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320d e91 poor fuel consumption - again

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320d e91 poor fuel consumption - again

Postby freefall_junkie » Sat, 04 Apr 2009 15:28:14 UTC

Had my 2006 320d for about 5 weeks now and the fuel consumption continues to be very poor. The last tank returned only 38.1 measured by the computer, about 41 by my own calculations on the actual fuel used between fill ups. This is with little driving in town and a generally gentle driving style. Based on lots of reading on the forums I had become convinced the MAf was the problem - the idle was slightly rough and disconecting the MAF made no difference to the way the car drove. So I replaced the MAF with a brand new Bosche one (20 minute job, but the part was £140) and........in the first 100 miles it makes no difference :evil: . Mega disappointed! It has been suggested to me that the ECU may take a while to 'learn' to talk to the MAF again. Sounds like tosh to me - what do you think folks? Running out of ideas, and the car is out of warranty, so any other suggestions gratefully received before I am forced to pay a BMW dealer huge sums to look at the car. Surely 38mpg can't be as good as it gets? My wife's petrol engined Megane estate gets more than that :(

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Postby tipper » Sat, 04 Apr 2009 15:50:02 UTC

I get up to 50mpg on my 2007 320d auto on a long motorway run, ie 200+ miles.

Drops really fast around town and seems only a little traffic driving drops it to to low 40s. After several local driving days it settles to 38-40 or so, so it really depends how much local driving you do.

Do a long run test to see what it really is capable of. Reset the computer once you are on the motorway and then you will see its best consumption.

Also mpg drops very fast the faster you go. The above figure is cruising at around 70mph. I find that 10mph faster equates to 10mpg lower mpg.

I also find varying consumption depending on travelling north or south on the M5....I assume that's to do with the prevailing headwind! :lol:
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Postby freefall_junkie » Sat, 04 Apr 2009 17:20:37 UTC

Thanks for the reply. I am doing a 300+ mile round trip from Scotland to Lancashire tomorrow, nearly all motorway, which will be the first really long drive I have done in the car. I guess that will reveal whether my expectations for daily local use are unrealistic or if the car does indeed have a problem. We will see...

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Postby speedman » Sat, 04 Apr 2009 21:17:50 UTC

Hi, I am from Malta and as such do almost always town driving and bumper-to-bumper driving (especially in the morning going to work). The average speed on the OBC is less than 26mph, but I have a constant economy of 47mpg.

Mine is a 320D E90 Dec 07 model (57 plate, built in Sep 2007, 177BHP)

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Postby freefall_junkie » Mon, 06 Apr 2009 09:28:24 UTC

Right, the results from my long trip have me even more puzzled. 150 miles on the way there, with the cruise control set at 86mph almost the whole way the OBC says the car returned a fairly dire 36.6 mpg. Exactly the same journey on the way back, it said 41.2 mph. Can anyone explain this? Driving style does not come into it as it was almost all on quiet motorway with the using cruise control set at the same speed both ways. The only difference was it was pretty cool on the way down - between 2 and 5 C, much warmer on the way back, between 10 and 14C, although I can't see why this would make such a difference.

So what to you reckon guys, does it sound like my car has a problem which I should get checked out by a dealer? And why the odd variation in consumption. Advice very welcome :?

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Postby freefall_junkie » Mon, 06 Apr 2009 09:30:04 UTC

That should be 41.2 mpg on the way back :oops:
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Postby tipper » Mon, 06 Apr 2009 10:48:34 UTC

Cooler air will produce more power but requires more fuel to do so. ( the basis is that cooler air is more dense and so contains more oxygen but more fuel will need to be injected to match the increase in oxygen in the combustion process)
Maybe that is the reason?

Also if you cruise that fast you will be using loads of fuel due to the increasing wind resistance as you go faster.

In addition I find cruise control is 'heavy footed' and I can achieve just as good speeds but much better economy with my own right foot feathering the pedal! It seems the human brain is better than the cruise control computer, if it has one which I suspect it doesn't.
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Postby agh » Mon, 06 Apr 2009 11:34:12 UTC

freefall_junkie wrote: - between 2 and 5 C, much warmer on the way back, between 10 and 14C, although I can't see why this would make such a difference.

I've owned my 320d through 3 winters and I recon your getting a 10% difference in fuel consumption just from the temperature difference.
I recon cold air rushing throught the engine bay is going to take away a lot more energy when the air is 2DegC compared to 14DegC plus your probably running the interior heaters more at 2DegC. That energy can only come from the fuel.

