BMW E34 M5 (S38) rough running and hesitation
E34 M5 hesitates or cuts out under hard acceleration - what could cause it?
The most common causes are in the ignition: distributor cap and rotor, cracked ignition coils or a rubber boot (the sleeve leading to the spark plug) that has come off a coil. The valve cover gasket can also leak oil into the spark plug wells. Check these first.
Why does the E34 M5 run rich or unevenly?
The mass air flow sensor signal can be too low, which makes the mixture rich; replacing the sensor removed the fault. Enrichment can also stay on if the coolant temperature sensor shows a wrong reading. Replacing the lambda sensor does not necessarily help.
Why is the E34 M5 uneven or rattling at idle and stalls under acceleration?
An intake manifold gasket leak leans out the idle; you can find the leak by spraying the joints. If the engine rattles and stalls under hard acceleration, the cause may be a worn crankshaft key and a loose mounting bolt. Have a workshop tighten it to the correct torque.