[elemgee]

Just to recap, I have a slightly rough idle until the engine completely warms up,and when in drive, the revs will bounce around between around 650 and 750 rpms.

I've since replaced my O2 sensors, knock sensors (I was getting a CEL indicating that the drivers side O2 and knock sensor were bad) and coolant temp sensor (this was the problem per another post by someone with a problem similar to mine), thinking some of that was causing the slightly rough idle when cold. But the problem still persists, so I bit the bullet and took it to a mechanic. His diagnosis was that he smelled engine coolant when opening the hood (I thought it might be due to losing some coolant when replacing the coolant temp sensor). He didnt find anything obviously wrong after hooking the engine up to his diagnostic machine; so he thinks it could be a bad head gasket, valve spring or something along those lines. He wont know for sure until he removes the head, which shows 12 hours of shop time, which I'm assuming is at least $600.

I'm going to take it to a different mechanic for a second opinion but thought I'd ask - would a bad head gasket cause this problem? The idle seems fine once the engine is completely warmed up (after driving for about 20 minutes or so).