I've always had this bad idle, and a slight missfire.I've done -

Injectors
Plugs
Leads
Coil pack
DFI module

But it's still there.Everytime it misses, the timing chain tensioner makes a clicking noise.Does that mean it's ****ed? If i replace the tensioner, do i need to do anything else? How long should a timing chain last? I have about 180,000 on the clock, and i'm not sure if anything has been replaced in that time.

How hard would it be to replaced the chain and tensioner? Do i need to know anything about adjusting timing?

What are these double row chains? I assume the VP is a single row? What advantages does a double row have?