Folks, there is lots of talk on forum about thermostats being changed because temperature was not upto spec and how MPG was poor. People then ask how do I change them and then change them.

No one seems to have come back and say hey yes temperature isnow 90 C and my MPG has shot up to x,y,z MPG.

There is one post fron SANJSANJ but the figures are much too high and people are questioning them.

I mean if people say they were getting 20 MPG before thermos change...surely after 3 or 4 weeks of their normal driving they will be able to get a realistic (average) figure post thermos change to get a like for like answer for post MPG.

If it does not make much differance to MPG and car runs at around 75-80 C then why bother with changing the thermos. DPF will be looked after as regens will happen around 75 C.

Just my humble 2 cents worth of observations

 

I just changed the thermostat in my '04 545i. I can report that it is a major inprovement in that my SES light is now off Really, I would think people change them simply because they fail and for no other reason.

 

Its only really the diesels that are affected by poor thermostats. Without having a temperature reading a lot of people are driving around with the engine temp in the mid 60s to 70 degrees instead of reaching 90 degrees were its supposed to run. At these temps the engine really runs inefficiently and the mpg will suffer, the engine will also run dirty at these temps and I'm sure that this is some of the problems with people also having early dpf failures. Now there are some people who don't check their cars temps and things and the first time they know the thermostats are playing up is because they are reading about poor mpg or they have other problems. Then there are people like me who every now and then check the engine temp and I noticed my stats were playing up when on a cruise my car wouldn't reach 91 like normal and sat between 82-84 degrees. Now by replacing my stats there is no difference in mpg because I haven't waited until there were issues. If the car was running in the 60s the engine will run a lot better and more efficient with new stats.

 

Roger that. I definately missed the part about it being a diesel thing. Makes sense.

 

It's not just diesels though. I had a stuck thermostat on my 525xi (US, gasoline). Temps would usually be around 60-70C, and I'd have trouble getting MPGs over 19. I used to get around 25ish. After I replaced it, they went back to their normal of 25-ish MPG and temps returned to the 90~100 C. Obviously not as dramatic of a difference as a Diesel with a stuck thermostat, but 5MPG on a daily basis still makes a difference.

 

Thats true all thermostats fail after time. The diesel ones are much more of a week link than the petrols. A lot of the petrol ones are controlled by the ECU so throw a fault if the ECU can't regulate the temp.

 

I have had my car for 5 months and when I got it I could get 44mpg on the motorway driving it like a grandad. About two months ago I got a diesel leak that was fixed by an Indy in Sheffield. I then got the DPF warning message on the i drive, the mpg has dropped to 34 and the car feels flat. After much reading on here I tried the hidden menu and the temperature was only getting to 72..I changed the EGR thermostat and the temperature went up to 81, I then changed the main thermostat and I'm now getting 90 but my mpg is still 34 and the DPF pressure is high so that might be the mpg problem.
In conclusion I think the thermostats failing prevents the DPF from regenerating, the first time you know anything about this is when the DPF warning message comes up and by that time your mpg is falling and its too late to force regeneration because the DPF pressure is too high...Oh and E61 Beemers are crap:O(

 

The DPF affects the mpg more than you think. After all, it is choking the engine.

Anyway, it's a shame you caught the regen problem so late. Your only way out now is to remove the DPF and clean it with a pressure washer. There's an awesome guy here called Anzafin who wrote a good DIY on it, and if you ask nicely he might even release the patent on his DPF Drying System® .

It's either that, or get a new DPF for £1500, or remove it completely and get the car reprogrammed.

 

I changed both thermostats on my '05 530d. Temps went from 65c to 90c and motorway MPG went from 36 to 42 at 70mph.

 

Very similar