In article ,
"Steve" says...
I have an up & over metal garage door that stays up when I open it most
of the time. The problem is that every now and again it drops down
again, by anything from about 6 inches to 2 or 3 feet. This often
happens some time I have opened it and carried on, happily believing it
has decided to stay open. It's already taken a lump out of the top of
my head and it's only a matter of time before it drops on to the roof of
the car as I back it out. This would not be a good thing.

Its the type that has a large metal spring just above the door, the full
width of the opening. At each side of the door opening is a vertical
track in which a nylon wheel attached to the garage door runs. There is
also a taut wire running from a reel on the end of the big spring to
help pull this wheel up.

The big spring was dry and rusty so I gave it a good soaking in WD40 and
that seemed to improve things for a while, but the problem soon came
back. I've sprayed everything I can see with WD40 but it's still not
right. Any suggestions would be welcome.

A stout bungee between the top of the door and an eye on the wall?
:-)
There's probably some way of winding more tension on the spring, but
as it's heavily corroded I'd worry that it might break, releasing a
considerable amount of energy when it did and possibly sending pieces
of hard steel flying through the air with some force.