anolin

I have a 1990 Audi V8 Quattro with around 100K miles on it. For a big V8 with all wheel drive it's always been pretty good on gas, around 19 - 21 mpg on the highway. Lately however, it has decided to not be so good on gas, and I'm lucky to get 13 or 14 mpg on the same commute, same driving conditions, same (or even more conservative now) speeds. I think this sucks, because gas ain't gettin any cheaper. The weird thing is that the car runs perfectly - smooth as silk, tons of power, no smoke or smells. I would think it would run like crap and smell funny if it had sensor issues and was running rich. Anyone have any ideas ?

audiqv8

don't. Check for codes to verify. Could also be a leak. Walk around the car and see if you smell gas. Do you have teenagers? Maybe they are syphioning your gas.

anolin

What about running rich? I heard that an indicator is a puff of bluish smoke when I hit the gas, sometimes this happens when I really nail it. This is different than the thicker smoke from burning oil, had plenty of nasty old cars that did that. What would make it do this and how would I correct the mixture ? Where is the O2 sensor located, manifold or cat ?

32vquattro

dirty or bad maf will also cause bad mpg

audiqv8

If you have more than 60,000 miles since the last change, then it is most likely the O2 sensor. Located underneath the car where the exhaust cross over pipes forms a "X", just before the cat. The O2 sensor is pointing to the rear of the car, this is a PITA to remove if the sensor has seized, especially lying on your back. This is a three wire sensor with two white wire and a black wire. The white wire is the heating element and the black is the data.

anolin

Thanks, I'm sure it's seized why wouldn't it be? I had a problem recently where the car was just pissing gas into the cat, which was practically glowing, would this damage the O2 sensor? It's never really been the same since. Also, pardon my ignorance of acronyms, but what's a MAF? I know Mass Air Meter, I know Air Filter, no clue what MAF references, tho I have often referred to it as the M-F'n Audi lately...

audiqv8

Just another name for Mass Air Meter, (Mass Air Flow meter). A bad MAF will cause a lost in power normally. The MAF use a heating wire element to measure the flow of intake air. As air flow over the wire it cools the wire which will cause the wire to draw more amps to maintain the wire temperature. The ECU will use the amount of current draw along with other sensors to determine the volume of air flowing through the intake manifold. Since the wire is covered by dirt (acting as insulation)the ecu will cause a lean fuel mixture (not enough gas).

When O2 sensors fail, they read in favor of a lean condition which the computer tries to correct by adding more fuel.

See link below for more info

"A dead sensor will prevent the onboard computer from making the necessary air/fuel corrections, causing the air/fuel mixture to run rich in the "open loop" mode f operation, resulting in much higher fuel consumption and emissions."<ul><li><a href="http://www.volksparts.com/o2sensors.htm">http://www.volksparts.com/o2sensors.htm</a</li></ul>

anolin

Huh. Interesting. Thank you for the detail. I assume the mass air flow sensor is located somewhere in the intake snorkle, I'll check and see if it's dirty. $130 for a Bosch heated O2 sensor, yikes ! Still, at 40+ $$ per day to drive the car with such sucky mileage, it'll pay for itself pretty quickly. I have noticed a very slight loss in power, enough to notice but not enough to cause alarm or concern. Betcha putting it on an emissions machine will tell great tales.

anolin

Do you happen to know the resistance (cold &amp; at temp) of the coolant temperature sensor ?

John Henry AIA

I have a power loss in mine, same model.
Mechanic suggested disconnecting O2 sensor.
Do you think it is the MAF then?