mileage drop

air filter, o2 sensor, Catylitic convertor, fuel tank cap.

Engine:
- Retiring PCV system.
- Air induction (Air filter, throttle body, secondary air pump if so equipped).
- Vacuum hose(s) disconnected.  vacuum leak
- Faulty thermostat not allowing the engine temp to stay at optimum.
- Blocked exhaust making the engine work harder to counter back-pressure.
- Induction/forced air plumbing leak for the turbocharging if so equipped.
*These may not be present, as your engine seems to be running smooth.

Tranny:
- Check the fluid colour/contamination/level.
- Check that the mode selector is in Economy and not in Sport.

Wheels:
- Check low tyre pressure.
- Check for uneven tread wear/have 4-wheel alignment checked at a reputable shop.

Other:
- A/C beingselected at Defrost, keeping the compressor on at all times.

换完spark plugs，顺便把throttle body也拆了下来，用cleaner清洗了一下，一开始
我不想拆这玩意，但是事实证明这东西非常好拆，特别是在你拆了intake后，用allen 
wrench卸掉4个bolts就下来了。发现内壁上一层嘿嘿的污垢，管进气的叶片因为这些污
垢都不是很滑畅。 后来一想都到这步了，顺便把MAF sensor也清洗了一遍，这个就是
小儿科了，不赘述。。
清理intake manifold（EGR）是肯定可以提升MPG的 因为能有效率的将废气回锅，废气也还是能利用的。所以MPG肯定上去