Stalling when fuel below 1/4 tank

[1fatcat]

I worked on a LeSabre once that stalled only on exit ramps and only when the tank was below 1/4. Turned out to be broken fluid baffels inside the fuel tank. The fuel was being pulled to the side of the tank and away from the centerally located pump by centrifical force...believe it or not. The problem was very evident once the tank was dropped and I could see the broken baffels laying in the bottom of the tank. I figured it out by using a fuel pressure guage taped to the windshield and noticed the pressure fall to zero when taking a hard, long, higher speed turn...like a highway exit ramp.

[enslow]

Thanks 1Fatcat. I've got a fuel pressure tester now. I'll stick it on one of these days and check the fuel presure. If the pressure seems on the low side I guess I'll have to drop the tank for sure this summer if it can wait. When I drove it last month at a 1/4 tank, it seemed as if it might stall just as I would try to accelerate.

[enslow]

I checked the fuel pressure on the 97 and got 45 psi with the ignition on, engine off, 40 psi with the engine running. The FMS specifies the pressure be 48-55 psi with the ignition on, engine off, and slightly lower with the engine running. I did not do an acceleration test to see if the pressure drops.

Does anyone have any experience that suggests fuel pump failure is imminent?

[1fatcat]

Sorry to say, but the test gets much deeper. You need to test the "dead head" pressure to determine if the pump is capable of making the pressure. That means you need to hook a pressure guage on the fuel system BEFORE it gets to the regulator. The pump should be able to produce at LEAST 60 psi unregulated...look for pressures more like 70 psi unregulated.

If the pump can make those pressures, look towards the fuel pressure regulator. It may be bleeding pressure off too low.

Also, the supercharged engines have a 2 stage fuel pump. The voltage going to the pump is increased under WOT conditions, so the pressure changes from idle to WOT because the PCM sends more voltage to the pump.

[enslow]

Thanks 1fatcat,

Well, I've looked at my FSM for further tests. I would have to buy more tools and adapters to do this diagnosis properly at this point. As a home mechanic and I've got to put a stop to buying tools sometimes. So, all I know is that my fuel pressure after the fuel filter change is 5 psi below what's specified, and there are at least 4 or 5 things that can cause the fuel pressure to be below spec.

I guess what I need to know:
1) Is the pressure low enough that I need to get it into a shop right away to have the problem diagnossed properly?

Keep in mind that before I installed the new fuel filter, it was stalling intermittently when the tank was 1/4 full or less. I did not measure the pressure before the filter install, so I don't know what effect the filter change might have had on the pressure. It has not stalled since, nor do I feel any hesitation in the acceleration.

2) Are there other simple things that I could or should consider doing as a matter of good preventitive maintenance with respect to the fuel system?

[1fatcat]

You can buy a brass "T" from Menards (under $2 bucks) and hook the guage in-line with some cheap fuel hose and hose clamps. Then watch the pressure while driving the car hard. If it falls below spec at WOT, then look further into the fuel system. If it stays within spec, then look elswhere.

[KL Mallender]

I'll kick in a few possibilities for stumbling with 1/4 tank and less, especially when making a right turn,
I have seen many a fuel sending unit read incorrectly when on lower levels, some were wrong from the factory and previous owner never let it go below a quarter tank, but was always amazed how good the mileage was after filling up, lol.
But most of the time it goes back to when the fuel pump was replaced, during the installation the float rod is bent/distorted giving a false reading, I have personally had this happen to me on my 91 Park Avenue.
I bought the car disabled and after driving it a few weeks, car would actually stall if I hustled around a corner(Michigan Stop) if guage was at 1/8 remaining, I tested the pump for volume and pressure through the return line, all was good untill 1/8 tank, then it would start pushing air bubbles through, flow stopped at a needle above empty.
The fix was to drop tank, remove sending unit and add arc upwards(bend) to the float arm, car no longer sputters when low fuel light comes on, and guage reads a couple of needles below empty before flow is interupted.
Has been working fine ever since(2001), also doesn't stay on full after fillup for 200+ miles either, lol
Good luck.

Kevin.

[enslow]

I've done some further testing, and compared the 97 and the 99 Lesabres. Here's what I have so far:

Key On, Engine Off
99 Lesabre: 50 psi
97 Lesabre: 48 psi

I wouldn't call that significantly different.

Engine Running at idle, fuel regulator vacuum attached.
99 Lesabre: 45 psi
97 Lesabre: 41-42 psi

Engine Running, throttle increase rapidly to about 1500 rpm.
99 Lesabre: pressure increases to 48 psi almost immediately, fluxuates back a little.
97 Lesabre: pressure increases maybe by 1 psi, then drops by as much as 3-4 psi almost immediatly.

Engine Running at idle, vacuum removed from pressure regulator.
99 Lesabre: pressure snaps up to 50 psi immediatly.
97 Lesabre: pressure snaps up to 48 psi immediatly.

Bleed down test:
While I didn't time anything, the pressure didn't seem to be dropping rapidly after shutting the engine off.

I know I didn't go WOT as 1fatcat told me to. I just don't have the time to set up the apparatus in such as was as I can see it while driving. The only way I would be able to see the head pressure would be to install a restrictor on the fuel return line. However, from what I've learned here and elsewhere, the fuel pressure should never drop like it did in the snap throttle test, and the result of that test suggests the fuel pump is indeed on its last legs.

I guess my question right now is this: are my test results still inconclusive? Or do I have enough evidence that I should be replacing my fuel pump this summer? Or do I have enough evidence that I should be replacing my fuel pump tomorrow?

[enslow]

...But most of the time it goes back to when the fuel pump was replaced, during the installation the float rod is bent/distorted giving a false reading, I have personally had this happen to me on my 91 Park Avenue.
Kevin.

Thanks Kevin. I don't think that is the issue with our 97 Lesabre. We keep and eye on both the odometer and the fuel guage. In my Lesabre I can get over 500km on a tank. I could feel her car (the 97) coughing when starting from a stop near the 400 mark. She usually fills up at this point (400km), and is probably adding about 50-55L. So I doubt the tank is empty. Could the float bar be bent? I suppose it's possible, but we're not running to empty when it stalls out.

[KL Mallender]

Hi Larry, when driveability issues arise from consistant conditions( ie: low fuel level ) I would test the vehicle in question for fuel pressure and condition, pressure test is made at the rail tap before regulator, condition would be looking for air bubbles in the return line coming off the regulator.
I've seen this happen with all makes of cars, not just the C or H platforms, it may be as simple as a pump not fully submerged and drawing air even though the tank has fuel in it, but if the pump is sitting too high it thinks and acts like tank is dry.
Test fuel pressure and check return flow with tank at higher levels, then again when low or problem returns, I'm betting the problem is related to pump/sending unit.
Good luck,

Kevin.