Can't Know, will give it a try this weekend. Are you referring to air being sucked in around the injector?

Zulfiqar, I ruled out the plug as I assumed this would show a higher voltage on the lambda instead of a low one due to unburnt fuel entering the exhaust. I have piston slap, and suspect cylinder 5 as the culprit again so have been eager to pull those plugs for a while to check for fouling due to oil....its just such a mission to get to them! The car has 215 000 km so nearing the point where you had to replace plugs.

One question I have is that if there is a spark problem and the fuel is sent down the exhaust with a full compliment of oxygen, will this show a high or low voltage? Lambda sensors are ideally designed to measure the amount of CO entering the exhaust?