On my engine Ford have designed an exceedingly tight fitting rubber plug cap. They are very difficult to remove. There are 3 or 4 separate little sealing fingers at the top which are a really tight fit into the bore. On their own they would make a formidable water seal. But first the water level would have to be deep enough to lap over the 10mm high upstand moulded into the rocker cover. That would be a big puddle of water sat on a hot cover and I’ve never seen a drop. But I do suspect that on mine the plug and the surrounding ali cavity below the cover is a condensation trap.
Roger, I have the same fittings as you and the leads were very difficult to remove. It's hard to understand how so much water got in and I've never seen any even when I have checked after being out in the rain. On this particular day the car had been running perfectly for hours before finally misfiring so why didn't the water boil off ?
