2nd turbo SOMETIMES doesnt work

[Macca-RX7]

I would say 50% of the time the 2nd turbo charger does not boost.
I have come across a troubleshooting site for the turbo chargers which looks very good, but as I have my boost pattern im wondering if anyone here could point me in the right direction what to look for rather than checking everything?

In 3rd gear, the 1st turbo will boost and hold around 10psi. Then at around 4500/5000 revs when the 2nd turbo is meant to start, the boost just drops from 10psi to a very low pressure.

Although when the 2nd turbo does work, i get the correct boost pattern of 10psi, drops to 8psi at 4500rpm then back to 10 at 5000rpm.

As it only does it 50% of the time i assume it just something silly?

Any ideas?
Cheers

[Macca-RX7]

I think I\'ve sorted it, just for anyone else who has the same problem it appeared to be a little check valve in the vacuum line between the turbo y piece and the pressure container. It was split, everything seems fine now

[chrisJDM]

Boost issues on fd\'s are usually down to a boost hose as there are so many of them and due to the age of the cars the hoses go hard, split or perish. I recently took my plenum off and found a couple broken hoses so replaced them