Hi,

My dad has a 2006 FFRR 4.2 Supercharged and it has a misfire on idle.

It's been in the garage and they replaced spark plugs and coils but that didn't work.

They plugged it in to get readings and cylinder 1 is misfiring quite a lot. He's just done a compression test and he said the reading for cylinder 2 was about 600psi whereas cylinder 1 didn't even get to 300psi.

Is it certain that it needs to be rebuilt?

Squirt some oil onto the cylinder and do the compression test again, if adding the oil raises the compression then it's the piston rings, if it makes no difference then it's the head, either gasket or perhaps a valve problem.

Any other symptoms? Water and oil ok? Overheating or pressure in the cooling system?

Water/oil seem ok.

Doesn't over heat at all and runs lovely except on idle..
They've had to put the car back together now as they need to get it out of their garage.