Strange mileage numbers. [Archive] - GrandAmGT.com Forum

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snawdjj
10-27-2005, 03:53 PM
I know there have been a quite a few posts about fuel economy recently. But here is a strange one. I seem to get good mileage on interstate runs (~29.5 MPG), but I get horrible mileage around town (~13-14 MPG). I’ve tested driving the car differently, i.e. no A/C and babying it, and I still only see ~14 MPG in city driving. Attached is a list of all my fill-ups since I purchased the car. I have done some performance upgrades as noted. The car has 37k miles on it.

I know I should check the normal suspects for bad mileage, but I guess my question is, should I be able to knock some suspects off the list? The only reason I think this is because of the good mileage on the interstate.

My gut tells me it’s something that is not working correctly while the car is cold, as I only drive 6-7mins at a time while in the city. Any suggestions on where to start?

http://216.135.38.249/temp/mileage.jpg
http://216.135.38.249/temp/mileage.xls

Mike Jung
10-27-2005, 04:02 PM
Your only driving 6-7 minutes at a time in the city.

It sounds normal.

The engine is not getting up to normal operating temperature, during that time.
When the engine is running cold, it runs a bit rich.
When the engine (& catalytic converter) gets up to temperature, the engine starts taking readings from the O2 sensor & adjusts the gas-to-air mixture.

It is only going to get worse as cold winter temperatures approach.
The engine takes longer to come up to operating temperature.

I have seen as bad as 10mpg in short city driving, when it was below -20°C/-4°F outside.


I sugguest changing the air filter & PCV Valve would be a good start.
Maybe try a different brand of gasoline.

snawdjj
10-27-2005, 06:02 PM
Thanks for the reply Mike. I never thought the engine would get such bad mileage when cold.

I’ll check the PCV valve, and the air filter is a new K&N inverted cone. I’m also thinking about changing the CAI to a WAI for winter. I’m not too worried about water, just trying to get the IAT reading warmer air in the winter. I’m not sure if it will help, but I’ll see.