Terry, this is a reply from Justin F about the bov, may explain the problem you found!
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Glad you like the dump valve, glad it arrived on time, this is the sort of
service you should normally receive from me.
The bolt at the back of the SQV is for fine tuning the spring strength.
On cars with MAF sensors (like UK supras), it is possible for the car to
stall
if the spring strength isnt enough (to increase wind the bolt in).
loosening the bolt, should increase dump valve volume and response, too lose
and
you may have stalling problems or (I think) it may leak boost pressure.
Try unwinding the bolt a little and see what the results are.
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