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Discussion Starter · #1 ·
I've recently
done quite a bit of long distance driving in <st1:country-region><st1:place>France</st1:place></st1:country-region>. During very cold conditions
I noticed my car was returning much lower mpg than in the <st1:country-region><st1:place>UK</st1:place></st1:country-region> (typically about 15 mpg lower). I monitored the mpg and found that when the ambient temperature was below 4
degrees Celsius the mpg would be
around 40-45 but when the temperature rose by a few degrees the mpg would increase to 55-60. This was
confirmed by the mpg meter.
The lowest mpg figures
were returned when travelling through a cold flat section of <st1:place>Northern France</st1:place> which is at sea level. The
average was taken over hundreds of miles not a short distance where the car
could be going up or down a gradient. We travelled at a constant speed (approx
75mph). We bought fuel from various service stations but the variations
in mpg were not related to where
be bought the fuel. We travelled non-stop; the car remained hot for the entire
journey and did not go through warm-up and cool-down cycles. Once we got back
to the <st1:country-region><st1:place>UK</st1:place></st1:country-region> (a blistering 7 degrees), the
economy has been excellent and I'm still using the same diesel that was in the
car when it was running inefficiently. <o:p></o:p>

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 but I cannot believe that
such an insignificant change in temperature would have such a dramatic change
in mpg.<o:p></o:p>

The car
doesn?t seem to lack power and as a precaution I?ve cleaned the MAF with a non residue electronic solvent cleaner - this hasn?t
made any difference to mpg or performance.
I?ve read the FAQ but its not clear that if I have a defective MAF or
coolant sensor would the mpg vary so dramatically if the ambient temperature
varies by a few degrees? Once the car has "warmed up" the coolant temperature remains at a constant 90 degrees.

Thanks!

<o:p></o:p>
 

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It's a common question and you'll find discussions here and elsewehere. One additional reason is that the car produces more power as it gets colder. The air mass sensor registers the denser air and more fuel is added.

I have had 4 Golf TDI's (110, 115, 130 and 150 PS), a Passat 130 TDI and a Bora 115 TDI.

All of them used between 10-15% more fuel in winter.
 

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Discussion Starter · #3 ·
Hi jabba,

I can appreciate that the air mass sensor would register a higher
reading as cold air is more dense than warm air, but would you expect a
few degrees change in air temperature to result in 15 mpg
difference? Is this symptomatic of a MAF failure?

Paul
 

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Low mpg can be a symptom of a MAF failure. Generally, with a MAF failure you would notice the car "holding back" - the "pull" will come and go throughout the rev range.
 

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Hi jabba,

I can appreciate that the air mass sensor would register a higher reading as cold air is more dense than warm air, but would you expect a few degrees change in air temperature to result in 15 mpg difference? Is this symptomatic of a MAF failure?

Paul
15mpg no - my 130 TDI was most affected swinging from 52 in summer to as low as 44 in winter on the same cross-country route.

I haven't had a MAF failure myself so can't comment...
 

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There are lots of variables.

I'm suffering a bit at the moment as well, and am convinced it's either the thermostat not operating as it should or more likely the water temperature sensor telling fibs to the ECU. The car is definitely overfuelling as I'm getting a fair bit of 'visible in the car behinds headlights'.

I'm going to change both this weekend, I'll keep you all posted on the outcome.

Also, I used to clean my MAF with dimishing returns, what made me change it was the really poor performance and then suddenly bad fuel consumption. Strangely I never got the flat spot at 3k revs but when I changed it, it was like having a re-map.

At the moment the car is running very nicely but the consumption is down quite a bit, I'll change the stat and water temp sensor and see how it goes and if that fails I will be looking at the combined Boost / Inlet temp sensor and then maybe fuel temp sensor.
 

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Discussion Starter · #7 ·
I would be fascinated to know how you get on.

If the thermostat is faulty, I presume the car would take a long time to warm up. Is this the case?

Do you know if the water temperature sensor is connected to both the ECU and the
temperature gague on the dash? If this is the case, a faulty
sesnor would be easy to detect as the gague would not work properly.

Its really difficult to know if the performance of my car has actually
diminished, I guess that its such a slow process its difficult to
detect. I do sometimes think it feels as if there is a flat spot at 3K revs but I don't know if I'm imagining it.
 

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I would be fascinated to know how you get on.

If the thermostat is faulty, I presume the car would take a long time to warm up. Is this the case?

Do you know if the water temperature sensor is connected to both the ECU and the temperature gague on the dash? If this is the case, a faulty sesnor would be easy to detect as the gague would not work properly.

