Andrius85

Hello all,

My Audi has 116,000 miles and about a year ago I noticed decrease MPG.
Normally as soon as I get on the highway they would go up to 34.5MPG without any effort.
Now even without hard acceleration I get about 30mpg tops on a highway. It's even worse in a city driving 18-20mpg.
I tried to calculate MPG manually during tank fill up, so whatever gallons I fill up I use it to divide my trip miles and that comes out at about 26-27MPG. Most of my driving is highway driving. Or more specifically about 80%.

Since then I tried to clean MAF, replace air filter, replaced spark plugs, battery was due so replaced that one as well. Also cleaned the grill filter.

I have been researching this topic without much success over a period of 1 year.
However, what I noticed is that during DPF burn out cycle my MPGs go up to the same numbers that I was used to seeing.
The Audi manual says to "check ash loading" at 115,000 miles which my car has reached and passed.
Could it be the DPF filter?
I was considering taking to the Audi certified mechanic to get the DPF cleaned. They said they can restore it to 60-70% by doing this maintenance but did not promise it will get my MPG back to what it was.
The service fee is $360 which I thought was not bad.

So my question would be - is it worth doing this maintenance item or are there any other suggestions that could help to get my MPG back to what it was?

1stGOAT

Getting DPF cleaned or replaced will help.

'10Q7TDI_Prestige'

You need a VAGCOM scan tool to view the actual DPF live data...ash loading, pressure differential value (backpressure), etc., as there are specific levels that tell you it's time to service the DPF...or not. For the price of a single diagnostic scan at many shops, you can own your own VAGCOM scan tool, etc. It is not uncommon on these TDI motors for the the EGR system to clog up the air intake very extensively with soot, and eventually resulting in decreased fuel efficiency. Clogged/dirty fuel injectors, clogged fuel filter, clogged DPF or one that is not regenerating properly are also items that will impact fuel efficiency.

Before you simply go pay for a DPF service to remove ash from it, be sure that is what you need...a simple look in engine module at the DPF sensors/data points will allow you to make an informed decision since it is really the combo of the clogged/carboned-up air intake and the DPF going about normal business that you are witnessing. On my 3.0L TDI (CATA) the intake was almost completely clogged from the EGR's exhaust recirculation at 108k miles, and was so clogged the Audi tech cleaned it while it was in for warranty service on the EGR Cooler. I was also getting reduced fuel economy as my primary symptom.

Additionally, change the lubricant in your driveline: front/rear diffs, transfer case, and...gasp...the transmission, which Audi likes to tell you does not require fluid changes. Great advice, Audi, as always.

Andrius85

10Q7TDI_Prestige, thanks a lot for such an in depth answer!

I have a few questions though:
1) would my ODBELEVEN do the job that VAGCOM scan tool would?
2) You mentioned that "Clogged/dirty fuel injectors, clogged fuel filter, clogged DPF" would impact fuel efficiency. I replace my Diesel filter every 2 years, so that is probably not the issue. I did consider fuel injectors to be part of the problem and honestly thought to focus on those next, if DPF cleaning does not help.
However, if my DPF is clogged, why doesn't my check engine light come up? Is it because the clogging did not reach the certain threshold, but it is still bad enough so that it affects the MPG?
3) You said: "On my 3.0L TDI (CATA) the intake was almost completely clogged from the EGR's exhaust recirculation at 108k miles, and was so clogged the Audi tech cleaned it while it was in for warranty service on the EGR Cooler. I was also getting reduced fuel economy as my primary symptom."
Is that a separate service than service on DPF? If so, what is that service called? And how would I find out about it from the scan tool if it's needed?
4) Can you explain what you mean to change lubricants on front/rear diffs, transfer case?
I understand the the transmission fluid but Audi dealership told me it's a sealed component and normally they do not recommend to replace the fluid on it. Are you still recommending to replace it?