Dummy question no.2

What's the EGR valve? Is it on all 190s around this age? I've never heard of it... Thanks for the other info too

Cheers, Martin

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1990 420SEL 217k Miles
1975 Chevy K10 400ci SBC power!
1995 Jeep Cherokee 4.0 Limited
1997 Suzuki GSXR750 SRAD
1989 Kawasaki GT550G4