# Thermostat Replacement



## dzlbimmer (Jul 16, 2017)

BB_cuda said:


> I changed the 335d when I got down to 78C/172F. After change out, temp increased to about 85-86C/186F. I didn't see any increase in fuel mileage. I've seen where many make point that lower coolant temp causes poorer fuel mileage. I didn't see this.
> 
> The X5 is showing about 79-80C/174-176F and I plan to hold off until more like 72C or lower prior to change. I hadn't seen any quantitative data showing cooler is worse so I will gather my own data. My 335d has ticked down to about 82-83C and I recall T/S was changed 2 yrs-2 months ago. I will have to pull the mileage traveled.
> 
> ...


The mileage decrease presumably is from the dde spending additional time in warm-up mode, this is under 75c. This increases your glow plug usage and increases injection quantity or injections quantity.

I will definitely be doing a water pump by 125k as I don't want it to be a weak link and a very expensive repair (relative to PM) if I get stranded out of town with kids.
In a warmer climate and less hardware, just under 75c going to be fine. Especially since you will be monitoring it and can do your own work.


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## BB_cuda (Nov 8, 2011)

dzlbimmer said:


> The mileage decrease presumably is from the dde spending additional time in warm-up mode, this is under 75c. This increases your glow plug usage and increases injection quantity or injections quantity.
> 
> I will definitely be doing a water pump by 125 as I don't want it to be a weak link and a very expensive repair (relative to PM) if I get stranded out of town with kids.
> In a warmer climate and less hardware, just under 75c going to be fine. Especially since you will be monitoring it and can do your own work.


"doing a water pump by 125" ? 125,000 miles ?


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