View Full Version : Crappy gas mileage, like 1/2 tank = 60 miles crappy . . .

02-06-2007, 10:23 AM
Wow, so I noticed after a recent fill up that my tank is showing half full after about 60 miles. Something that dramatic makes me think this could be serious, but I have no idea where to start. What could cause such a dramatic change?

My CEL is on showing a bad O2 sensor and a weak signal from one of my knock sensors, and my temp gauge has been all over the place (never above half way, but dramatic jumps from barely/not registering all the way to half way in a matter of minutes). Any ideas?

02-06-2007, 11:02 AM
you pretty much answered your own question.
its probably the O2 being bad, probably all you listed that is bad is whats doing it.

02-06-2007, 11:08 AM
Dodgy temp sensor, the poor car is probably running rich as it thinks the engine is cold.

02-06-2007, 11:16 AM
Ok, good to know - just wanted to make sure there wasn't anything more I should know. Replacing the O2 sensor seems pretty straightforward, anyone know if the knock sensor and temp sensor are relatively simple DIY's?

02-06-2007, 11:20 AM
Yeah you gotta be running pig rich. Replace only the front O2 sensor, as the post cat one doesn't do anything in terms of A/F mix. The temp sensor is another good idea. I know on fords the temp sensor can go bad and your gas mileage will totally suck.

02-06-2007, 11:24 AM
Think the temp sensor thing might be related to a bad thermostat? This thread sounds pretty familiar to what I'm seeing:

http://www.audizine.com/forum/showthread.php?t=110048 ('http://www.audizine.com/forum/showthread.php?t=110048')

02-06-2007, 11:55 AM
what codes are you pulling when you scan it?

02-06-2007, 12:05 PM
Nothing related to the temp sensor or thermostat:

2 Faults Found:
16716 - Knock Sensor 2 (G66): Signal too Low
P0332 - 35-00 - -
16544 - Oxygen (Lambda) Sensor B2 S2: No Activity
P0160 - 35-00 - -

02-06-2007, 12:12 PM
i'd say Def O2 sensor

02-06-2007, 12:16 PM
this is a good thread.. yeh samething is happening to me i put like 8gallons ,thast little more then half, of 93 octane and i get like 93 mile to it but i have no cel or check engine light

02-06-2007, 12:23 PM
Does the bentley manual specify which O2 sensor is which? I did a search and someone mentioned this particular code is for one that sits after the cat - any way I can tell for sure?

EDIT: Guess I could systematically unplug each and watch to see which codes pop up, but that seems excessive (and potentially a waste of fuel, though I seem to be wasting quite a bit already [;)] ).

02-06-2007, 12:31 PM
The one after the cat doesn't determine the air fuel mix. It only senses if the cat is operating properly. Replace the one before the cat.

02-06-2007, 12:33 PM
That's why I want to make sure - if it turns out to be the sensor after the cat, the gas mileage problem may be rooted elsewhere. I suppose I have to replace the sensor either way, but it's probably good to know anyway.

02-06-2007, 01:02 PM
sensor 2 is downstream so your cel is for the post cat. You might have a clogged cat or your o2 is shot.

02-06-2007, 01:26 PM
So is that the post-cat driver's side? When shopping for a replacement, are all 4 different, or are they basically the same?

Makes me wonder about the gas mileage though . . .