Car dies on half a tank - Chevelle

My Chevelle recently started dying when the tank gets to half-full. She won’t start again until more gas is added. She’ll run fine (like a rocket) on a full tank, but then dies just when you get to around a half-tank.
The problem started when she was at the mechanics getting the O2 sensors replaced (LT-1 engine), and actually occurred the first time the mechanic took her out for a test drive. I can hear the fuel pump spinning up (external Edelbrock), and like I said I get plenty of fuel on a full tank. There is no vapor lock that I can tell, and the fuel filter has been replaced recently. Ideas?

Does just taking the gas cap off help?

No, there’s no sucking noise that I can tell. Like I said, I don’t think vapor lock is the issue.

Where is the pump located in relation to the tank? The pump should be at or below the bottom of the tank.

Yeah, it’s mounted behind the rear seat below the tank. It ran fine for 3 years before all this started.

How is your pickup tube doing in the tank?

That’s one thing I don’t know about. I’ve never done a rebuild on a gas tank. I’ve been looking around and some people say that the sock around the tube can have issues. Any advice on getting this stuff out to take a look at it?

What year Chevelle is this? Since I assume it’s an old vehicle from the 60s/70s what about the possibility of a tank vent becoming clogged?
At some point a clog could cause the tank to develop a vacuum because of the pump removing fuel from it.

The older Chevelles were usually vented through a pinhole in the gas cap.

He tried the cap off already

Based on the response to texases comment the part about the gas cap is not real clear to me. If the pump is trying to pull a vacuum on the tank that doesn’t mean the vacuum will exist for minutes or hours.
If the engine dies at speed the vacuum could be gone by the time someone coasts to a stop, gets out of the car, and then goes around to pop the cap loose.

What I’d like to know would be if the engine runs fine with a 1/4 tank of fuel and the gas cap lying in the back floorboard. If not, then we need to move on to the oddities such as the possibility of a fuel pickup tube being clogged shut and gasoline being pulled from a higher, and further upstream, rusted out hole in the tube.

Yeah, my test would be to drive it untill it stalls, pull over, remove the cap, and see if it starts.

Also, this is unrelated to vapor lock, which is when the gas boils somewhere (fuel line, carb, etc) and blocks the flow of the liquid gas.

A Chevelle…LT-1 with oxygen sensors. Please tell us more…Injected? Carbureted? model year? What?

I’m confused too. Was there ever a Chevelle with an LT-1 and O2 sensors?

Nope, my '71 got the engine out of a '95 Z-28. 4L60E trans, custom air, custom fuel (the front half, anyway), electric fans blah blabbity blah blah and Hotchkis suspension. She’s been rebuilt twice, the first time by someone else (he put the motor in), and the second time by me after I flew her across a 12 foot median.

Anyhoo, I’m going to siphon most of the gas out of the tank tomorrow and see if she’ll start, if she will, then I’ll put some more in and let her run til she cuts out. THEN I will check the cap for vacuum.

Monitor your fuel pressure while you experiment. When the pressure drops, find out why…Is the pump sucking air? Find the air leak.