My 1991 740 runs great. No smoke, idles great, low miles, ect... reciently though, I lost boost. I dont hear the turbine making its normal sound. Everything else seems fine. To fix the problem I put in a K&N filter, new sparkplugs, fuel system cleaner... what next??? PLEASE HELP!!!

no boost. in that case check the vacuum hose that comes from the inlet manifold on the engine and goes to the BOV on the turbo, as this is most likely whats causing it not to boost as the pressure generated by the turbo will just push the BOV open all the time.

Failing that ... check the waste gate actuator as it could be stuck open not spinning the turbine giving you a no boost situation as well.

IF that is all ok, then check the system for leaks, eg turbo to inter-cooler pipes, inter-cooler and its seals, inter-cooler to throttle body pipes etc... just give it a spray with soapy water when the engine is running and get a friend to hold her at like 3krpm. any air leaking out will form bubbles at the spot where its leaking.

Matt

Thanx I'll try that and let you know what happens.