Previous owner replaced the alternator and battery before I got it. Car will start but the gauge will read no higher then 11.5, If it sits overnight it will be dead in the morning and wont even turn over. Any common issues with these cars?

 

Would indicate battery voltage only voltage and the Alt is not charging at all .Should go 14V+ on a cold start

Plenty of documented power drains only identified by measuring the battery current draw while pulling out the fuses one by one.
If that does not identify the problem circuit , there are then power feeds direct off the battery to be tested

 

Sure...
the ONLY common maint issue that I've ever heard of was the absence of maintenance.

What DOES happen is that there are LOTS of places for something to stick, short, ground or get chewed thru in a 25 yr old car that's 100% computer managed.

Start by cleaning every electrical connection you have. You WILL need the FSM to show you where the grounds are. Learn the elec system and maintain it accordingly. Electrical connections are extremely sensitive and the level of resistance in the system effects the performance of the engine. Crappy connections can equate to misfiring or dead batteries..take your pick. Most often the poor charge issues come from dirty connections at the jumper post, the alt plug-in, or the alt pos cable contact point. If its got a discharge overnight you need to get your DVM and start checking each fuse circuit to see which has something happening when the key is off and the car is parked. Keep in mind, that there are MANY "hot-always" circuits on a C4 that are NOT managed by the key.
underhood lites
fuel pump relay
IAC
cabin lites
blower motor
??????

could be almost anything but those I have seen.

 

including the Antenna, check it doesn't click on and off while the car is parked.

 

start the car hook a volt meter to the battery what is it reading? should be between 13 and 14 volts or so. if it is not hook voltmeter to large lug on back of alt should be between 13 and 14 volts or so. if it is you are loosing voltaage between here and the battery (fusable link). if you do not check and see if you have 12 volts going into the #1 term that plugs into the alt do this with the key on it you do not have 12 volts here this is your problem if you do you have everything you need for the alt to charge so you need to replace your alt.

 

1 check your battery, check the fluid levels and make sure it will maintain 12 volts overnight (disconnect the battery to make sure)
You can also get the batterys specific gravity checked (correct acid balance)

Note a new battery can be ruined with a massive discharge buckled plates, also fully discharging a battery can result in it not holding a full charge or not being able to charge the battery to 12 volts.

2 The 86-91 alternators are fairly reliable but there are many bad NEW ac delco ones, i went thru 2 with bearings not sized correctly that could not grip the armature correctly resulting in back and forward movememnt which caused the brushes to touch the wrong areas resulting in erratic charging volts to 0 charge. You could tap the alternator with a socket wrench and the volts would rise to 13-14 volts and then not charge. Warranty was no good as they shipping cost overseas was almost as much as the alternator.

I went to a POWERMASTER 86-91 alternator ($160) which has a 140 amp output (std has 105) and also 80 amps at idle, had no problems since. (check the corvette central website) or ebay

Check your antenna motor
I had my antenna motor drain my battery over a week due to the grease drying up and not lubricating causing it to jam.
the antenna armature moves when the antenna is fully retracted to break a contact and stop the motor. because it was jamming it would cut in and out every one to two minutes ... listen for a click at the left rear ..


dont forget the glove box light, under hood light etc which LEESVET mentioned.