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Under light cruise or decelleration, never at idle or high load conditions.
The vacuum hose at the egr valve runs to a switch-over valve, there should be 2 switch over valves, one for egr and the other, right next to it, for the air injection valve, so you have to be sure someone didn't swap hoses around.
The ecm decides when to switch it on and for how long, but is driven by driving conditions, not any preset sort of time element. If you are coasting along and the egr valve is open and you suddenly accelerate, the ecm will close the egr valve, for example.
As a test you can apply vacuum to the egr valve with a hand vacuum pump and the engine will idle roughly. If it doesn't then you know the problem is the valve or passages and you are on the right track. If it DOES then the operational side of the egr system is fine, the problem is in the control, either what I mentioned, the vacuum hose is going to the wrong switch over valve, or the (switch over) valve is bad or the wiring is bad. I "think" the wiring can not be reversed between the AIR s/o valve and the EGR s/o valve, the connectors are different to prevent this from happening, but the vacuum hoses CAN, damhik!
Gilly
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