I didn't know where to post this but whenever the car nears a quarter tank it starts to stall out and drop its idle when I'm at a slower speed like parking lots or stop signs it starts dying, I can hear what sounds like the fuel pump struggling but it only happens on a few days, if I don't fill up and drive the next day it doesn't do It again. I think it's on days I drive a lot not on a just back and forth home to school day. It's an 89 iroc z if that's helpful in any way.

 

I would wager that your fuel gauge sending unit is less than reliable. Might have less fuel than you think once it reaches a certain level. Only way to know for sure is to fill up when you reach a 1/4 tank and see how much it takes. If it's more than what should be 3/4 of the tank's capacity then you know you have a wonky unit.

If that isn't it the pump could just be tired. A pump in a full tank doesn't have to work as hard as a pump at 1/4 tank because the higher the fuel level the higher the fluid head on the pump intake.

 

I'm battling a similar issue in my 89. I plan to install a new fuel pump and figure out if the tank is baffled when it is apart. My check valve is bad too since the fuel rail won't hold pressure unless the return line is pinched. Anything less than 1/2 tank will stall out under heavy throttle for me. Fuel gauge seems to work based on the amount of fuel it takes to fill.

 

What year is the car? It may or may not have baffles.

 

Perhaps pump gets hotter when fuel level drops and what you're seeing is the signs of a failing pump.

 

Next time it starts struggling, try taking the gas cap off. It may be building up a vacuum in the tank that is causing the fuel pump to have to work harder to overcome....

 

If pinching the return line maintains pressure, it's more likely the regulator leaking though, not the check valve in the pump.

 

No smell of gas through diaphragm vacuum line and brand new diaphragm installed while troubleshooting with no change.

 

It's leaking through the seat or sealing surfaces of the return line, not the rubber parts

 

Ive filled it when this issue happened to me and it took a little over 8 gallons. A few weeks prior it didn't make the sound ever and it took 12 well below a quarter tank so I don't think that's it but I'm assuming 150k miles 33 years later it's probably tired because I'm assuming it's factory

 

Would that also cause it so when I take off the cap for it to hiss loudly because when I fill up it does that usually

 

There is a "ufo" looking device under the car that is a vent.