Bad Fuel Economy, Low Temp The Cause ??

A3 T

A3 T

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I've been asking some of these questions in another thread (thanks cookstein for your help so far) but thought i should start my own thread and not hijack someone else's .


It seems that my DIS is showing that I'm only getting 11.5 MPG :


img2011053000024edit.jpg




cookstein asked if the cars temp is at 90° so today while on a long drive i kept my eye on the temp gauge and it seems that it only gets to 90° if I'm either stuck in traffic or if after a run i turn the engine off for a few minutes then turn it back on again , it goes straight up to 90°.


While driving the temps sit about here:


50827470.jpg



So it looks like i need a new thermostat but could this be the cause of me only getting 11.5 mpg? and if so how?


Or could it be that the on-board computer is out of sync ?

I've scanned the car for faults and nothing has come up.




Thanks for any help given ;)



 
In a word no, regardless of what your DIS says...are you only getting 11 mph, your wallet should tell you that.

DC
 
My temp gauge hardly lifts from 60 as my themostat of well FUBAR, I still get 30-33 mpg
so as above nope, work out your mpg the old skool way

fill your tank and zero the trip and take note of the miles

drive around normaly untill the tank is past half or nearly empty and refill to the top,

take note of the litres, convert to gallons ( divide the litres by 4.5 to give gallons )
take note of your milage covered between fill ups

divide miles by gallons = average mpg.
 
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as for your question about how does it drop your mpg...

your engine is/thinks its cold... so puts choke on... aka injects more fuel :)

normaly find idle lifts a bit too, basicly like starting from cold :)
 
i had a problem with this a while ago, where the seal on my stat was crimped, so it wasnt working properly, so the car was running at 60 all the time. as a result my mpg went down as the ecu over fuels the car, cos it thinks its running cold all the time, which made it run ritch too!!!
but i would also check the temp sender too, that could be playing up!
 
normally if temp sensor fails it goes into fail safe mode which is fans on all the time, i would say stat tbh
 
I had a similar problem - only ever got to 90 when stuck in traffic. I changed the thermostat and it fixed the low temp problem and did help increase the MPG a bit, but I wasn't getting 11.5, more like 22 average over a full tank and now it's around 28/29 (based on the DIS readout).

If you're really getting 11.5 I'd say there's probably something else wrong too.
 
Might be wise to do some VCDS logging? Sure it reads the lambda values but this could be a cause although I wouldn't put it down to be that bad...

My 'stat has gone and I average 31mpg currently.
 
or do as i suggested and work out your mpg properly?

as for the comments on low temp causing the engine to run more fuel, what do you base this on?
surley if the " choke " was on all the time the idle would be at 1200 rpm all the time, surley the cat warmer would run all the time,

ive been running at 60 degrees for over a year now, even in winter it would never go through more fuel, i actually dont want to change the stat now due to lower running engine temps in summer, i beleive it is actually an advantage now:sm4:
 
bazil said:
or do as i suggested and work out your mpg properly?

as for the comments on low temp causing the engine to run more fuel, what do you base this on?
surley if the " choke " was on all the time the idle would be at 1200 rpm all the time, surley the cat warmer would run all the time,

ive been running at 60 degrees for over a year now, even in winter it would never go through more fuel, i actually dont want to change the stat now due to lower running engine temps in summer, i beleive it is actually an advantage now:sm4:
Click to expand...

colder engines use more fuel as more is squirted in, this is the case with most, if not all injection engines, no it wont "high idle" unless the sensor is reporting like 20 degrees so if it is reporting 40 degrees its not cold enough to sit on "high idle", but not warm enough to use the "up to temp" settings.

also not sure how your car runs cooler if it has a faulty temp sensor ? that seems like a silly thing to say unless you are basing it on it dumping more fuel into the engine?
 
Looking at teh top picture you posted that shows your Average MPG not Instantaneous MPG

I don't think my average readings are ever correct. Might be worth checking if they are actually working properly.

