Subaru Legacy: EJ20 2.0 twin-turbo sequential changeover and exhaust control valve faults

The sequential system runs the small primary turbo at low revs and brings in the larger secondary turbo higher up, managed by a set of vacuum-actuated control valves including an exhaust control valve that should stay shut at idle. The actuators, vacuum hoses and solenoids perish with age, and when the changeover misbehaves the car surges, hesitates or simply will not make full boost. It is rarely catastrophic on its own but it is genuinely difficult to diagnose, few New Zealand workshops know the system well, and a car sold as "just needs a tune" often has a perished vacuum system behind it. Budget diagnostic time as well as parts.

• Surging, hesitation or a sudden flat spot around 4,000 rpm where the second turbo takes over
• Boost that feels inconsistent between drives
• Engine warning light with boost-related codes
• Audible hissing from the engine bay under load

The test drive is the real test here. On a clear straight road, accelerate firmly in a mid gear from about 2,000 rpm all the way up and pay attention to the middle of the rev range: the pull should be continuous, with at most a mild step. A pronounced flat spot, a surge, or boost that arrives and then falls away points at the changeover system. Repeat it two or three times, since the fault often varies. With the bonnet up and the engine idling, listen for hissing and look over the many small vacuum hoses across the top of the engine for hoses that are cracked, brittle, oil-soaked or obviously replaced with generic tubing. Ask whether the car has been modified, remapped or had a boost controller fitted, and whether any original vacuum plumbing was removed. If a workshop inspection is possible: have the vacuum system pressure-tested and boost logged, and the fault memory read.

Parts involved: Turbocharger.

$400 to $2,500 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 car been remapped, modified, or had a boost controller fitted?”
• “Has any of the vacuum or turbo control plumbing been changed or removed?”
• “Does it ever hesitate or surge in the mid range?”

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.