2001 A4 2.73 47,000 miles (long block replaced about 3,000 miles ago under warranty).

I used to get about 30mpg highway and 17-18mpg city average.

It has just kept dropping over the past 2 years and I am down to 11-12mpg city average?

• New tires 32-psi
• New Iridium Plugs Red Gm Performance wires
• New Air Filter
• Spray cleaned MAF and Throttle body.
• No codes
• Mobile 1 oil changed every 6 months
• Use only Chevron w/Techron Premium fuel

Car seems to idle/run fine and I drive it relatively mild, so what else might cause this mpg drop? I did get a mail order tune last December and dropped about 1-2 mpg in town, but was experiencing the loss prior to the PCM retune.

Any ideas what might be causing this radical drop over time? Even after getting a new Long Block (under warranty), mpg continued to drop?

 

Lazy o2 sensor(s) ?

 

I'm from the "other" Forum...C-4. Thats almost the stock answer over there.

 

Alignment. When my tierod was loose and made the car pull left I lost 4 mpg.

Or some combo maf/02

 

Your engine operates in TWO modes. Open loop mode and Closed Loop mode. Closed loop is used during cruising and uses the O2 sensors as a feed back to the PCM for controling your cruising Air Fuel Ratio. It tries to maintain a 14.7:1 AFR for optimum fuel efficiency.

During warm up and WOT the PCM operates in OPEN LOOP and the AFR is controlled by various preset tables.

Using aftermarket data logging equipment, you should read the O2 sensors and see if there properly functioning. They cycle high to low, low to high, high to low...100ma to 900ma.......... to control the overall average AFR. If the O2 sensor values ar hanging out at one value there NOT functioning and that would cause poor fuel efficienct issues.

You also need to read your Long Term Fuel Trims (LTFT) they are the indicators as to what the PCM is doing to maintain that 14.7:1 closed loop AFR. The PCM can only correct +/- 25%. If the LTFT is significantly off, it is an indication that something is wrong that is causing a rich or lean condition that the PCM needs to correct.

- How are you reading the DTCs???

- Are you using the built in DTC routine?

- Are you reading the DTCs before you shut down the engine??


Some common things that effect Engine efficiency are:

- Dirty MAF
- Defective O2 sensors and or wiring
- Dirty Air Filter
- Vacuum leaks
- Poor tune up (you already addressed that issue)
- Some other engine sensor not functioning correctly (normally seen with a CEL)

Hope this helps.

BC

 

Ethanol causes lousy gas mileage. Use Startron with every fillup and it will help. Your problem is probably the 0-2 sensor, I agree with the other poster.
Capt Bob

 

WAIT, WOT is open loop (no sensor readings)?

When I had on oiled up MAF sensor, I would get a huge hesitation at 4500rpm WOT.

When I cleaned it with cleaner the hesitation cleared up at WOT.

Sorry to thread jack.

PM me if you can.

 

I have just been checking for codes on the DIC using the OPTION-FUEL button routine and not seeing any current or history codes?

Can the O2 sensors be bad and still not throw a code?

 

yes, they can just be lazy but still be in acceptable range.

Only way to real check is with a tuner.

 

Guess I am off to find someone with a Tech II.

 

ANYONE with EFI Live, HP Tuners, Aftermarket scanner OBDII capable or TEcH II can data log.

You can do what I did for my C6. Its a CAN Buss system but the Aftermarket Scanner that I purchased (ACTRON CP9180) can data log a C5 or C6 and it was only $130.