Why am I getting such horrible gas milage?
10-12-2012, 10:46 AM
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My Ride: 530i
Brake cleaner will ruin a MAF. Only use a MAF cleaner such as CRC. Cleaning your MAF may or may not work. Cleaning didn't help mine, but when I went ahead and replaced it there was a noticeable improvement in power and performance.
I didn't see your mileage mentioned. Normal mileage on an '04 would be about 120K which is time to change all your O2 sensors. The pre-cat O2s can definitely affect gas mileage and performance. When I changed mine at 90K my mileage improved 10%.
Your coolant temperature sensor is another possible culprit along with the thermostat. If either the thermostat isn't warming the engine properly or the coolant temperature sensor isn't telling the ECM that the engine is warm, the engine control algorithms will not switch from open-loop to closed loop control and your fuel economy will suffer.
VANOS o-ring seals are important too. VANOS gives a 10% improvement in fuel economy. When these o-rings harden up over time due to heat soak (i.e. engine temp rises after shut down). They become stiff and flattened instead of rubbery and round. This keeps them from sealing the VANOS pistons which are pushed by engine oil pressure. As a result the pistons move erratically, or not at all, resulting in a loss of fuel economy, less power, hesitation at 2-3K rpm, and offtimes a rough idle.
Another idea: tire pressure? If it's down 2-3psi it will reduce fuel economy significantly without triggering a tire pressure warning.
When one of these systems (and the others that have already been mentioned) fails completely your car will throw a code and possibly turn on a warning light. But the performance of each one can degrade quite a bit before that happens. On an older car without a lot of aggressive preventive maintenance it's likely to be more than one thing contributing.
I didn't see your mileage mentioned. Normal mileage on an '04 would be about 120K which is time to change all your O2 sensors. The pre-cat O2s can definitely affect gas mileage and performance. When I changed mine at 90K my mileage improved 10%.
Your coolant temperature sensor is another possible culprit along with the thermostat. If either the thermostat isn't warming the engine properly or the coolant temperature sensor isn't telling the ECM that the engine is warm, the engine control algorithms will not switch from open-loop to closed loop control and your fuel economy will suffer.
VANOS o-ring seals are important too. VANOS gives a 10% improvement in fuel economy. When these o-rings harden up over time due to heat soak (i.e. engine temp rises after shut down). They become stiff and flattened instead of rubbery and round. This keeps them from sealing the VANOS pistons which are pushed by engine oil pressure. As a result the pistons move erratically, or not at all, resulting in a loss of fuel economy, less power, hesitation at 2-3K rpm, and offtimes a rough idle.
Another idea: tire pressure? If it's down 2-3psi it will reduce fuel economy significantly without triggering a tire pressure warning.
When one of these systems (and the others that have already been mentioned) fails completely your car will throw a code and possibly turn on a warning light. But the performance of each one can degrade quite a bit before that happens. On an older car without a lot of aggressive preventive maintenance it's likely to be more than one thing contributing.
Last edited by Bimster530i; 10-12-2012 at 10:51 AM. Reason: changed % to psi
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