Why has my Toyota Wish VVTi system failed? 

Hello Paul, I drive a Toyota Wish with a VVTi engine. It has been running well until recently when it started reducing engine power or hesitating. A computer diagnosis reported it. Can the VVTi solenoid cause poor engine running? How long can I drive the car before replacing it? Jackson 

Hello Jackson, to appreciate the role of the VVTi solenoid, you need to understand how the VVTi (Variable Valve Timing intelligent) technology works in your Toyota engine. Variable Valve Timing is a technology most car manufacturers have introduced to their engine air intake systems (valve trains) to improve fuel economy, engine low end and high speed torque and performance. It works like your mother’s traditional cooking stove (sigiri). 
The more you improve the management of air intake in the cooking stove by varying the amount of air entering through the air vents and door, the better you will manage the economy of charcoal as well as the efficiency of burning (fire). In a VVTi engine, this technology uses oil pressure to continuously vary the angle or lift of the camshafts in the air intake system to ensure fuel economy, low end and high speed torque and better engine performance. 
The VVTi solenoid is an electrical component that plays a central role in this process because it actuates the oil pressure needed to adjust intake valves. Critical for the operation of the VVTi solenoid is timely service of your engine with the manufacturer recommended oil grade. Counterfeit or low quality engine oil will build up lumpy sludge in the engine, which blocks the VVTi solenoid and prevents smooth operation of the VVTi system. 
The VVTi solenoid will cause delayed starts, hesitation, misfiring, poor acceleration and poor fuel economy. You can limp on like this but cause damage to other ignition system components. The first step towards rectifying the VVTi system problem is to service the engine with recommended engine oil grade, which can sometimes restore its operation. Should the VVTi solenoid fault persist, especially if it reports circuit failure, you will need to replace it with a new one. 
A bad VVTi solenoid is not repairable.