Car feeling weak and sluggish

Not enough fuel flow into the combustion chamber is one of the most common reason why car feel sluggish and weak. This is usually related to fuel system failure such as 

1. Clogged fuel injectors
2. Weak fuel injectors
3. Weak fuel pump
4. Clogged fuel pump filters
5. Clogged fuel filters
6. Clogged fuel line
7. Clogged charcoal canister
8. Faulty fuel pump

Another reason is not enough air flow which cause by

1. Clogged air filter

The 3rd reasons are

1. Bad / faulty spark plugs
2. Bad / faulty spark plugs wiring
3. Bad / faulty ignition coils
4. Bad / faulty spark plug cables

There are fundamental factors in combustions which is fuel, air (oxygen), fire (spark plugs). If any of these are faulty or worn out it will affect the car performance.

The root causes of cars' poor fuel economy or high fuel consumptions

When we think of cars' fuel consumptions usually we ought to relate it to the spark plugs, fuel pump, fuel injectors, oxygen sensors, map sensors, maf sensors, catalytic converter, or engine wear and tears. These many different factors have made us forget about what driven the fuel consumptions in most engine and that is including cars, generators, motorbikes, etc and that is the gas pedal and loads in needs to overcome. 

Most of us overlook what actually driven the engine to consume more fuels because the gas pedal is press more frequently especially in city driving. And of course you have to press even more when the load of the car is heavier or weaker in order to move the car at the pace you want.