An egr should only be open above idle, on a warm engine.
If it opens at idle/partly open= stall or very rough running
If it doesn't open = may cause pinging on acceleration, Nox emissions will increase.
Apply vacuum to the valve with the engine idling. If it stalls or is noticably rougher, then you know that it is 1) closed at idle and 2) the vacuum diaphram is ok.
Your PCM applys vacuum at the correct time by energizing the vacuum solenoid in the vacuum line. Check for vacuum at the valve at idle, there should be none. If you always have vacuum, then the solenoid is defective or there is a problem with the PCM
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