Under powered and is consuming a lot of fuel 2013 Toyota Yaris 1.3L

2008 Toyota Yaris

My Yaris is very under powered and is consuming a lot of fuel.
When cruising in 6th gear changing down to 3rd or 4th and flooring it provides almost no extra acceleration at all even well into the torque band, the engine revs as it should but the power is simply not there, the engine itself also sounds a bit rough, like a faint scratching noise when accelerating hard. The fuel consumption has increased by 2 litres for every 100km. (I’m from South Africa, sorry for the metric units).

It has 400 000 km on it. No check engine light comes on whatsoever. I’ve replaced spark plugs, oil, filters and cleaned the MAF sensor. The exhaust system has been checked for leaks and there are none.
I’m trying to look at any other cause before taking the car to have the engine rebuilt. Do you have any suggestions as to what could be causing low power with high fuel consumption?

The Top 5 Causes of High Fuel Consumption in Petrol Engines

Below are the top 5 causes of high fuel consumption for vehicles with petrol engines.

  1. Carrying Excess Weight – One simple cause of high fuel consumption is when a vehicle is carrying too much weight, whether it has a lot of items in the trunk or in a trailer that it’s hauling. The more weight that’s being carried around by the vehicle, the more demand that is being put on the engine to generate enough power to move all of it. Whenever the engine must generate more power, it needs to burn more fuel in the internal combustion process. Therefore, you end up having to pump more gas because the vehicle uses more gas to satisfy its power needs.
  2. Bad Spark Plugs – When spark plugs get worn out, you will have a lot more engine misfires with your vehicle which will use up more fuel. You need to be sure to use the best quality spark plugs possible, so you can get the most miles out of them. Iridium spark plugs and platinum spark plugs are the two most preferred types.
  3. Dirty Air Filter – There are many reasons to have a clean air filter, but one big reason is to have a strong fuel economy. If your engine is filled with dirty air, then its internal combustion mixture won’t create as much power for the engine. This means it’ll use up more fuel just to perform the most basic driving needs. So, remember to change your air filter regularly.
  4. Low Tire Pressure– The tires on your vehicle need to be inflated to the proper amount of air pressure, usually between 32 PSI and 34 PSI on the majority of economy cars. If you’re driving a vehicle with low tire pressure on any or all the tires, it creates more wear and resistance for those tires. This causes the engine to work harder to make up for the pressure loss, which means more fuel will be used to power the engine for this purpose.
  5. Bad Oxygen Sensors – Your engine has oxygen sensors which keep track of the mixture of air and fuel in the internal combustion chamber. Based on what it senses, it tells the powertrain control module to either take away fuel or add fuel. But if the oxygen sensor does not work properly, the system will automatically add more fuel even if the engine doesn’t need it.

Source: https://oards.com/causes-of-car-high-fuel-consumption/