My average time always seems to say 99.99h and my average consumption sometimes seems to always read the same. I will have a look at mind 2mz and see what the case is. Mind you in thw two years of ownership I don;t think I have ever paind attaention to my averages and I have a feeling it was down to what I mentioned above.
 
Thanks for all the replies everyone, I didn't think i would get so many on this topic :)

Ill try and cover as much as i can in this post.


This is a quote of my post in the other thread:

A3 T said:
I never let my petrol run down too low so the only example i can give of my fuel consumption is.......

When i first bought my car it hardly had any petrol in it so on my way home the fuel warning come on the dis, I only had £25 on me so stuck that in and reset the trip counter, I then drove the car until the warning come on the display and when i looked at the trip counter, it said i had done about 80 miles.


Does that seem about right?
Click to expand...

My idle seem fine and drops below 1k after about 10 seconds and the fans only come on if I've been driving the car for a while.


I made a video showing my instantaneous MPG as it seems to be a bit erratic:




Is this only meant to change when the car is moving? as when I'm at idle it just shows the last mpg before the car come to a stop.


I suppose my questions now should be.......

If the DIS is just giving me the wrong information, is there an easy way that i can recalibrate it with vagcom?

I'm sure this could be the problem as the last time i looked at the average speed reading it was showing something like 5mph and i spend most of my time on the north circular ....lol

But on the other hand , if it is giving me a true reading, then where should i start looking ?


Thanks again guys ;)
 
Last edited by a moderator:
well, it sounds like your thermostat is broken which will affect you mpg. When they break they stick open meaning that you coolant is always diverted to the radiator so its constantly getting cooled. It rises when your in traffic because there is no air flow over the radiator, the temp will rise and eventually the fans will come on forcing air flow over the rad.

So you should get that fixed, but whether its causing your mpg to fall to 11.5 mpg i very much doubt, but i may be wrong. I would get that fixed and get it scanned.
 
colicabcadam said:
also not sure how your car runs cooler if it has a faulty temp sensor ? that seems like a silly thing to say unless you are basing it on it dumping more fuel into the engine?
Click to expand...

re-read my post, my THERMOSTAT is stuck open, the temp sensor works fine, the engine runs at 60-65 degrees and upto 90 when driven hard,

What I will ask you is this, does your car have a faulty thermostat? Have you covered over 10,000 miles with your car runing at 60 degrees or less? have you noticed that in the last year your average fuel consumption hasn't changed?

After 8 years and 70,000 miles of S3 ownership I beleive my post and my comments are valid to this thread and not "silly things to say"
 
Does my vid look normal ?
 
yeah the Vid looks normal, its instantaneous MPG so its all over the place
 
audi upgrades
I reset the the average mpg a couple of days ago and I'm starting to think its just my driving.....lol


I've kept the DIS showing the instantaneous mpg and while driving around I've been keeping my eye on it.

It seems that i pend a lot of time in the lower gears (because of traffic lights, roundabouts...etc) and the mpg spends a lot of time showing 11-13 mpg. As soon as I'm in 3rd 4th the mpg goes in to 20+.

I'm going to do another vag scan tomorrow and maybe some logging .


What do you guys think?
 
maybe, but still give it a scan and some logging. What are you going to log?

My instantaneous whilst in town is 15-25 i think. Can get it to drop to around 5 though
 
Cookstein said:
maybe, but still give it a scan and some logging. What are you going to log?

My instantaneous whilst in town is 15-25 i think. Can get it to drop to around 5 though
Click to expand...

I didn't get a chance to do anything to the car today but with a bit of luck ill have some time tomorrow :(

If i floor it in second i can get it as low as 6 but if i come off the accelerator in about third it goes up to about 140 -160.....lol
 
I just did a quick scan and it come back with two faults:


16804 - Catalyst System: Bank 1: Efficiency Below Threshold

P0420 - 35-10 - - - Intermittent

Not too bothered about this one as its intermittent.



