Common faults
The turbo system on all Biturbos is actually quite simple. Pressurised air from the turbos is fed via a couple of large diameter pipes and hoses to a plenum chamber - more simply, a box that covers the carburettor or injection throttle bodies. A number of small diameter hoses provide control and information feeds to various ancillary parts of the car.

The rubber hoses and seals should really be time limited as they realistically have a life of only between 2 and 4 years. A tiny pin-prick size hole in any of the hoses or a poor connection anywhere in the system will cause problems. Whenever you have problems with the turbos in your car (other than obvious clouds of smoke from the exhaust) the first thing to do is check all the hoses and connections.

Rule Number One is: If in doubt, replace that hose!

Poor boost pressure
The most common fault that owners complain of is insufficient boost. The system is susceptible to leaks in any of the joints from turbos to plenum chamber. The turbos themselves are buried down beneath the engine and can be difficult to access without a hoist but the good news is that most leaks occur at the top of the engine. The plenum chamber - with its large gasket - is the most likely culprit followed by the various hoses leading to and from it. Only the smallest pin-prick of a hole is needed for boost pressure to fall markedly and/or to cause rough idle running.

Rough running or vibration under boost
Under full boost it is important that the waste gates on both turbos operate equally to keep the pressure on both sides even. The waste gates have springs that can be adjusted but this must be performed with the correct gauges following the workshop manual procedures.

If the system becomes unbalanced, symptoms can include loss of boost pressure, rough running or even marked engine vibration under boost.

Boost pressure too high
Unless your engine has been modified for additional boost this problem has one of two causes. All Biturbos use a similar mechanism for controlling boost pressure. Sensors on the engine measure engine speed, manifold pressure and 'pinking' or pre-detonation (the dangerous result of too much boost). Early cars use a box called MABC (Maserati Automatic Boost Control) while later cars have more sophisticated integrated engine management systems. For the purposes of this discussion, both work identically.

When manifold pressure rises above an absolute setting (about 0.7 bar) or (on the more sophisticated systems) too high for the engine speed and load OR any pinking is detected the system opens the waste gates and dumps the turbo pressure until a safe condition is obtained.

How do the electronics operate the waste gates?

The waste gates are operated pneumatically. Take a look at your plenum chamber and you will find two small diameter rubber hoses leading to a small, cylindrical box attached to the rear bulkhead. This is the turbo-control valve (known to its friends as the AMAL valve). One of these hoses feeds pressurised air to the valve, the other allows it to return to the plenum chamber.

Another hose leads from this valve to the front of the engine where it meets a plastic T-piece from which two further hoses lead to the individual turbo waste gate servos on either side of the engine. On top of the AMAL valve are two electrical connectors which are connected to the MABC or engine management system.

MORE INFO - http://www.maserati-rc.org/techtips/Techtip052.htm