A bad distributor can cause rough engine idle and even cause the engine to fail to start.
The distributor takes the voltage from the battery, alternator and the ignition coil and distributes that high voltage through the engines cylinders as it rotates.
If the distributor is not working properly then your engine may idle rough, may stall out or may fail to start.
However there's also another part on the distributor or near it called an Ignition Control Module and when that Ignition control module fails then the engine will start idling rough and then eventually the engine will sputter and die out and not restart.
Overtime the distributors bearings can wear out and make it tight for it to turn so the bad distributor can also short out new ignition control modules.
I had a customer who came in with his truck and said that he replaced the ignition control module on his distributor 10 times and within a month each one failed.
I checked the distributor and it was getting really hard to turn and put a new distributor in for him and the modules have stopped failing.
Rough engine idle can also be caused by bad spark plugs, bad timing, bad fuel pump, low fuel pressure, bad spark plugs, bad gasoline etc.
A bad ignition coil can also cause rough engine idling and even engine misfires.
The most common sign of a bad ignition coil is an engine misfire.
The misfire could be on one or more cylinders or if you have one ignition coil for the entire engine and also have a distributor and distributor cap and rotor then the one ignition coil failing could cause engine misfire in all or one of the cylinders.
Also if you have one ignition coil and it goes bad your engine may run rough, may start but then stall and then start up again.
Or the engine may not start at all.
When the ignition coil is not working properly it's not sending the proper amount of voltage to the engines spark plugs which is needed to start the engine and keep it running.
You can have Autozone or other auto parts stores run a check engine light test on your vehicle if you check engine light is coming on and you're getting hard starts or engine misfire and it can tell if you have a bad ignition coil or not.
Engine misfires, rough idle, and loss of power are very common signs and symptoms of a failing ignition coil or coils.
One of the most common symptoms associated with a faulty ignition coils is engine performance issues.