Need Help Diagnosing

Hello all,
I have a 1994 Cavalier with the 2.2L engine, 97K miles. Recently it has started to knock & ping, and the fuel economy has dropped 15-20%. No other symptoms present,
and no change in driving style/conditions.
I've been told it could either be a faulty EGR valve, or a plugged catalytic converter. I changed the spark plugs, no effect. Any thoughts ?
Thanks in advance...
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Make sure the EGR is not jammed - try pushing up on the diaphragm underneath, you should be able to move it with firm finger pressure. Also, if you do this with the engine idling, it should run rough or stall - if not, the EGR passages may be clogged with carbon.
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Robert Hancock Saskatoon, SK, Canada
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