I have an 86 vette that idles up and down between 700 and 1000 when stopped.  There don't seem to be any vacuum leaks anywhere and I believe the oxygen sensor was replaced.  What else could it be?

Also, everything in my dash works perfectly, but my gas gage always reads full.  What can I do to fix this issue?

Thanks  

Ok, first, the gas gauge always reading full means your sending unit has probably gone belly up. You'd be better off to go ahead and change the fuel pump while you've got it out of the tank to change the sending unit.

Your idle problem is one of two things, if not both. A fluctuating idle like yours means that either the idle air contorl valve has gone bad, or the MAP sensor is going bad.  If you don't have any vacuum leaks, I'd start by changing the idle air control valve.  

Thanks for the info.  The vette originally belonged to my father, so I asked him about the sensors.  Turns out that he replaced both the MAP sensor and the idle control valve trying to fix the problem.  It has funny idle mostly when it is cold, though I haven't driven it enough to see if it would ever do it hot.  Right now, seems ok hot.  Turns out he also recently replaced the fuel filter.  Makes me wonder if there's a wire loose somewhere now on the sending unit.  

Nah, changing the fuel filter has nothing to do with the sending unit.

On the IAC valve, if they're not installed properly they can lead to idle problems. No knock on your Dad, but I've seen it happen. another thing is to check your vaccum lines, and see if any look suspect and need to be replace. An old vacuum line that collapses under pressure can cause trouble.

If it idles funny when cold but not hot, I'd also check the Mass Air flow sensor. This could affect your cold idle as well.  

Sorry for the delay in responding.  I was having a difficulty logging in.  When I said filter, I meant pump.  Turns out that there is a code 32 that comes up on the vette.  So, how hard is it to replace the EGR valve?  btw it turns out that it only idles up and down when it is cold.  When it is hot it idles high for the first minute of a stop light and then idles down.  

The EGR valve on your '86 is on the intake manifold. It should be towards the back, underneath the runners for the TPI induction.

Before you change it, soak down the bolts with a good penetrating oil like PB Blaster. Sometimes those bolts can stick pretty hard because of the heat.

Still sounds like you might want to change the IAC valve out again also, just to be sure.