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Just to illustrate my comment on a cold engine using more gas, this morning I did a few tests. There are some good places for me to Pulse and Glide, so while my car was in neutral idling, I checked the temperatures and fuel consumption. There were three cases:
Car was cold, only running about a minute. Engine was at 32 F and Intake 32 F. Car consumed 0.70 – 0.75 gallons per hour coasting in neutral.
Car was partially heated up, running about 8 minutes. Engine was at 160 F and Intake 36 F. Car consumed 0.27 – 0.30 gallons per hour coasting in neutral.
Car was fully heated up, running about 30 minutes. Engine was at 185 F and Intake 68 F. Car consumed 0.24 – 0.25 gallons per hour coasting in neutral.
Here is the best test that I can think of to check that your car is functioning properly. Only takes an hour and costs about $10. Go to an interstate or highway where you can drive 60-70 mph. Fill up your car at a fuel pump until it clicks off 3 times. Drive for 30 minutes on the highway in one direction, turn around, and drive back. Try to be “nice” to the car, moderate acceleration using cruise control and going with the flow of traffic. Don’t hyper mile, just drive nice. Come back to the exact same fuel pump (important) and fill the car to the same level (clicks off three times). Compute your MPG. This won’t be extremely accurate, but should be close. Compare this number to the EPA highway rating on your car. If your MPG on this roughly hour highway only trip doesn’t meet or exceed the EPA highway figure, I would bet there is something wrong with your car.
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