BMW E34 jerks or hesitates under acceleration
E34 jerks or hesitates under acceleration - what usually causes it?
The most common cause is the ignition: ignition coils, spark plugs or ignition wires. Other causes are the throttle position sensor, mass air flow sensor, vacuum leak, fuel delivery and lambda sensor. Read the fault codes first, they guide the search.
Why does oil in the spark plug wells cause hesitation?
Oil leaking from the valve cover gaskets runs onto the plugs and coils. The spark then escapes to ground and the engine hesitates, and the coil may eventually fail. Replace the gaskets. Careless engine washing can cause the same if water gets into the plug wells.
Can sensors and the mass air flow sensor cause jerking?
Yes. A faulty throttle position sensor, a dirty or broken mass air flow sensor and the lambda sensor can all cause jerking. You can test the mass air flow sensor by unplugging its connector and revving the engine: if the running does not change, the sensor is probably faulty. A dirty idle control motor can also cause hesitation.
Can fuel or a vacuum leak cause jerking?
Yes. A clogged fuel filter, a weak pump or a faulty injector reduces fuel delivery. A cracked intake boot or another vacuum leak leans out the mixture. Check the intake hoses and the intake boot all the way round, including by squeezing them.