motors

[young-raymond]

please can any one tell me how to tell wich plug on my car is miss fireing by removing the leads,does the engine slow down or speed up, any help please.

[ChuckFickens]

It won't change when you remove the one that is missing, it will slow down when you remove any of the others.

Remove the leads with a well insulated set of pliers.

[sara3]

shall I get an ambulance on standby?

[ChuckFickens]

Meh... it's only about 60000volts. being blown 5 foot backwards across the garage is nothing :)

[sara3]

↑ this from the man who has all but beheaded himself!

[AlBags]

Plastic pliers .. engine ticking over.

Mind you ... won't work on coil-over engines .. but you could try a disconnect of the LT connection. Might cause a MIL light in itself tho : )

[Hopkirk]

Why not just replace all the plugs?

[AlBags]

Because he is possibly considering an injector issue .. or a valve problem ; )

[AlBags]

(or low compression because of another reason)

[Hymie]

If an engine is running rough due to a plug mis-firing, pulling the lead from the mis-firing plug will make little change to how the engine is running. Pulling the lead from a good plug will result in the engine running much rougher – if not stalling, due to having two plugs not working.

Most modern cars employ a small transformer incorporated within the connection to the spark plug such that there is no high tension within the actual lead.

Many years ago, I conducted such a test on an engine running slightly rough – and I am still here to tell the tale. Even so, I would advise that you employ some form of additional insulation between you and the HT lead – just to be on the safe side.

As others have said, there could be many reasons for the engine mis-firing/running rough.

[beso]

It isn't actually good idea to remove HT leads from the plug. It puts increased stress on the insulation in the coil because the voltage climbs even higher without the plug attached.

Bad pugs usually fail shorted rather than open circuit so a misfiring plug is not the same as an open circuit.

[Cowtipper]

If you get really good and wet, lean up against the car and start grabbing the boots on the plugs you can generally tell which ones are working properly. I use a timing light sometimes rather than trip a fault code.