engine revving up and down quickly

[Royal_rasta]

i took my valve cover off to look to see what was rattling in it. i take it off and found a broken plastic piece with a hex head screw in it along with the hex head bit to fit it. i looked around and didnt see anything broken inside. i removed them. put the cover back on and now the engine revs up and down quickly at idle and when im stopped in drive. Help please??

[bosers]

That's a bit worrying finding something rattling about in there. If you haven't renewed the rocker gasket then there could be a vacuum leak that will cause the engine to idle erratically. Also if there's any loose hoses etc.
I'd definitely try and find where the loose bit has come from as well.

[HairyScreech]

What engine?