Been having a lot of fun trying to get the wife's Ford Puma Millennium to run today. We were out and about in it yesterday (looking for new cars ironically) when it started misfiring, progressively getting worse as we drove around. We stopped into a Halfords to see if I could diagnose the problem and fix it cheaply. Removing the HT leads one by one revealed the problem to be with cylinder number 1 (removing any other HT lead made the car idle even worse). A new set of plugs didn't sort it and neither did a new set of leads so we limped it home and I've been working on it today.

I have swapped the following items one at a time and tested them on my 2.0 litre Focus, which I know runs absolutely spot on before I start swapping stuff.

Coilpack - Focus runs fine, Puma still runs on 3
HT leads - Focus runs fine, Puma runs on 3
Spark Plugs - Focus runs fine, Puma still on 3 cylinders.

So I have now completely swapped all the ignition system and proved that the coilpack, HT leads and plugs that were in the Puma are all absolutely fine but the Puma still has a problem because the Focus coilpack, HT leads and spark plugs are all known to be good.

Then I borrow a compression tester, thinking cylinder 1 is dead, which reveals strong compression (150psi) on all 4 cylinders. I then remove injector number 1 and swap it for injector number 3 - the car still runs on 3 cylinders so it's not a fuel problem and removing HT lead 1 doesn't change how the engine runs, removing HT lead 3 takes the car down to 2 cylinders.

I am now completely at a loss as to what the heck is going on. I have changed the coilpack, HT leads and spark plugs so it's not an ignition problem, I have ruled out a fuel issue by swapping the injector and its not a compression problem either. MAF, ISCV, TPS and Lambda sensors have all been changed for new items within the last 12 months. The only thing I have noticed is that when removing HT leads 2,3 or 4 the car runs like a bag of nails, when I remove HT lead 1 the engine runs exactly the same but I am getting electric shocks through the lead as I am holding it. This may be a red herring, especially as I've tried 3 HT leads so it's not the lead itself that's the problem.

ANY help on where to go from here is most appreciated.