2.0 TDI Ultra Serviced. Poor MPG, feels underpowered and engine sounds rough

[Jon B]

Hi all,

Been a long time since i’ve posted here which is a good sign!

My Audi A6 Avant with the 190bhp 2.0 TDI Ultra engine has got to 92k miles no problems at all.

Took it into my local audi specialist a couple of weeks ago for a major service, new brake discs / pads and new front tyres.

I’ve noticed since that my MPG on long journeys has dropped by about 10mpg, the car feels slightly under-powered and engine seems noisy. I am also getting this periodical sensation where it feels like while in cruise control the throttle is lifted off for a second and then re-engaged. This also happens without cruise control.

I plugged into my Carista dongle - there are no codes coming up, no lights on dash. I’m not sure what numbers are normal on turbo etc etc, so not sure what could be wrong.

I know MPG drops in winter, but it never usually drops to below 50mpg on a 70mph long motorway run.

I know the easy answer is to drop into the garage, but i’m on holiday and then it’s christmas. I’d just like to know if anyone has any ideas?

Thanks,
Jon

 

[spartacus68]

Treat the car to some premium diesel and see if the situation improves. Cold temperatures do reduce efficiency. Any issues with the battery? Again at this time of year, especially with batteries, then a weak battery will draw more resource. You should be sitting around 12.6V before starting car. A cheap multimeter is an essential bit of kit. There’s a year code stamped on the negative terminal if car is new to you.

 

[Jon B]

Thanks for the reply.

I connected my car up to Carista. Voltage only showing 11.4V before starting engine. When engine turned on it does eventually get to 14.7V, but once battery gets to 75% full the voltage drops to 13.2V and that’s when the car starts to perform really badly.

Also had a low inflation light come up this morning on motorway, but all tyres were fine.

I feel like battery is the issue, but worried could be an alternator issue, also.

 

[spartacus68]

I connected my car up to Carista. Voltage only showing 11.4V before starting engine.

11.4V, it’s a wonder you could start the car. You could try putting the battery on a trickle charge. Interesting video on Dave Sterl’s YouTube channel about batteries, as you can charge them, but do they hold the charge under load? That tends to sort the wheat from the chaff so to speak.

The tyre pressure TMPS issue is probably related to battery too, especially if you’ve had no issues with tyre pressure readings lately. With the car running you’d expect 14.5V for healthy alternator.

Chances are it’s an AGM battery, and not cheap, but hey it is what it is. You need VCDS to register to car.

 

[elan2s]

If that is the original battery then it almost certainly is a contributing factor, if not the issue itself. If you do a direct like for like replacement then you can usually get away wothout having to recode the battery the car

 

[Jon B]

The original battery was replaced under warranty by the previous owner. To do with a recall I think?

Current battery is nearly 4 years old - but also covered 60,000 miles on it.

I think i’m close to discounting an issue with the alternator as start-stop works absolutely fine. Just a nice £250 to spend on an AGM battery, unfortunately.

I just find it hard to believe a nearly dead battery could cause the overall running of the car to be so poor.

 

[spartacus68]

If fuel filter not changed - then that would be worth a look too. They are due every 40k miles. Ideally you need VCDS to prime the fuel filter. On an A4 Avant it's under the driver's side and a bit of a pain with plastic undertray trim to renew, not sure where it is on the A6, but suspect same location. On previous Audi diesels if car was running poor, then you can use Forte Diesel Fuel conditioner prior to swapping over the filter, but it does need changed.

 

[Jon B]

Car was serviced literally a month ago with fuel cleaner, fuel filter etc all done. Which is why it is so odd. S-tronic gearbox oil changed 12,000 miles ago.

Will see if change of battery makes a difference and will report back.

 

[Jon B]

Just as an update.

I booked car in for battery to be replaced. Garage said the voltages were all over the place and to go to an auto-electrics specialist.

In meantime, went on a 400 mile journey and car seemed much better by end of it (typical). Took it into auto-electric place - they said fault wasn't present, but were faults relating to "misplaced signals" - could possibly be transmission or ABS issue, but fault will need to be much more permanent to be able to confirm.

So in meantime, new battery is being fitted and will see what happens.