Toyota Voxy: 1AZ-FSE D4 Direct Injection Intake Valve Carbon Deposits

The 1AZ-FSE engine uses Toyota's first-generation D4 direct injection system, which injects fuel directly into the combustion chamber rather than the intake port. Because fuel and its detergent additives never wash over the intake valves, oil vapour from the crankcase ventilation system accumulates on the back of the intake valves unchecked, baking into hard carbon deposits over time. These deposits progressively restrict airflow, disrupt the fuel-air mixture, and can cause misfires; severe buildup prevents the valves from seating fully, leading to compression loss in affected cylinders.

• Rough or lumpy idle that worsens when the engine is cold
• Hesitation or stumble under light throttle acceleration
• Persistent misfire fault codes (P0300–P0304) with no ignition or fuel system cause found
• Reduced fuel economy compared to the engine's rated consumption

1. 1

   Check for rough idle, misfire DTCs (P0300-P0304), and smoke at idle.

2. 2

   Request any history of walnut blasting or chemical intake cleaning.

3. 3

   If a workshop inspection is possible: Remove the intake ducting and use a borescope or mirror to view the back of the intake valves through the inlet ports; look for grey-black crusty buildup.

Parts involved: PCV / oil separator.

$550 to $1,600 at New Zealand independent workshop rates. That is for this job on its own, not for the car.

An estimate, in NZD, for the repair as normally carried out. What a specific car costs depends on which part has failed and what it damaged on the way — treat this as the floor for a negotiation, not a quote.

• “Has the intake ever been walnut-blasted or chemically cleaned, and at what mileage?”
• “Are there any current misfire warning lights or rough-idle conditions?”
• “Does the engine hesitate when pulling away from a standing start?”

The failure described above is documented in published sources — owner forums, technical service bulletins, recall notices and workshop write-ups — held in our fault database. The inspection instructions and the cost estimate are KickTyres’ own interpretation of them, and it is a starting point for an inspection rather than a manufacturer statement.