I have now got a test gauge to check my compression. The plugs out of the suspect back were full of carbon. I have disconnected the fuel pump before I start and have read I need to disable to leads that go to the coil pack, where do I need to unplug these from I can't seem to find the end?

 

Put a squirt of oil or 3 in the offending cylinder. If the compression goes up, the majority of the problem is rings. By the rest of your post, if that plug is so black, that cylinder is probably getting too much fuel, so you may have a leaking injector......Mike

 

Thank you mike, do I need to unplug the coil leads from somewhere before doing the compression test or will they be OK with the coil packs removed alone?

 

Chris, what you are wanting to do is install a jumper wire (with a built in switch) between the battery terminal of the starter and the small terminal on the starter that engages the solenoid on the starter. By flipping the switch, this will cause the starter to engage and roll the motor, but not activate any of the fuel system and/or the spark plugs. That way you are in complete control of what the motor is doing. Just make sure to use a switch capable of handling say 10 amps. Getting one that spring returns to the off position is a nice thing to have too.

 

Not sure if our cats do this but if you floor the gas peddle when cranking the engine most cars cut fuel and spark from the engine for just such a test.