Arbre ou roulements d'allumeur usés

Wear in the distributor's shaft or bushes letting it move where it should turn true. It applies to older engines with a mechanical distributor, and it matters because the distributor carries the ignition timing reference — so shaft movement is timing scatter.

The signature is a timing signal that wanders rather than fails. Play in the shaft lets the reluctor or pickup gap vary as the engine runs, so the module receives a signal whose edges move relative to the crankshaft — and it reports a reference or synchronisation fault while the pickup itself is healthy.

It gets worse with revs and load, since those are what deflect a worn shaft, so the complaint is a misfire or hesitation that appears under acceleration and clears at idle. Check the shaft for radial play by hand with the cap off — any perceptible side movement is too much. A worn drive gear produces the same scatter and is worth inspecting at the same time, since replacing the distributor without checking it repeats the fault.