BMW E90 glow plugs, glow relay and glow light
How long does glowing last on an E90 diesel and when does the glow light come on?
Pre-glow is 0.5 s when the coolant is below 25 C, and it lengthens as it gets colder up to at most 2.7 s below -25 C. The glow light is shown only when the coolant is below 0 C. Post-glow happens below 30 C and improves idling. With a warm engine the light barely flashes.
What does a fault in the glow plugs or glow relay cause?
A glow fault affects above all starting, and usually not the smooth running of the engine. Each glow plug gives its own fault code. A short circuit in one plug can burn the fuse wire of the control box. The glow relay's internal wires often burn through, and they can be repaired by soldering new ones in.
Is it worth replacing the glow plugs one by one?
It is worth replacing them all at once. If the intake manifold has to be removed, replace its gasket at the same time. In some cars the glow relay can be replaced without removing the intake manifold, but in others the relay is too tight, so how hard the job is depends on the car's version.