Hesitates when tank is 1/2 empty.
 

Hello again.
Timothy here with my 91 Buick LeSabre.

When I fill up the car, it seems to work fine. Accelerates ok, doesn't hesitate. Fuel pump pressure is 35 PSI.
When the fuel guage gets to 3/4 tank to 1/2 tank, troubles start. When accelerating from a complete stop or under 5 KPH, the car hesitates, stumbles, then throttles up and acts normal until you stop again. When i fill it up, it only takes 30-34 liters, and this is a 63 liter tank..

Any Ideas fellas? spark plugs and wires were changed to AC delco, and beldon wires.
Thanks a lot..'
Timothy :)

im thinking something is busted in the tank.

That was my first thought, that maybe some baffles are missing or something along those lines.

35PSI seems a little low, even for our cars. What is the PSI when the tank is full, and when the trouble starts?


-justin

Hmm! The FSM says that the fuel pressure should be 40-47PSI.

Yep, that sounds about right. Seems as though your Fuel Pump has taken a dump.


-justin

Fuel pressure in the manual is with the vacuum disconnected. The fuel pressure should drop approx. one psi for every two inches of vacuum Hg, maybe 8-9 psi at idle with a good running engine. The 35 psi is about right at idle. If you blip the throttle while idling the fuel pressure should rise in response to the decrease in engine vacuum. The injector works on differential pressure between the intake and the fuel so the spray pattern stays constant. It does sounds like the fault may be with the fuel pump where the greater head pressure of the fuel above the pump works until the pump actually has to lift the fuel or a small leak above the pump is uncovered and the leak gets worse? Tape a fuel pressure gauge to the mirror or windshield to see what happens when the problem starts. Have you changed the inline filter lately?

I think it's not the fuel pump. If the pressure is a bit low (and it sounds like it might be, just a bit), the first thing to throw at it would be a fuel pressure regulator. FAR cheaper than a pump.

Replacing the fuel filter is a good idea anyway, so I'd do that too.

But I'm just not sure that any of the above will actually fix it. I like the idea of being able to see the fuel pressure as you drive, so that you can verify that it is or is not a fuel pressure issue.

I just woke up this morning with a thought: I wonder if the fuel pickup fell off of the pump assembly or something, so that it's not sucking from the bottom of the tank anymore?

Just a thought...

Pump is mounted vertically. Sock is horizontal. If sock fell off the difference would be a a 1/2" at most.