Thermostat

Jono8

Jono8

Jono8

Caporegime
Joined
20 May 2007
Posts
41,032
Location
Surrey
The other day, i noticed my mpg was perhaps 1-2mpg lower than normal for no apparant reason.

I didnt think much of it, but i know a possible cause of this is a dodgy thermostat ( i.e. car not staying at full working temperature).

The car does warm up within what i believe to be a normal time and the temp guage goes to bag in the centre within 6/7 minutes or 2 miles, sometimes sooner and the guage will be in the centre the whole time im driving....EXCEPT, yesterday i noticed that on the long (1 mile+) steep downhill stretch on my commute where i am off throttle for all of it the guage dropped slightly (moved about 2-3mm to the left of the centre). When at the bottom, and back on the throttle the guage will go back to the centre and stay there.

Now, from my research, this indicates the thermostat is getting stuck open/partially open. Correct?
 
Yeah sounds like stat to me. Considering almost all water temp gauges are damped, the temp could be falling quite a lot for a small movement on the gauge.
 
Sounds perfectly normal operation to me!!
Of course the temp is going to struggle to stay up to temp if the injectors are shut and you're coasting down hill! Nothing is being burnt and no heat is being produced!
The likely drop in mpg is more to do with the weather conditions in my opinion!
Traffic going slower along with puddles creating more drag.
 
Hm. The same mixed responses that i am getting from looking over the web :p

I was of the understanding that once up to temperature, a thermostat should keep the car at the correct operating temperature regardless of what is happening?
 
Jono8 said:
Hm. The same mixed responses that i am getting from looking over the web :p

I was of the understanding that once up to temperature, a thermostat should keep the car at the correct operating temperature regardless of what is happening?
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It can't if the engine is receiving air cooling from wind and not creating any heat. It's normal. Mine wavers at times too. As long as it is staying around 90-95 especially when you trash it, I wouldn't worry. Or if it takes forever to warm up. The differences you talk about are minimal.
 
So you are saying that as long as it warms up ok and only drops a bit when off throttle/coasting in gear for a long period everything is ok?

Ive tested and last two times i have been up the car has got up to temperature within 5/6 minutes and 1.5-2 miles
 
Jono8 said:
So you are saying that as long as it warms up ok and only drops a bit when off throttle/coasting in gear for a long period everything is ok?

Ive tested and last two times i have been up the car has got up to temperature within 5/6 minutes and 1.5-2 miles
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Nothing to worry about. I had to replace mine because the temp would drop 20 degrees C on the motorway at 70mph, which is not good for the engine or the wallet.
 
Jono8 said:
So you are saying that as long as it warms up ok and only drops a bit when off throttle/coasting in gear for a long period everything is ok?

Ive tested and last two times i have been up the car has got up to temperature within 5/6 minutes and 1.5-2 miles
Click to expand...

It depends entirely on the car.
 
If the thermostat was faulty enough to reduce the mpg it would probably not reach temperature without sitting around for a long time, the last one i had to replace only ever reached a quarter up the gauge.

1-2 mpg is nothing to be honest, especially if its a dash read out, they're never accurate enough to prove that change anyway :)

Personally i wouldn't bother changing it unless it got worse or some other symptoms came up.
 
Tons of other normal things can cause an mpg drop, old oil/filter, air filter, plugs, poor injectors, tyre pressure, bad alignment
 
bledd. said:
Tons of other normal things can cause an mpg drop, old oil/filter, air filter, plugs, poor injectors, tyre pressure, bad alignment
Click to expand...

I know, but i just though that this dropping of temperature slighlty when coasting downhill for a while might be indicitive of something.

The consesus in here seems to be that it isnt and this is normal.
 
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andrewdodd13 said:
Only 1-2mpg in winter? My old car used to have a variation of about 5mpg on short trips between summer and winter; consider yourself lucky!
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Don't forget, in the winter you are more likely to be using lights and wipers heated windows etc etc for extended periods. The electricity to power them is not free! :cool:
 
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