Here are some faults I have encountered when using Weber-Marelli engine management which are worth mentioning as they can cause strange symptoms.
1. Fuel Pressure Regulator leaking
Here are two pictures of a disassembled Weber fuel pressure regulator, with the bottom one being a close up of the diaphragm. The problem here is that the material that the diaphragm is constructed from will start to break up with age and become porous. This allows fuel to enter the upper part of the regulator which is directly connected to the inlet manifold. Initially this may not be noticeable, but the mixture will become richer and richer for no apparent reason and will be even worse on idle with a vacuum helping to pull the fuel through. In my case it became so bad that petrol would pour along the pipe and cause a big puddle in the throttle body elbow causing a massively rich smoky idle.
2. Sticking throttle / TPS badly adjusted
If the throttle does not fully return to its closed position, then the ecu will assume it is being held open by the driver and so will not try and control the idle speed. The same fault can be caused if the TPS is not correctly adjusted. This may not be too much of a problem on a running engine but it does create a strange fault when starting. When the ecu is powered up it sets the ISCV to fully open, which helps the engine to start without using any throttle, the ecu will then quickly close the ISCV down to the required level to maintain the required idle speed. If it thinks the throttle is not closed then it will leave the ISCV fully open until it does, causing the engine to idle at about 1500rpm when cold and increasing to nearly 3000rpm as the engine as the engine gets hotter and easier to turn. This fault can be intermittent so you have an engine which will suddenly rev wildly for no apparent reason. The ISCV itself can also sometimes clog up with carbon particles from recirculated blow by gases etc and will need cleaning from time to time to keeps its operation smooth.
3. Leaking Dump Valve
The standard bosch dump valve diaphragm can develop a split causing it to vent boost pressure back to the air filter. Although the boost pressure may be correct, the turbocharger is working harder than it should be doing to maintain that level and may fail prematurely.
The picture of the split diaphragm shown above has a screwdriver in it so it can be seen clearly in a photo. The valve relies on boost pressure acting on both sides of the diaphragm to keep it pushed closed, and the split will reduce the pressure at which it will start to relieve boost. The best remedy is to replace the valve with one which uses a solid piston.
4. Blocked Fuel Pump
The fuel pump outlet is always filtered before it gets to the injectors but the supply to the pump is not always filtered in any way. The pictures below are of my pump which seized solid and I opened up to see why.
The problem arises when a foreign object gets between the faces of the roller cell (see above) which reduces in size as it turns to create the pressure. Once it is jammed in here there is no way of getting it out again and the pump needs to be replaced.