Its really difficult to know if the performance of my car has actually diminished, I guess that its such a slow process its difficult to detect. I do sometimes think it feels as if there is a flat spot at 3K revs but I don't know if I'm imagining it.
Yes longer warm up and in my mind due to the efficiency of the TDI could cause cool running if the stat is wide open causing more fuel etc

Temp sensor is a duel output jobby, one output to ecu and one to gauge in car, all might appear well on the gauge but the ecu output might be shot. I've read of loads of instances on here of failure of the sensor.

The performance drop in my case was not sudden and as a result took ages to diagnose.
 

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My TDI 110 Bora is absolutely identical to this!!! Once the weather gets a bit warmer the MPG rises & it would seem that it pulls better also - I thought it was my imagination but obviously not!!!! [*-)]

Let me know what you find once you've changed a few things - I know its not the MAF as I've changed mine as a matter of course!!! [y][y]
 

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Discussion Starter · #10 ·
I think I might have found a problem that might contribute to the poor mpg of my Bora. One of the three vacuum hoses (shown in the picture going behind the air intake hose) has become disconnected. It is the largest diameter hose and isn't very long but I can't find where it should be connected. Any ideas?
 

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If it's one of the larger vac hoses around the bulkhead area (at the back of the engine bay) and it's disconnected, it's more than likely to be the one that connects just upstream of the MAF (look under the air intake).
 

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Discussion Starter · #13 ·
I tried to upload a picture but it didn't appear with my post. I've found that the hose should be connected close to the MAF on the air filter box, thanks for the advice.
 

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Discussion Starter · #14 ·
Hi ct.p,

Did you manage to change the thermostat and temperature sensor? Did it change the economy of the car?

The mpg of my Bora in the recent cold weather has been terrible,
averaging a pathetic 42mpg (with some motorway driving in my
journey). I got better mpg out of my MKII Golf GTI! Now the
ambient temperature has increased to 10 degrees, its rocketed to
60mpg. It appears that as soon as the ice warning indicator on
the dash is illuminated, the mpg goes to pot.

Anybody interested in buying a Bora?
 

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Hi ct.p,

Did you manage to change the thermostat and temperature sensor? Did it change the economy of the car?

The mpg of my Bora in the recent cold weather has been terrible, averaging a pathetic 42mpg (with some motorway driving in my journey). I got better mpg out of my MKII Golf GTI! Now the ambient temperature has increased to 10 degrees, its rocketed to 60mpg. It appears that as soon as the ice warning indicator on the dash is illuminated, the mpg goes to pot.

Anybody interested in buying a Bora?
I did indeed, changed both the thermostat and the temp sender and it's made no difference at all.

I blame entirely on having big heavy 17" wheels with 225 wide rubber + remap as checking my fuel stats this time last year I was consistently getting high 40's, even the odd tankfull in the low 50's.

However, this year it has been a lot colder comparitively and my conclusion is that TDI consumption really really suffers in the cold. If this is the cold start warm up period in the lower ambient air temp, I don't know. I do enough miles to even things out and must say this year I have been in the low 40's and struggled to get better.

The wheels and re-map have knocked my consumption the most though. I shall be conducting an experiment very soon by putting back on the stock avus2 wheels and seeing if my suspicions are correct. However... A remap is supposed to better your consumption not make worse so therefore it's all the wheels fault... we shall see, hope to get the avus 2's back on sometime this week.
 

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Discussion Starter · #16 ·
I wouldn't have thought that the wheels would make such a big
difference but if you've checked everything else, what else could it be?

Perhaps the ECU is programmed to inject more fuel when the air
temperature is below is certain threshold? I'm thinking of
"artificailly" adjusting the air temperature by heating the sensor to
see if it makes a difference to the economy. Have you any idea where
the sensor is? I would have thought the logical place would be at
the front of the car or in the air intake section somewhere (as it
draws "cold" air from outside the engine compartment).

I could try changing the MAF but I suspect this will be a waste of money.
 

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I know this post has been dead for a while, but if It can be
of any help to anybody else I had a faulty thermostat on my Mk4 1.8T AUM the
car would take ages to rise off the bottom spot of the temp gauge and would
never completely warm up to 90 on the gauge, on a motorway run the average temp
would be around 75 - 80?c and my average motorway miles out of half a tank
(25litres) dropped from around 150miles right down to 100miles sometimes less.(I
do have a tendency to floor it now and then) The money I was throwing away
because of this faulty ?13 thermostat was crazy I drove like this for a couple
of months until I changed the temp sensor twice to no avail and finally realized
it must be the thermostat. Changing the thermostat wasn?t the most pleasant job
to do mainly because its pig awkward to get to unless you have the proper
tooling, but if I had changed it sooner I could have saved a small fortune in
fuel bills. I?m guessing the ECU over fuels a fair bit when the temperature is
below 90.[*-)]
 
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