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hello
i would check and see if the wastegate is stuck or unhooked. maybe the wastegate actuator is gone bad.
or check the small hose that goes from the turbo compressor to the wastegate actuator, i replace them from time to time.
seems like the wastegate is not dumping the extra boost.
the cbv is there to prevent overboost, on my car it seems to open up at 16 psi and recurculates the extra boost back through the turbo.
shouldnt be too big a problem.
i think the 1990 740 turbo has a overboost cut out switch but im not sure.
good luck
mike
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