2009 G37 Decline in MPG

[william44]

My 2009 G37 has had a significant drop in mpg, from 22 to 16 over the past year. I do drive less mileage on the highway, although it was at 80mph when I was on the highway.

I've got 55K miles and it gets treated right, never red-line the engine, always use synthetic oil changed regularly .

I also seem to notice the smell of gas, particularly at startup, but it does seem to be more smell than a year ago. I recognize that when first started, the engine noise is so cool, I like it loud, but it has not changed. But I do seem to smell gas, sometimes it may be from a truck in front of me, other times I wonder if it is from my car.

In addition to the MPG, any comments on maintenance and comments replacing the spark plugs and wires? Any premium aftermarket plugs and wires that will make the care go faster?

I did replace the air filters with K&N a year ago, but that did little and I was disappointed. Anyone try a cold air intake? I live where there is snow, most years but not this one, and don't want an intake pipe near the ground where it can scoop up snow or water easily. I did have one on my previous VW Jetta VR6, and it was great. I loved the sound, I believe I had good increase in HP, and enjoyed the car with no regrets regarding the cold air intake.

[Jesda]

Could it be running rich? Get it checked out before you ruin a catalytic converter.

[M37xfan]

Did the smell seem to appear around the time you put the K&N filters in? That may be part of the issue

[ArmedAviator]

K&N filters work by catching smaller particles in the oil present on the filter. Often times the oil is applied too liberally and the oil will travel up the intake air tube to the MAF sensor (not far from the filter). This Mass Air Flow sensor tells the ECU how much air is coming in the engine and therefore it can calculate how much fuel it should add for an appropriate mixture. Of course, O2 sensors should also tell the ECU to trim fuel if it senses too much or not enough O2 in the exhaust, but if these are old, these may not be functioning correctly/fast enough to be efficient.

Since you have the K&N filter, I'd go back to the paper filter for a bit and clean the MAF liberally with proper MAF cleaner (spray can at an local parts store). Do NOT touch the MAF with anything other than just spraying it with the cleaner...the wire element is fragile.

[audtatious]

^^+1 on the over-oiled filter clogging the MAF's and possibly contaminating the TB's as well. While you have not hit 100k miles the car is a '09 so it would not hurt to replace the plugs, at which time you can wipe out the TB's and lightly clean the MAF sensors. I would also suggest completely resetting the ECU and let the car re-learn. When done with the above test mileage by measuring gas use between fillups and not via the cars electronics.