Why is my Harrier engine performing badly?

Thursday March 5 2020


By Paul Kaganzi

Hello Paul, my Toyota Harrier with a 2.4 litre petrol engine lacks power, bellows smoke, has a hard start and consumes a lot of fuel. Recently it overheat and mixed coolant with the engine oil. My mechanics refaced the cylinder head and replaced its gasket. However, it has not improved. What could we be missing?


Hello Kasozi, your mechanic needs to carry out a compression test to determine the health of your engine. Compression in car engine terms is the pressing of the fuel air mixture into smaller volumes by the pistons in the cylinder chambers.

This builds pressure which is important for prompt ignition by spark for petrol engines or higher compression for diesel engines.
Recommended compression also gives your engine good performance, fuel economy as well as ample power. Compression can be reduced when your engine cylinder walls or piston rings are worn out or damaged due to poor lubrication. This will leak compression from the cylinder chambers and lead to a cross flow of fuel air mixture to the crank case (cause of poor fuel economy) and engine oil to the combustion chamber (burning oil blue smoke and drop of oil levels).

Engine lubrication (oil) can be compromised by delay to service, use of low quality or counterfeit oil as well as infiltration of coolant fluid or water when the engine cylinder head and gasket are damaged during an overheating episode like in your car. Renewing the head gasket or refacing the cylinder head may not help much if the coolant mixed with oil has led to damage of the piston rings or cylinder walls.

The Toyota 2AZFE engine in your Harrier has an aluminium cylinder head which is not easy to repair after it is damaged by overheating. It may remain an escape route for engine compression even with a new head gasket.


A compression test with a special tool will help determine whether this level of damage is not there.

Hello Paul, what does the coast button do and when do I use it? Just changed my car and this button is new to me. Ronald

Hello Ronald, those buttons work with the cruise control system on your car. Cruise control allows you to remotely set and lock car speeds as well as accelerate or decelerate using the buttons on your steering wheel. In case you are not familiar with the use of the cruise control function, read the car user manual or get a technical person to show you how it operates before you attempt to use it.

When you press and release the “SET/COAST” button while the cruise control is switched on, you will be able to reduce the preset cruising speed. Each tap of the “SET/COAST button decreases the speed by 1.6 km/h. On the other hand a tap of RES/ACC will reset and accelerate back to the set speed or a new speed.

Ordinarily, when you apply brakes, the cruise control feature will be cancelled. Cruise control gives you a more hassle free drive and better fuel economy because of the controlled gear shifts and power demand.

Hello Paul, my 2003 Toyota Prado has had the front brake pads replaced but it wobbles and shakes violently when I brake. Could the problem be the new brake pads? The mechanic who fitted the pads says that we should have the front discs replaced. Please help us resolve this.


Hello Samson, it seems you need to replace the front brake discs (rotors). When brake discs are unevenly worn out or warped, they will rattle, wobble or shake violently when you brake. In most cases, you will feel vibrations to your foot as you apply the brakes. This is consistent with the current symptoms of your brake system.

Using a vernier calliper tool, one can measure the surface of the disc rotors to detect inconsistencies. Consider replacing those brake discs as your mechanic suggests and your car brakes will work well.