Odd.

So you are looking for a common mode failure.

I have a recollection from the dim and distant past (racing motorcycles, I think) of hearing about simultaneous plugs failing because of the formation of an invisible but conductive glaze on the internal insulator, caused by *something* chemical getting into the cylinders. Can't recall what. Oil from turbo? Antifreeze leak?

Maybe the computer recognised a fault, and refused to let the car start. I had this happen on a US Pontiac, two bad injectors, out of six, and it would not allow the engine to fire.

It's pretty crude injection on a car this old - fuel only. I doubt it's that clever. ;-)

Bit of a grasping at straws, but could it possible be a failing coil / HT side of things on *both* cars and the "working" plugs having a slightly smaller gap?

Ian

The Pontiac was a 1991 car. It had a brilliant four-valve per cylinder, four-cam V6 engine producing 210 bhp. The engine was still mechanically great at 125,000 miles, but the engine computer kept on losing all communication with everything; it needed to idle at 3,000 rpm to stay alive. Then the driver's seat fell back 3 notches one day, and I eventually had to let the car go. One that I still miss.

What happens if he tries three "good" plugs and three "bad" plugs per car...?

The manual 520 in question is well north of 250,000 miles. Much of it towing a big caravan. Only major work done to the mechanicals was a cylinder head overhaul after a failed head gasket. I bought it at auction direct from a lease company and passed it on to my brother. The auto is much lower mileage, and he's just the second owner.