First off, does the car have an aftermarket blow off valve, or just the stock bypass valve blocked off? Whatever the case, the ideal setup would be to recirculate the bypass valve, but if it was tuned with it blocked you will probably need to have the tune tweaked once it's hooked up correctly. If it is the OEM BPV it is probably leaking like crazy because the vacuum pulled from the intake aids in keeping the valve closed. Although none of this really gets at your problem.
Honestly, the first thing I would do is have a compression and leak down test performed. Once you rule out engine failure then we can go from there. Data logging would be beneficial in diagnosing the problem, but I would get compression and leak down tests done first. If the engine is on it's way out it would be best not to run it anymore to prevent further damage to some components that may be salvageable.