Was recently given a 2005 Tahoe LT 5.3 that had been sitting for a few years (4-5). It ran when parked and was started intermittently. I immediately changed the fluids before doing anything, but after starting it I got a random misfire code. Now, I know this could be many different things, so to save time here is a brief list of what I've already tried:

New fuel
Fuel pump/filter
No leak in fuel lines
New plugs/wires/coils
Intake manifold gasket
All 4 O2 sensors
MAF Sensor
Checked for vacuum leaks
Checked relays/fuses

I'm getting no codes at first. Then after a short idle I'll get P0300 Random Misfire. This free ride is becoming very expensive. Please help!

 

How about the gas? You said you changed fluids, and it has been setting for sometime. I'd suspect bad gas.

 

 

Just here for that meme