This one has me stumped. I keep careful records on the Z and I just noticed that I had a consistent 2 MPG mileage drop start last April and I can't find anything to pin it on. It has just over 80K and is running as good as it ever has. Tire pressures and filters are all up to date and it wasn't that long age since it had new plugs and wires. No CEL, of course-it passed tech in May on it's annual inspection. I'm still driving the same route every day. I was wondering if there was a change in the gas blend but I see no similar change in my wife's car so that couldn't be it. I do the air filter at 12K and it is coming up soon but it went that far last time and there was no drop. What can I be missing?

 

my first thought was that the o2 sensors are starting to go out...possibly throwing off the a/f

other possibilities could be old trans and rear end fluids, causeing small increases in friction...dropping efficiency.

 

my question is, why is this posted in the ls1 forum when you're an lt1.

 

Run any fuel injector cleaner lately? Try seafoaming it ...

I even went WOT once and saw a 9 MPG increase and thats no exagguration - went from 26 MPG to 35 MPG ... These were 100% Freeway miles though but none the less I was doing 70 MPH - 75 MPH throughout the entire commute during that time period... ...When I went back to 50% city / 50% Highway driving though, It went back down to 24 MPG...

 

I dunno, Brain fart? You'd think I'd know better by now. Come to think of it I used to run Techron through it every 3 mo. and I haven't in a long time-That or something similar is worth a shot. You'd think if the oxygen sensors were going out the second sensor would notice and show a code. If It's rich, I can't smell it, anyway. If it ever quits raining, I'll drag out the Autotap and take a look at the sensor readings.

 

I would think your answer lies in the season.. its summer..

 

I would agree with you if it wasn't a change from same periods in 2004-2005 and 2006 which were all the same numbers during the same time period.

 

I hooked up the Autotap last night and the oxygen sensors appear to be functioning just fine. I guess now I'll try some Techron and see if there is any improvement.

 

I had a gas mileage drop before, because a spark plug had wiggled loose. It wasn't out all the way but i could spin it with my fingers.

 

The after-cat sensors are only there to monitor the efficiency of the cat. A pre-cat sensor that is "off" due to age is not going to affect the after-cat sensor to the extent required to set a code.

How do you know the O2 sensor readings are correct? If age has caused them to read a few mVolts "low", the PCM is going to add fuel the engine doesn't really need, the O2 sensor volts will come back up, and the readings will "appear to be functioning", when in fact that are causing the engine to waste fuel.

Is there any significant difference between the left and right bank long term fuel corrections, for the mid-range Cells, where the engine operated at cruise?

 

The voltage readings, per Autotap, on the pre-cat sensors were where they said they should be and the sensors also appeared to be reacting quickly to changes. I'll did a little deeper and see what I can find.