Poor VW Bora TDI 115 mpg during cold ambient temperatures.

I don't have one of those meters but you just reinforced my reasons for not wanting one.

Thanks for your reponse Joseph but I was hoping answer to my specific questions not a personal opinion whether mpg meters are a good or bad thing. Perhaps you would like to start another topic related to mpg meters? I would be happy to post my personal opinions on them ;-)

I intended that as a answer to the the question as I understood it. Those meters are not known for their accuracy. I believe you have just found one factor in their inaccuracy.

Sorry about the misunderstanding, the question was not related to the accurancy of the meter but the mpg returned by the car.

The meter is accurate (within +/- 3mpg) and corresponded with the amount of fuel and distance between refils. The car was significantly less fuel efficient when temperature was below 0 degrees. As I said in my post "... there was a definite correlation between the ambient temperature and the mpg returned by the car". I want to know if anyone has noticed this and whether the ECU is programmed to inject more diesel when a certain minimum temperature threshold is reached or whether I have a problem with my car.

Thanks, Paul

It is usual for cars to become less fuel efficient in cold temperatures. It involves many reasons. In some cases there are differences in fuel in addition to the issues directly related to the car.

My 2.0 GT TDI Golf MkV seems to use more fuel anything below 5 degrees c. On my journey to work rather than getting 55mpg I get around 47/48MPG according to the trip computer. So I would say it is normal for your car's computer to presume you are using more fuel. Plus I do tend to get 50 miles less out of a tank of fuel. Roll on Spring :)

Rich

I'm relieved that your mpg observations correspond with mine. I presume therefore that the ECU is programmed to inject more diesel when the temperature sensor detects low temperatures. I wonder if there is a way of "fooling" the ECU that the temperature is greater than 5 degrees? This might induce starting problems so would need to be "switched on" when the engine has reached a normal operating temperature.

My understanding of temperature sensors is that their resistance reduces when temperatures increase so introuducing a parallel resistor would reduce the effective resistance of the sensor.

Do you think this would be feasible? Would it damage the engine in any way? A saving of 7-8 mpg over the winter months is quite considerable!

I do not know much about this, but could it be that the fuel sold in the cold areas and seasons has lighter hydrocarbons that make starting much easier, but do not have as much energy?

We bought fuel from various service stations on the outward and return journeys but the improved fuel efficiency of the car (confirmed by the mpg meter) was limited to stretches of the journey which had higher ambient temperatures. Once we got back to the UK (a blistering 7 degrees), the economy has been excellent and I'm still using the same diesel!

Well, let's not forget, that an diesel engine has no "throttle", so it gets all the air it is able to suck (with a little help from the turbo :)). So, if you inject more diesel - you are going to accelerate. In other words, ECU does not need to care, you will do all the adjustments with your foot.

In general, the colder it gets, the denser is the air, which makes your engine more efficient, as each cycle gets you more "punch" (that's the intercoolers are for). So, you get more power.

But there is the negative side too. The colder it gets, the more energy gets lost in the drivetrain. The tires, CV-joints, gearbox: everything is getting stiffer and harder to turn.

Which is probably the main reason for a worse mileage you saw.

In France? I doubt it very much. It's not the country known as the "cold area" :) But in general, yes, the "arctic" diesel is sometimes less energy

One more idea: Going to the skiing area, you were probably going uphill more. Coming away from the skiing area, you were going downhill more. No?

How about the cold areas were hilly, and the warm areas were flatter?

How about this... When it was colder outside did you keep the heater on high, so the glow plugs in the coolant ran all the time? I bet they suck up a lot of power.

I don't believe ambient temperature has any real effect of fuel economy, and have never seen that reflected in my fuel comsumption. (And I've been driving VW diesels for 25 years.)

I do believe there is a difference due to snow tires, and due to the winterized diesel fuel we get here in southern Ontario. And certainly driving on snow covered roads sucks up a lot more fuel.

Have you taken wind conditions into consideration?

And difference in altitude?

I often travel to a town 150km from my home and back. There is a altitude difference of about 700 feet between these towns and this contributes to a considerable difference in consumption.

To put this to bed. When it is colder your car uses more fuel, your engine takes longer to warm up, and it uses more energy to stay warm! Plus you also take heat away from the engine to use your heater to keep the cars cabin warm. Having said that in the summer you also use your Air Con which also uses fuel as it takes power off the engine.

I have noticed a difference in the winter months in other makes of car, and also in my new Golf this year. I was getting better MPG going to work in the summer on my daily commute to work, than I do now in the winter doing the same journey - Fact.

Rich

I can fully appreciate that cars use more fuel in winter due to heat loss, viscosity of the oil, friction of bearings, differential, gearbox etc.

The lowest mpg figures were actually returned going through a cold flat section of Northern France which is at sea level. Once the temperature dropped below zero the mpg averaged 45 but when it warmed up to 2 degrees the car averaged 55. The average was taken over hundreds of miles not a short distance where the car could be going up or down a gradient. I cannot believe that such an insignificant change in temperature would have such a dramatic change in mpg. If for every 2 degrees increase in ambient temperature the mpg improves by 10 mpg, the car would be super efficient at 20 degrees.

The only explanation I can think of is that the ECU is adapting the fuel/air mixture to take account of the environmental changes. Have you ever pulled out a choke on a petrol car when the engine is at its optimum temperature? Its performance certainly doesn't increase.

I agree maybe the VW ECU does put more fuel in to make the engine run sweater in the cold, however they do this for a reason, so I would suggest not to mess with it ;)

Rich

I think Tom is onto something here. I've heard/read that fuel mixtures in colder areas during the colder regions have a lower BTU rating than the summer mix... I believe the reason is so that cars start and idle easier.

I've heard this about regular fuel, and perhaps the case is the same with diesel. I do know on my regular fueled cars that I do get appreciably less mileage during the winter months.

Michael

If that's degrees Celsius, then quit yer complainin'. -9 C here in New York last night, and a balmy -8 this morning. Global warming, my ass. ;-)

-- Mike Smith

Don't know where you live, but if you're in the States, especially the Northeast, then the reason you see worse mileage in the winter is most likely because gasoline sold here is mixed with 10% ethanol between October and April.

-- Mike Smith