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E39 (1997 - 2003)
The BMW 5-Series (E39 chassis) was introduced in the United States as a 1997 model year car and lasted until the 2004 when the E60 chassis was released. The United States saw several variations including the 525i, 528i, 530i and 540i. -- View the E39 Wiki

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  #1  
Old 03-03-2011, 11:37 AM
stroatenmoaker stroatenmoaker is offline
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Does a defective thermostat affect your MPG

Thermostat will be replaced soon, gauge is not going further than 10 o'clock position, never reaches 12 o'clock. Does anybody know if a defective thermostate (always fully open) affect your MPG ?

Thanks.


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  #2  
Old 03-03-2011, 11:43 AM
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ahull ahull is offline
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I would guess yes - slightly, in that the engine does not get up to optimal operating temperature.
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Old 03-03-2011, 11:44 AM
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DominguesE30 DominguesE30 is offline
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If your engine is cold then yes, it will use more fuel. My dad once took the thermostat out of my mums magna and just run it without one. Ran like sh1t because it was always cold and the consumption went through the roof!


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  #4  
Old 03-03-2011, 11:46 AM
bobdmac bobdmac is offline
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It will hurt it somewhat because thermostats keep the engine at its most efficient operating temperature. Maybe someone with some first-hand knowledge can quantify it in terms of MPG per degree of deviation.
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Old 03-03-2011, 11:53 AM
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AnotherGeezer AnotherGeezer is offline
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Yes it will.
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Old 03-03-2011, 01:15 PM
cn90 cn90 is offline
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It depends on the mode of failure.

Stuck HOT: Engine overheats ---> $4-5K damage

Stuck COLD: Engine runs cold, fuel mixture is rich ---> lowering the fuel mileage.
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Old 03-03-2011, 02:37 PM
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bluebee bluebee is offline
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Quote:
Originally Posted by stroatenmoaker View Post
Does anybody know if a defective thermostate (always fully open) affect your MPG ?
Unless all your driving is under five or ten minutes, I can't imagine that first five minutes of slightly cooler engine temperatures mattering all that much to your gas miles per gallon given all the other variables in the mpg equation.

BTW, do you have the high cluster?
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Old 03-03-2011, 10:57 PM
stroatenmoaker stroatenmoaker is offline
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Quote:
Originally Posted by bluebee View Post
BTW, do you have the high cluster?
Yes, I do have the high cluster
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  #9  
Old 03-05-2011, 09:35 PM
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bluebee bluebee is offline
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Quote:
Originally Posted by stroatenmoaker View Post
Yes, I do have the high cluster
Then, even with the averaging gauge busted, you should be able to read two different KTMP figures (from the dual temperature sensor) on your cluster as shown in my post above.

- High instrument cluster check of the cooling system temperature KTMP (1)
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