Hi Jassper
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what is the function of the flap in the intake?
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The flap is to increase the vacuum in the intake manifold when the EGR valve opens.
On the 1992 602, the flap operates in tandem with the wastegate : when vacuum is applied to the flap, the flap closes, no vacuum goes to the wastegate, the wastegate is open and no turbo boost is builded. At the same time, the EGR valve opens and more exhaust gaz is recirculated.
The opening of the EGR valve and the closing of the flap is controled by the Electronic Diesel System unit (EDS) : depending on engine speed and quantity of injected fuel, the EDS compare the actual intake pressure to a stored reference pressure to achieve optimum engine operation in respect of performance, NOx, HC and particle emission.
The EDS knows that the EGR is functionning when it sees by mean of the MAF sensor a reduction of fresh air admitted in the engine.
Overboost is controlled by the same set of components : when pressure is too high, the flap is closed and the wastegate is opened and the EDS sees a rapid drop in fresh air admitted.
When one of the component is not doing its job, the EDS cut all the vacuum to the wastegate in order to protect the engine. That's why you cannot just block the EGR valve on a 602.
When the engine is idling, the wastegate and the EGR valve should be closed, and the flap wide opened. Gradually increasing the revs to 2000 rpm, the wastegate will open (check the movement of the control rod), the flap wil close and the EGR will open.
The EDS unit stores error codes : plugging and unplugging various component can leave you without turbo boost until you erase the stored fault.