1990 rst sluggish between 3000 and 3800rpm then preasure suddenly drops to 0 psi with a loud hissing noise then it hits 4000rpm hissing stops preasure jumps straight back up and car takes off like nothing will catch it.any ideas please.have checked hoses for leaks and cant find anything.cheers

 

If I were to say that it may be the small vacum pipes that are fitted either end of the plenum would that make sense to you?

They are usualy held on with jubilee clips or cable ties!

Its likely to be conected with air escaping from 1 of them.

That would be my initial impression.

 

is it likely to be the waste gate solenoid control valve

 

Simple if it has a dump valve?
The dump valve fitted is leaking at peak boost. Bin it. Job done.

 

removed the dump valve and the preasure gauge and still got the problem.just renewed all the vacume hoses on the inlet manifold today and still no better

 

Im Amazed... sounded so mush like that problem...

You need to get her somewhere that can pressure test the system for you as it sounds 100% like you have a leak at peak boost.

When you say you removed te dump valve, you didnt just remove the pipe to te top of it did you? You took it OFF teh car and bunged up the big 25mm hole it left?

 

yes fully removed it from both ends and sealed them