how will gas mileage react to a stuck open T stat?

[Mordan]

looks like my T stat is stuck open as the needle doesn't go over 11h30.
or it is the engine temperature reader who is not doing well. ( i did an engine swap without checking it)

the top hose is burning hot so I'm not worrying too much. I just feel it drinks more gas. for educational purposes, how exactly the DME knows the engine has reached operating temperature (in order to stop sending more gas)? Is it a switch once the temperature reaches 12 o'clock?

thx

[Ross]

The gauge and ECU use different temp sensors. It will be less efficient if overcooling. I couldn't quantify how much.

[Mordan]

The gauge and ECU use different temp sensors. It will be less efficient if overcooling. I couldn't quantify how much.

ok so who is using what? what's the difference between

the Remote thermometre sending unit (part #12 62 1 710 512)

and

the Temperature sensor part (par #13 62 1 709 966)

[whiskychaser]

ok so who is using what? what's the difference between

the Remote thermometre sending unit (part #12 62 1 710 512)

and

the Temperature sensor part (par #13 62 1 709 966)

Not sure which engine you are talking about, but pull the plug on each sensor in turn and see which affects the needle. They are both just resistors but the resistance varies according to temperature. The ECU one is probably a lot more sensitive and wired in a loop-the ECU probably measures the voltage it sends out against what it gets back. The gauge one will be more primitive-the 'return' will probably just go to ground.

[e34.535i.sport]

looks like my T stat is stuck open as the needle doesn't go over 11h30.
or it is the engine temperature reader who is not doing well. ( i did an engine swap without checking it)

the top hose is burning hot so I'm not worrying too much. I just feel it drinks more gas. for educational purposes, how exactly the DME knows the engine has reached operating temperature (in order to stop sending more gas)? Is it a switch once the temperature reaches 12 o'clock?

thx

You should be able to tell by feeling the top hose as you say you have been. It shouldn't be getting hot until the engine reaches operating temp which is when the thermostat opens up... If it's starting to get hot straight away, which sounds like the case, the thermostat needs changing.

Well if it is the thermostat, you'll be using a lot more juice. Mine got stuck open for a while and it took a long time ro reach operating temp but it did eventually... My mileage went down enough for me to want to change it quickly! Luckily the new OE thermo has been brilliant, the car reaches operating temp within 1.5miles of 'light' driving, i.e shifting before 3000 revs.

Not a difficult job to do and well worth doing.

[Mordan]

You should be able to tell by feeling the top hose as you say you have been. It shouldn't be getting hot until the engine reaches operating temp which is when the thermostat opens up... If it's starting to get hot straight away, which sounds like the case, the thermostat needs changing.

Well if it is the thermostat, you'll be using a lot more juice. Mine got stuck open for a while and it took a long time ro reach operating temp but it did eventually... My mileage went down enough for me to want to change it quickly! Luckily the new OE thermo has been brilliant, the car reaches operating temp within 1.5miles of 'light' driving, i.e shifting before 3000 revs.

Not a difficult job to do and well worth doing.

yea I guess it is worth it if the car drinks 2 liters more per 100kms but still would like to understand the exact mechanism used by the ECU for reading the temperature.

[Tiger]

In colder climate, the engine runs cooler and the engine would burn more fuel as the ECM thinks the car is still 'warming' up.

On hotter climate, because the engine fluctuates from too cold to too hot, engine still burns more fuel... colder as in warming up...

Either case would also increase the chance your cat will burn up too.

Broken or stuck thermostat is usually very easy to tell... the needle swings like ... too cold on highway... too hot local driving.

[Mordan]

Either case would also increase the chance your cat will burn up too.

good point, which makes it an even more pressing issue.

thx wisky for your explanation. I will do the check by removing one plug at a time.

[e34.535i.sport]

IIRC... It is the sensor with the blue surround that tells the ECU (Blue for brain).

Yep... this confirmed: http://www.bimmernut.com/forum/showt...erature+sensor