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I'm trying to figure out how the fuel tank breather valve works.
There doesn't seem to be any consistency when its open, although I'm not getting any manifold pressure warning codes. It seems that when its closed the engine runs with lower HC's, i.e. when more air is being drawn through the engine breather hose. Breather hose is brand new.
Any help in understanding how its controlled is much appreciated.
 

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I'm trying to figure out how the fuel tank breather valve works.
There doesn't seem to be any consistency when its open, although I'm not getting any manifold pressure warning codes. It seems that when its closed the engine runs with lower HC's, i.e. when more air is being drawn through the engine breather hose. Breather hose is brand new.
Any help in understanding how its controlled is much appreciated.
below pick is typical of the system the tank is sealed by fuel cap the vent form the fuel tank is fed to carbon canister then vented via a ecu controlled purge valve the purge valve also allows a controlled amount of air back in to balance the pressure, the purge valve also as a diagram in it linked to ecu to monitor this pressure, if you have a leak somewhere fairy liquid and water in a spray bottle will show leaks when bubbles appear when spray the whole system. on as i would do a smoke test ie pump smoke in to the system see where it leaks,
the ecu is a bosch mev17.4 its a clever one for sure does many things has 3 big connectors runs switched earths for all sensors even the thermostat heater is via the ecu and earth as is the tank breather control, checking you have voltage as the plug is a start 0v to 11volts and when car running it should move up and down, find the earth first make sure its erath to deck then the reference wire ie voltage wire at plug if no volts then check it at ecu pin out, below is pic of of pinout for mev17.4 ecu two pics one for the earths, the view is set to terminal side so unplug the plug and count the rows for right wires, what you do is use a couple of paper clips and probe the rear of the wire in to the plug at each point of each connector as such, then use a multimeter set to 20v volts or 12v if yours does that,
 

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