Diesel thermostat definative answer

[Frank Incensed]

Originally Posted by crashmarks

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]

Originally Posted by Frank Incensed

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]

Interesting!

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.

[James.uk]

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..

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

[Matt1960]

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.
Matt

[Jules]

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!

[Matt1960]

Thanks Jules,
But I have changed maf sensor, intercooler rings, filters, pcv, egr clean, etc. to try to improve matters. I can get 40mpg at 60mph on a motorway, but nowhere near 50mpg.
I also drive with a fairly light foot, and my area is not hilly.
Yet to test the injectors for leakback, so this may be an option. But I did try injector cleaner.
Matt

[Jules]

Mmm yes you must have less hair than me now Matt!
The performance and economy varied widely even when they were new.

Some were lively, and others were sluggish.
However recently we cured a very sluggish ZT 131 with 50K miles on it.

After much deliberation I advised the owner to go for a punt & to re purchase the XPart 131 remap CD for his VIN number.
We programmed it in and guess what ...............CURED ...like new again.


So it seems either it wasn't mapped properly from new

Or

The mapping had somehow in it's life got corrupted.
I suspect theres a few similar owners Cars driving around with the same Mapping problems

[BarryH]

Originally Posted by Matt1960

Thanks Jules,
But I have changed maf sensor, intercooler rings, filters, pcv, egr clean, etc. to try to improve matters. I can get 40mpg at 60mph on a motorway, but nowhere near 50mpg.
I also drive with a fairly light foot, and my area is not hilly.
Yet to test the injectors for leakback, so this may be an option. But I did try injector cleaner.
Matt

Hi Matt,I improved my mpg from 39 to 44 by having a full service,changing discs and pads all round(needed doing anyway) and renewing the maf sensor.I live in a rural area which is quite hilly and although I can`t get up to 60mph very often I am very pleased with 44mpg.Would love to take the car on a long run to see if I can improve on 44 but not possible at this time.In my opinion changing the stat won`t make any difference to mpg.
Regards
Barry

[Livbrown]

I have fitted the Renault 5 inline thermostat and works a dream.
It's a bit fiddley to get into the top hose. But once it is in its great and it only cost £11.25 including 2 50mm jubilee clips.