Why do mechanics dislike D-4 engines, Toyota Voxy?

1) Just what is wrong with D-4 engines?
2) Also what is wrong with Toyota Voxy?
These two are highly discouraged by mechanics in Uganda.

The D4 (Direct Four) Toyota engine and its companion technology the VVTi (Variable Valve Timing intelligent) have been demonised and declared a no go area by some drivers and mechanics when it fails. This is the car engine technology of today and it is not about to go away.

To appreciate why this technology is problematic you need to understand what it is. Evolution of car engine technology has been driven by the pursuit for better fuel economy, engine power and less environmentally harmful emissions. The first step was the shift from carburretor engines which mechanically mixed and delivered fuel to the combustion chamber.

The carburretor was considered wasteful because it delivered unprecise fuel amounts and its efficiency depended on the condition of its mechanical components or one’s driving style. This was followed by the more efficient and conventional Multi Point electronic Fuel Injection (MPFI). The MPFI system relies on an engine computer and sensors to inject fuel into the inlet manifold (ports/delivery tracts) where it is mixed with air before delivery to each cylinder for combustion.

The shortcomings of the MPFI system include its wastage of some fuel which stays in the delivery ports especially during cold starts and goes unburnt as well as limited precision of fuel delivery timing. Automotive engineers and car manufacturers have tried to overcome the above inefficiencies of MPFi by introducing direct fuel injection technology.

Toyota call it D4 (Direct injection four), Mitsubishi call it GDi (Gasoline Direct Injection), Mercedes call it CGI (Charged Gasoline Injection) and Volkswagen/ Audi call it FSI (Fuel Stratified injection). The D4 engine uses an engine computer to regulate the injection of petrol under high pressure (increased fuel atomisation / charge pressure) through a common rail fuel line directly into the combustion chamber of each cylinder.

The D4 fuel delivery timing is more precise, as a result the engine runs leaner (uses less fuel), reduces harmful emissions while producing a higher power output. To push the engine efficiency and performance envelope automotive engineers at Toyota have combined MPFi and D4 with another technology VVTi (Variable Valve Timing intelligent). This is the technology in the Voxy, RAV4 and other Toyota vehicles with the 1AZ-FSE engines.

Other car manufacturers like BMW, Honda and Mercedes have combined the multi-point and direct fuel injection technologies with Variable Valve Timing intelligent in their vehicles. 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. The outcome of this effort is better efficiency and performance.

However, some of the Toyota engines with the D4 and VVTi systems such as the 1AZ-FSE in the Voxy or in the post 2000 RAV4 have had performance related problems. The issues range from cold start difficulty, erratic idling, loss of engine power to outright failure. The potential cause of D4 and VVTi system failure is the buildup of carbon deposits on the engine intake valves which affects airflow to the cylinders and cuts down engine power.

This is a result of using adulterated or leaded fuel which does not burn completely and forms deposits around the valve necks and seats. This can be avoided by fuelling with unleaded fuel from reputable fuel dealers. The periodic replacement of fuel filters and spark plugs with genuine Toyota parts will help. Fuelling with Shell Fuel Save unleaded petrol which has additive chemicals would keep intake valves clean by preventing accumulation of deposits.

Accumulation of dirt and soot along the air intake walls and the throttle valve housing will affect the engine performance or damage internal moving parts. This can be avoided by regularly maintaining or replacing dirty air filters. The use of high quality multigrade engine oil like the Shell Helix HX5 15W40 or the fully synthetic Shell Helix Ultra 5W30, Shell Rimula R4 or 5W40 are recommended. This oil is blended with chemical detergents to clean after burn deposits and soot on pistons before transporting the chaff for filtration. It is therefore important to service with genuine Toyota oil filters.