sensor can detect over heating due to stuck closed position of thermostat Jul 1 '15 at 21:06
  • yes but it just shows as overheating and not overheating due to a specific reason that is one of the many possible. i don't see why the specificity is so hard to understand
    – amphibient
    Jul 1 '15 at 21:21

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