This said the fuel consumption you are getting does sound a bit low. 85mph cruising Carlisle - Manchester - Birmighan is something I've done and seen 45mpg and I have the same 163bhp engine as you.
Short trips around town 38mpg or worse is to be expected. I'm doing a short trip 6 miles each way to the railway station on open road right now and seeing around 41-42mpg, was 38-40mpg then the weather was colder Jan/Feb.
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Postby Peter » Mon, 06 Apr 2009 21:14:11 UTC

freefall_junkie wrote:Right, the results from my long trip have me even more puzzled. 150 miles on the way there, with the cruise control set at 86mph almost the whole way the OBC says the car returned a fairly dire 36.6 mpg. Exactly the same journey on the way back, it said 41.2 mph. Can anyone explain this? Driving style does not come into it as it was almost all on quiet motorway with the using cruise control set at the same speed both ways. The only difference was it was pretty cool on the way down - between 2 and 5 C, much warmer on the way back, between 10 and 14C, although I can't see why this would make such a difference.

So what to you reckon guys, does it sound like my car has a problem which I should get checked out by a dealer? And why the odd variation in consumption. Advice very welcome :?

Mark


In my experience (and others who have run tests in various ambient temperatures), many diesel engines are running just 'too cold' at low ambient temperatures to be most efficient. Don't mix petrol air ramming/cooling theory with diesel's compression ignition characteristics. Chilled cylinder walls can throw any air flow theory sideways, when it comes to AIT. I'd put my money on most diesels running best for economy at around 10 - 14 degrees centigrade ambient. Assuming the OEM intercooler is clean, inside and out, and no airflow restrictions. Personally it's my experience and anyone I know who bothers logging efficiency/mpg.

I'd like to be convinced otherwise, but do know some 'Big Rigs' in the colder parts of the USA and Canada run AI heaters to get combustion efficiency in colder winter months. Or they lose power and run smoke.

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Postby sanjeev » Mon, 06 Apr 2009 22:18:26 UTC

I have a 2004 150bhp Manual, and in the recent almost arctic conditions we have had, it has been returning around 28.5 MPG. My journey and driving style is the same, I.E. 17 mile round trip daily in 100% town driving with OBC computer showing an average speed of 17 M.P.H.

But recently as the weather has been warming up, my MPG has risen dramatically to about 34 MPG, so in my case the ambient temperature does play a significant part in my fuel costs, also worth checking that your tyres are correctly inflated!

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Postby shennas » Tue, 07 Apr 2009 10:12:27 UTC

sanjeev wrote:I have a 2004 150bhp Manual, and in the recent almost arctic conditions we have had, it has been returning around 28.5 MPG. My journey and driving style is the same, I.E. 17 mile round trip daily in 100% town driving with OBC computer showing an average speed of 17 M.P.H.

But recently as the weather has been warming up, my MPG has risen dramatically to about 34 MPG, so in my case the ambient temperature does play a significant part in my fuel costs, also worth checking that your tyres are correctly inflated!

Regards.
Sanjeev.


I have a 318D, thu i suspect mpg should more o less be the same. During the summer I manage an average of 50mpg i summer and winter it's been 48.7. Thats a mix of 70% (85mph) motorway and 30% town.

Check your tire pressures is the easy one.

Make sure the brakes are not stick on (feel hotter than expected after a long trip)

After that it's servicing. You could check the air filter and the EGR. Testing the injectors is a bit harder and is worth a trip to a good Ind. Physical clean of the EGR and injectors may well improve matters, I am not one for the magic solution added to your fuel tank.
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Postby freefall_junkie » Thu, 09 Apr 2009 13:26:04 UTC

Quick update on this. Just filled the car up and worked out the real MPG for the last 500 odd miles. The OBC indicated a deeply unimpressive 38.7 average, the actual figure is a whole 4mpg better at 42.7. Still not brilliant, but I guess not too bad either considering 300 miles of that were on the motorway with the cruise set at a highish 86. Odd that the OBC is so far out - I thought these thing were usually on the optimistic side if anything, and surely you would expect the OBCs to be consistently out on the same model of car :?

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Postby pirhoney » Thu, 09 Apr 2009 16:09:29 UTC

I wouldn't say there was anything wrong there. 42.7 when 300 out of the 500 were at 86mph. You've done pretty well IMO.
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Postby woody37 » Thu, 09 Apr 2009 17:10:10 UTC

I have a 2003 320d. I'm averaging between 38-50 miles depending on driving type and style. I have done a few short motorway journeys this week and average over 50 mpg this week.

I have put a new inlet manifold on and fitted an EGR blank pipe on, but it was still pretty good before then.
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