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crashmarks 9th September 2010 22:20

Diesel thermostat definative answer
Regarding my enquiries with the UK technical manager into whether BMW ever used a hotter thermostat in the M47R engine, this is the reply I have received. This comes from the BMW development engineers in Germany.

The M47R, as we called the engine, was different to the engine for the
320D. For the Rover ( 75) and Land Rover ( Freelander) application we
used the first common rail injection system.
I went back into my documentation and have spoken to the "older"
colleagues in the development about the "cold" running engine.
We have never used different Thermostat during the life span of the
entire M47 engine ( D and R), so we do not have different thermostats to
choose from. We have introduced different thermostats for the late M57
Anyway, we are sure, you cannot change the behavior of the engine with a different thermostat. At the time when we introduced the direct
injection diesel engine we had problems to get the engine to proper
working temperature, because it was so efficient.

We remember that we had big problems to get enough heat for the
passenger compartment heating, therefore we had additional heating.
Basically it would not help to get a different thermostat, because the
engine will not deliver more heat and therefore the operation
temperature of the thermostat does not matter.

I assume the cars are mostly driven in cities and will not get too much
load on the engines. There is no other way to get the engine to hotter temperatures as to put load on them ( faster driving or uphill driving !).
Sorry for this disappointing news , but if they want to play with
thermostats, then I would recommend that they will take one out of the
car and go to a spare part depot and have a look if they can find
another one from a different car, which will fit. At that time, the
supplier used mostly the same design for different car manufacturer.

BMW Group UK

So thats it boys back to the drawing board 9th September 2010 22:26

Hmmm... So what about partially blocking off the radiator? An old trick I know, but one that has worked for me in the past on other cars? :shrug:

Jules 9th September 2010 22:48

I bet the UK manager hasn't got a solution to the engine whine noise on the overrun either :shrug:

Thanks for speaking to him

crashmarks 9th September 2010 22:53

Having now pondered the reply i have just sent them another note asking what cars they put the M57D in. Having regard to the last paragraph of their reply, if the thermostat manufacturer used the same design and there was a 92 degree for the M57 engine it may fit ! Perhaps somebody who has a spare / old thermostat (which I dont) could pop into the BMW dealer and compare them. mmmmmm food for thought.

Frank Incensed 9th September 2010 22:58


Originally Posted by crashmarks (Post 585895)
Regarding my enquiries with the UK technical manager into whether BMW ever used a hotter thermostat in the M47R engine, this is the reply I have received.

Well done on getting the response, even if it's not what we were hoping for.

I'm still trying to find an alternative thermostat and I'm off to my local parts dept tomorrow, with what I've found. But BMW's reply that it would'nt make any difference doesn't bode well. This is born out by tales of good mpg obtained by members with stuck-open thermostats.

As for the motor being too (thermally) efficient, then why does it use so much diesel for the power output? As T-Cut said, it seems the cooling system is the thing that's too efficient! James' idea of restricting the air flow to the radiator has been put to me by others and currently seems the best solution. Perhaps someone can devise a way of diverting some of the air blasting through the grille into the air intake.

Jakg 9th September 2010 23:14


Originally Posted by Frank Incensed (Post 585930)
As for the motor being too (thermally) efficient, then why does it use so much diesel for the power output?

Anecdotal, tbh. What are you comparing it to?

There are 50 million reasons why car x will give more MPG than car y

Comparison wise, the VW Passat with the 1.9 (only 5% smaller engine...) TDI gives 50 MPG in 100 BHP guide, and 48 in the 130 BHP model... very similar to the slightly more powerful and slightly heavier Rover 75 / ZT.

HarryM1BYT 9th September 2010 23:33


My 2L 1993 petrol Granada seemed to be very thermally efficient. No matter how hard I drove it, I could never get it to fire up its cooling fans. One day, on an extremely hot day I did a fast 100 mile motorway run, then decided to check its cooling fans did actually work. I ran it straight into my garage, blanked the radiator up completely and left it ticking over. It took a further 30 minutes of this, before the first fan cut in.

Getting back to the 75's CDTi, I can understand that if it is very thermally efficient, it will simply take longer from cold to get up to temperature before the stat opens. The stat should then maintain the temperature by varying its width of opening. What I'm seeing via the OBD is the temperature rise up to the opening set point, the temperature then fall several degrees as the cool water starts to flow in, then rise back up to its stable running temperature. This is exactly what I have witnessed on other cars with a properly working stat and a temperature gauge which shows an accurate temperature.

I think radiator blinds fitted to the CDTi will have absolutely no effect. They were common in an era where radiators could freeze in cold weather and thermostates were not as efficient or accurately made as they are these days.

So the basic problem remains the CDTi's high thermal efficiency and its lack of waste energy to bring itself quickly up to temperature. 10th September 2010 00:30

The simple fact that loads of CDt's are fitted with FBH, and many have a non working fan, but still failed to overheat and sieze, did point us in this direction anyway.. :shrug:

So even if we fitted higher rated thermos all that will happen is it will stay closed more. ..

Matt1960 10th September 2010 07:49

I think its great that you got a reply from BMW, and not just a stock one, but one where engineers were spoken to.
However, I have long had doubts whether a higher rated thermostat would improve efficiency. I changed mine last year and my original temp was in the 60's (guage on 1/4). A rise of 20c, has not made much difference to mpg, if any. I doubt whether adding another 4c or more will be the answer to more efficiency. But I think its good for people to try.

Jules 10th September 2010 08:09

HI Matt you must have other issues with your engine ancillaries, as most other members have seen typical increases of 5 MPG and some have attained the magical figure of 50 MPG once again just like when the their Cars were new.

Back in 2005/6 I often used to see 60 MPG at 60 MPH driving many fairly new 75's back from lease companies.

Our Wedgie Blue Tourer AUTO attained 51.3 MPG on a French trip back in June!

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