Ask the mechanic: Why does my car overheat?

I drive a Toyota Kluger with a petrol engine. Recently, it started overheating. We have changed the water pump and the thermostat but it starts to overheat when I switch on the air conditioning. The engine electric fans seem to work well and there are no signs of leakage on the radiator. What could the problem be?
Jacob

Your mechanic needs to bleed or prime the cooling system. When the coolant in your engine cooling system is released by the thermostat at about 95 degrees centigrade, it is pumped from the radiator into the engine. As it flows around the coolant passages in the cylinder head and engine block, some of the coolant passes through the interior ventilation system’s heater core (exchanger) and back to the mainstream cooling system.
When any component in the cooling system fails and it overheats, this introduces pockets of vacuum in the cooling system, especially in the heat exchanger. The vacuum will prevent free flow of coolant fluid especially when you switch on air conditioning. It should be evacuated after necessary repairs are carried out. Occasionally this is not done by the repair technician. You will need to run the engine with the interior cabin heater selected and fans at position two or three. The coolant reservoir cap should be off for a few minutes as you evacuate the vacuum. It is important to ascertain the condition of the release valve in the coolant reservoir cap. This too can cause overheating. Your car should run normally after that.

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I would like to know the difference between VVTI and direct injection in fuel saving.
Mary Nandhego

VVTI means Variable Valve Timing with Intelligence. Toyota developed this engine technology in 1996 as an extension of an earlier version VVT of 1991. VVTI hydrau-mechanically adjusts the relationship between the camshaft drive chain or belt and the intake camshaft. The camshaft position is adjusted by an actuator using engine oil pressure to achieve an optimum valve timing point. This results in greater engine efficiency. Direct injection engines like the Toyota D4 (Direct 4 cylinder) are the ones where fuel is injected under very high pressure into the combustion chamber itself. During the combustion cycle air is pressurised or compressed and at a very precise moment, fuel is injected directly into the combustion chamber. This technology allows the engine to run more efficiently producing less harmful emissions and lower fuel consumption. The opposite of D4 is the multi-point or Multi-Port fuel injection (MPFI) which replaced the less efficient single point or carburretor fuel injection technology. In MPFI the engine fuel is injected into the inlet manifold to mix with the air prior to entering the combustion chamber. This engine fuel delivery system is less efficient when compared to the D4.
It should be noted that some of the MPFI and D4 engines are fitted with the VVTi technology. Toyota and other car manufacturers have evolved the fuel injection technology or combined different technologies (e.g. VVTI + D4) to push the engine efficiency bar further. It is important to note that this engine technology will only work well when it is maintainedwith high quality engine oil and fueled with good quality unleaded petrol.

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I own a Mitsubishi Airtrek Sports gear 2003. Its idling is rough when cold and occasionally the engine stops. It also makes some clicking sound around the timing belt area. All these go off when the engine warms up and the car runs normally. What could be done to sort this out.
Ambrose Olaa

You will need a computer diagnosis to determine the efficiency of your Mass Air Flow sensor (MAF) and Intake AirTemperature sensor (IAT). These sensors help inform the engine computer when to enrich the fuel injection during the cold starts (cold start enrichment) by metering and measuring the volume and temperature of the cold air coming into the engine. The engine coolant temperature sensor will also aid this function. These sensors may fail as a result of delay to replace your air filter, prolonged exposure to soot or corrosion damage and age (coolant temperature sensor). Symptoms of failure are poor cold starts or erratic idling during cold starts and general poor performance driving uphill or during acceleration. Also check for any faults related to the idle air control valve or idle speed control valve.
This valve regulates the engine idle speed by controlling the amount of air that bypasses the throttle plate in cars with a fuel injection system. This feature adjusts the engine idling speed as required.
The performance of the IAC valve is usually affected by accumulation of soot or carbon due to age or use of poor quality petrol. Symptoms of failure include erratic idling or intermittent engine surge and stalling. This valve can be cleaned with petrol or a carburretor cleaner by a qualified car repair technician.