Poor(ish) MPG vs. a bit of white smoke on cold start..

[Alfonzo]

Hi All,

2003 A4 1.9 (AWX) Engine.

Not owned this car long. It's generally in good order and nice to drive. During this recent cold weather I've found the car starts but runs for 30 seconds or so with a bit of rough running. It also produces a bit of white smoke for a while, although clears after a short time then runs fine. I occasionally get a small whiff of diesel smoke in the cabin (seemingly randomly).

I've also just finished the first full tank of diesel (£89) and managed about 640 miles (inc. 500 motorway). To be honest, I was hoping for better. I know driving style plays a big part, but I nurse my cars for good mpg and know how to get the best from them.

I've also noticed the dashboard temp gauge tends to run about 70-80 deg c, a bit cool? and also seem to 'hunt' a bit more than I'd expect. Not sure this is related..?

Be interested to hear if you think there's any connection between the above, or any other comments.

Cheers guys.

[Google44]

I've got same issue. TBH your mpg is even better then mine. I use to get 69 - yes 69 mpg out of B5 Avant with AFN eng. As it looks now I cannot go over 42 with 80% motorway drive. There is no error codes in ECU and car drive OK.

As far as your temp gauger you will need to replace thermostat or temp sensor, or both. This does affect MPG.

I've done it on mine but mpg is still bad. :-)

[JimC64]

Hey Alfonzo,
Google is right about the thermostat and or possibly the temp coolant sensor ( neither of which ar terribly expensive or difficult to do )

Depending on your mileage if its fairly high and or you aresuffering from hesitation / hunting, it could be 02 lambda sensor related?
I've just changed 2 sets on my sons Astras for similar reasons and they've cured the hesitation issues and poor mpg........Do a search on lamda sensors mate.
They can be good for 30k - 60k perhaps depending on circumstances, but can fail sooner ....they should really throw a code but perhaps they're on their way out and haven't totally failed as such yet.................definitely worth checking

Astra G 04 plate had 68k miles and both were changed......Astra H 06 plate with 49k miles and both were changed.

Hope it gives you some food for thought at least

Good luck

[Alfonzo]

Google - 69mpg is seriously impressive, was that actually a brimfull calculation of from the trip computer? And 42 on motorway driving doesn't sound right unless you're caning it! I will look at my temp gauge sender & thermostat once winter has left us, too damn cold out there for driveway car repairs at the moment!

Cheers for the thoughts there Jim, I don't have any hesitations or engine hunting, in fact it drives nicely but I take your point. I guess if the lambda sensor is broken the ECU is never going to fuel the car properly. The truth of the matter is that diesel engines in particular are so sophisticated that you end up with loads of different failure possibilities that can lead to incorrect fuelling. Reading other people's experiences, perhaps 47 ish MPG isn't a mile off but I had expected better and certainly get better out of my old Focus Tdci.

I'm not sure about the white smoke though?? I see from the paperwork that came from the previous owner it had a tandem fuel pump replaced as it was leaking. Maybe the new pump is playing up again.

[Google44]

I've got 69 mpg on display out of AFN engine in AVANT model 1998. The real mpg was probably about 63. The most economical car I've had. Only replace it as it was very small inside comparing to B6. I did expect 130 Avant sport to take a bit more but claimed mpg is over 50 on motorway doing 70 mph. I cannot get that. The car has no issues in pulling or accelerating, no codes in ECU and coolant temp is OK now. It had full service. I will start checking tandem pump when weather lets me do it. Think I've got spare pump of B5 engine. Not sure if they are the same.

[Alfonzo]

Well I doubt you'll ever see 63mpg from your Avant, but should definitely be getting better than what you presently do. They're funny beasts are diesels, they do rely on everything being in good fettle to return decent MPG's, and that's compression, injector spray pattern, fuel pump, egr, turbo, thermostat's etc etc. quite complicated really!

[Google44]

I know that 63 is not possible. I've got a set of pictures taken over the 400+ mile the car did with mpg displayed. The friend who use to work in audi said that AFN was 5-10 % more efficient then new Bluemotion or whatever they are called now.

My current MPG is bad. No error codes. I will clean EGR valve when weather lets me do it. after that - not sure. Engine mount needs replacing but dont think that would affect mpg.

[steveshort]

My A4 was very smoky on start up , and for a good few miles after . The MPG was averaging high 30's , I serviced it this weekend and changed the thermostat , temp sensor and put a bottle of Red X in the thank . Its now running much cleaner with no smoke after the car warms up and less smoke on start up , my MPG has so far impoved slightly .