Vehicle
Alfa Romeo 156, 2.0 L Twin Spark — surging idle and occasional stumble on take-off.
Symptom
Surging idle once warm, worse with the air conditioner on. Plugs, air-mass meter and throttle-body clean / idle adaptations had already been done elsewhere.
Diagnostics
Bosch KTS showed no stored engine codes. The system is Bosch Motronic 1.5.5. Idle air is controlled by a side-mounted motor on the throttle body (about 10° of flap at idle); with the accelerator cable applied that idle motor is out of the loop.
Checks covered:
- Spark / coil output
- Live data (coolant temp, air mass, throttle degrees at idle)
- Valve timing and belt tension
- Smoke test for manifold / PCV leaks (dipstick O-rings were leaking)
- Full adaptation reset and base idle / TPS switch setting
Throttle actuator continuity between pins 6 and 7 was about 4 Ω (OK). PicoScope on the actuator drive should show ~7 V peak PWM from the ECU per the KTS tech diagrams. We saw that — and intermittent 12 V peaks, which meant the DC stepper was going open-circuit under load (resistance normally pulls the peak down to ~7 V).
Summary
Faulty idle throttle actuator / throttle body. New units were hard to source at the time of writing; the customer was advised to import from the UK, fit the unit, then return for idle adaptation reset.
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