Low fuel economy bluegt?

[jb182]

Hi all

After some advice please. My polo bluegt has been running at around 50-60 mpg which is great, however in the last few weeks the economy has dropped to high 30s very low 40s.

I've checked all the tyre pressures all of which are fine, it's recently had a dealer service etc. I can't find anything as to why the economy is so low.

A/C is generally off, fuel is super unleaded.

Any thoughts appreciated.


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[boydeee]

Has it been colder where you are recently? I'd expect a drop when the weather gets colder

[jb182]

It has been quite cold for the past couple of months or so. I've mainly noticed the drop in the past couple of week, the temperature has been slightly warmer in the last week or so than it has been.


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[Ricmondo]

I have found, over the years that my cars are more economical and produce more power when it is cold. The margins are not huge but in the days when I kept logs it was measurable. My Blue GT has not bucked that trend but on Turbo cars the better the intercooler the smaller the effect of digesting air which at ambient temperatures is denser.

I think the key event was the garage visit, it is not unknown for mechanics to screw up.

[RUM4MO]

Ricmondo, I am surprised that you have found that, in general, you find that a petrol engine will give best economy at higher ambient temperatures, even direct injection ones. Yes the colder weather will allow the air going into the engine to be slightly cooler, so you should be able to develop more "max" power, but other issues like the oil taking longer to fully heat up as well as the engine to heat up will steal more in the way of fuel efficiencies than the lower inlet temperatures would possibly gain. I think that in most places in Northern Europe, the petrol fuel is slightly different in winter than summer which tends to add to this.

I run a normally aspirated manifold injection petrol engine car, and a turbo charged direct injection petrol car and a supercharged direct injection petrol car - and all three of them give lower MPG figures in colder weather - which I expect.

[Ricmondo]

I think the oil viscosity question may have been a factor back in the days we used Castrol R but is not much of an issue with modern smart synthetics. I still love the smell of R though.

I could throw another variable into the melting pot by suggesting that engines are more efficient in the rain. I never measured that although I have played about with water injection with some sucess.

But getting back to the point surprising or not.

My father started keeping records in the thirties, I acquired my first vehicle way before I could drive on the road in the late 1950s, out of respect to the old man I continued keeping his records until around 2004 when I decided that there were more exiting things in life. The record was then donated to the National Motor Museum archives.

There is no doubt that we achieved better mpg in the winter.

Now life is full of surprises but look far enough and many of them can be explained as measuring the same thing with a different ruler. A lot of our family cars didn't have a heater, many did not have roofs and too many were not very safe in bad conditions. In the summer there was the insentive to do lots of short jaunts and a car, any car was a private place to "get to know a girl friend". In the winter such antics could lead to frostbite and you tended to avoid going out for a drive for the sake of it. Winter drives tended to be of necessity, longer and on less congested roads. All good for economy. Go to the seaside in the summer and you risked jams, fuel vaporisation and boiling you nuts off in a non air conditioned car, we were even forced to use the heater to stop some cars boiling over.

Oddly there are still folk you regard those as the good old days.

So lots of factors why seemingly objective comments may in fact be subjective.

When building or modifying cars Ive always gone to great lengths to cool the charge by ducting etc or fitting bigger intecoolers. If an ambident temperature intake was just as efficient perhaps I should have not bothered and spent more time down the pub.

All interesting stuff but, at the end of the day every journey starts cold and ends hot and the differences we are discussing whilst measurable are slight.

Meanwhile we have an under achieving BGT out there and the poster needs positive help rather than the ramblings of an elderly petrolhead, I must admit that I do enjoy the informed technically informed postings you produce, keep them coming.

[skymera]

I've never realised how much the cold weather affects MPG.

Recently, my 1.4 TDi has struggled to maintain 55MPG on my work commute where 70MPG was easily done in the summer.
The oil takes a considerable amount of time to get to temperature and the start/stop and idle speed return to normal within 10miles or so depending on traffic and speed.

To add a bit more, I tend to check my average MPG around the same points on my commute to and from work. It's sad, I know.
My first point is 2miles from my house where I expect to be 50-55MPG but in the very cold weather it's actually about 24-28mpg.

Chilling at 55 on the A404 helps bring it up

[Oli3000]

I think this is too big a dip to attribute to cold alone. Windy + cold maybe.

My 1.0tsi has dropped by about 2mpg (calculated) from September to these colder months. I do 40miles each way, 30 on the motorway.

[jb182]

Thanks for all your comments much appreciated. Slight development on the BGT, went to start it this morning and on comes the engine management light. Booked into dealers tomorrow for them to sort it out, service advisor reckons lambda or O2 sensor making it run too rich on fuel. Will post when I know more.....


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[Ricmondo]

Fingers crossed for you.

[jb182]

Hi all

Thanks for all your input, the issue is resolved now, dealer found that one of the sensor plugs had Broken slightly causing it to run rich.

Clip replaced and economy back up to mid 50's

Thanks again


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[boydeee]

Hi all

Thanks for all your input, the issue is resolved now, dealer found that one of the sensor plugs had Broken slightly causing it to run rich.

Clip replaced and economy back up to mid 50's

Thanks again


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Great news! [emoji106]