I have a 2003 cooper S, I have a external water temp monitor which used to stay within 90-93C (200F)after fully warmed up( about 5 min drive),
but today which show only 60-65C (150F) on it, and I am pretty sure the monitor work fine as I noticed the guage on the speedo is only 1/4( which normally stay in the middle all the time), I've check the coolant reserve tank and which looks just normal, coolant level is fine, seem no leaks ( I think the water will goes up even if it is leaks i think ) so I have no Idea why would the water temp drop so sudden and seems it is always 30c cooler than it should be....
I did re-set ECU and it doesn't help, would it be any failure in sensor ?
what should i check first? and what should I focus on??
Anyone got same trouble before?

Many thanks

 

sounds like a thermostat stuck open. And IMO anything below 165 is not good, too cold for good combustion , is your MPG going down ?

 

What running too cold does is lets the oil sludge up. Over the long term this will really screw up the engine. Time for a thermostat & a coolant flush.

 

didnt notice any change on MPG as it just started going wrong yesterday

 

Should i simply replace the thermostat ? Is #11537596787 the correct item i look for?
Should i replace the water temp sensor as well ?