Firing on 3 cylinders...

[eihtur]

...instead of 4. How can you find the one that isn't firing?

 

[Troon]

On a petrol with one plug per cylinder and old-fashioned HT leads, pull the leads off one at a time. The lead that doesn't make it fire on two cylinders is the dodgy circuit.

Obviously, take care with HT lead voltages and running engines.

 

[Stratman]

I'd be reluctant to touch an HT lead while the engine is running.

I'd stop the engine, remove the spark plugs and look for the wet one. Failing that, one may be a bit cooler than the others.

Or you could buy a spark indicator.

 

[BlackC55]

The simple way. Plug it in.

 

[Bellow]

Removing plug leads on a running motor is frowned upon nowadays as the higher Voltage deprived of its earth path will try and escape its insulation - damaging something (lead or coil) as it does so.

An ad-hoc method to identify a duff cylinder is start the engine from cold and observe the exhaust manifold heating up. The slow to warm branch is from the lazy cylinder. (Flicking a drop of water helps, but only if it lands on the manifold).

 

[eihtur]

I should have said, this is a VW new beetle...the plugs are almost completely inaccessible without removing things. But thanks for the suggestions.

 

[DSM10000]

I should have said, this is a VW new beetle...the plugs are almost completely inaccessible without removing things. But thanks for the suggestions.

The individual coil packs are known for failure on these engines. If you remove the plastic engine cover they are fairly easy to get out. With the engine off you can then unplug the electrical connector to each in turn and see if gets worse or remains the same.