BMW E28 hard to start when cold
E28 starts poorly when cold but normally when warm – what causes it?
When cold the engine needs extra fuel and extra air. If a part related to enrichment is faulty, the fault shows only when cold. The suspects are the cold start injector, the thermo-time switch, the coolant temperature sensor and the idle control valve. Test them one at a time.
How does the cold start injector work and how can I test it?
The injector is at the end of the intake manifold and sprays extra fuel during starting when the engine is cold. Its operating time is controlled by a sensor in the thermostat housing, and the injector receives 12 V through it. If the injector gets voltage but the car does not start, the injector is faulty. If no voltage arrives, the fault is in the sensor or the wiring.
How do the idle control valve and the temperature sensor cause cold symptoms?
The idle control valve is open when cold and closes as the engine warms up. If it is stuck closed, the cold engine does not get enough air and you have to pump the throttle. First check the vacuum hoses for cracks. If the temperature sensor or its wiring is faulty, the engine gets no information that it is cold and does not enrich, so the idle only settles once the engine warms up.