And this one:

01039 - Coolant Temperature Sensor (G2)

30-00 - Open or Short to B+


I think i should change the temp sensor and see how it goes.
 
maybe your car is running very rich and knackering your catalytic converter giving you the CAT system fault? But as you say, its intermittent
 
Does anyone know if i need the 2 pin or 4 pin sensor ?

I put my reg in to euro car parts and it come back with both.....lol



Maybe i should give audi a call :ermm:
 
I phoned audi today and they put me through to the parts department.

When they put me through i asked the guy if he can give me the part number of the sensor if i give him my reg and he said “were not meant to do that”, I told him that I've got part numbers from them loads of time and they even cross reference them for me, so he said ill do it this time but we are still not meant to do it.

Anyway.....


My part number is 059919501A and its the 4 pin green one.

And looking at the one fitted to the car, its already got the green one fitted.

I cleared the code yesterday and after a run today i did another scan.......

The:

16804 - Catalyst System: Bank 1: Efficiency Below Threshold

P0420 - 35-10 - - - Intermittent


Has gone and the other one has changed to intermittent :

01039 - Coolant Temperature Sensor (G2)

30-10 - Open or Short to B+ - Intermittent



I still think i should change it as its must be faulty in some way.
 
On talk of temps effecting consumption.

When I was queing to get into and out of Oulton Park over the weekend my car was reading a near constant 70degrees. I thought this was a bit weird coz I'm sure it's normally 90degress. I'm getting the car logged tomorrow evening so hopefully it may bring something up.
 
Hmmm.....

I've just been doing a bit of reading and its got me thinking.


If the temps go up when I'm in traffic and then come down when i start moving, then wouldn't this be the cause of the thermostat being at fault not the sensor?

If not can someone explain how it could be the sensor?
 
...
Cookstein said:
well, it sounds like your thermostat is broken which will affect you mpg. When they break they stick open meaning that you coolant is always diverted to the radiator so its constantly getting cooled. It rises when your in traffic because there is no air flow over the radiator, the temp will rise and eventually the fans will come on forcing air flow over the rad.

So you should get that fixed, but whether its causing your mpg to fall to 11.5 mpg i very much doubt, but i may be wrong. I would get that fixed and get it scanned.
Click to expand...
 
So do you think its both the thermostat and the sensor that's at fault (as the scan shows the sensor being the problem)?
 
I'm hoping my thermostat is OK after reading this. Only had a new Neuspeed one fitted 4 months ago.
 
not sure about your sensor, maybe yeah, but i would have thought an intermittent fault with a sensor would give erratic temp readings?
 
Cookstein said:
not sure about your sensor, maybe yeah, but i would have thought an intermittent fault with a sensor would give erratic temp readings?
Click to expand...


The error wasn't intermittent in the first scan ;)


It seems like I'm getting bad thermostat symptoms with bad sensor fault codes.......lol
 
Just lately my car hasn't been running much over 70degrees and it is a thermostatt issue - basically it stays wedged open. Because your car is still running at 'start-up' temperature it is basically telling your ECu to add more fuel.

Luckily my thermostat is less than 4month old so getting it changed under warranty - ballache like but gotta get it sorted. Plus I'm also getting MAF fault readings but need to get car re-logged after Thermostat is fitted.

My car showed a few errors during logging ast night yet no fault codes.
 
I took the car to a mechanic today and he watched the temp gauge ( and did a few other things like checking the coolant temp....etc) and he says he thinks its the sensor.


I just need to know if the sensor i posted will be ok or if its got to be a oem part?
 
I'm only getting 23mpg put a new temp sender in 2 day so will see this week if it works.
 
Does you temp never get abover 70degress (for example). Does it climb, reach a 'peak' and them drop back down as if it was climbin to 90degress (optimum idle temp)?

Does anything flag up on diagnostics? Any engine lights?
 
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G60DAN said:
I'm only getting 23mpg put a new temp sender in 2 day so will see this week if it works.
Click to expand...



Did you get an oem sensor or the one i posted ?
 
So does anyone know if the sensors on ebay are any good?


I really would like to get one sorted out sometime tomorrow but not sure if a oem part is a must :(
 
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