Fuel pump failure mode and life, mean time to failure (MTF)?

[Gene in NC]

Fuel pump failure mode and life, mean time to failure (MTF)?

'89 525a 166k quit twice on same 100 deg day. Both times restarted and ran well after aprox 3 hr cool down. Didn't have the fuel pressure gauge rigged to the windshield then so couldn't tell if no fuel, but attempted restarts when hot would get engine to fire occasionally but not run.

I could shot gun this problem but that doesn't help wifey's confidence. Need to find/prove the failed part of the system, fuel, which I'm sure it is, or ignition. If I can't do that I'm in for a new car.

So the question is:

Can the pump fail and reenergize itself after cooling down, and

At what mileage/life would a prudent man replace the d thing anyway to avoid a bad experience of "breakdown maintenance"?

Of course I'm running the car now, and of course it has not failed again, but temp has not been over 90.

[Ross]

Sounds like the fuel percolating in the lines. The cool down period allows the vapor to condense. I think a pressure regulator that isn't allowing return to the tank could cause this as well as a weak pump unable to overcome the vapor pressure. Seems to me a pump this weak would show other symptoms like poor high speed running.
A poor electrical connection to the pump could also be the culprit.

[Gene in NC]

Ran the extreme heat test again today. Have been driving around town w/o incident since the 24th.

Today ran from home on Beltline at 65 to 70 to Hillsborough St and back to Six Forks exit ramp before it failed (about 8 mi).

Test rig:

FP gauge rigged to windshield for observation of possible intermittments which have been experienced. Fuel pressure w/in specs before this failure.

Toggle switch to ground the ground post in the fuel relay in case the system loses ground for the pick coil in the fuel relay.

B+ at fuel pump wired to test light on console.

Failed with no fuel pressure and no power to fp/test light. Toggle switch no effect.

Started diagnostics in about one hour. Too soon for cool down.

Fuel pump fuse #23 blown.

Replaced fuse. Power to fp, but pressure less than 20psi then no pressure, but test light shows power to FP.

This seems like a repeat of the first two failures except for the blown fuse.

Is there a failure mode in the pump that could blow the fuse? Fuse didn't blow again in testing that lasted about five minutes.

[The Stretch]

When my pump went, the pressure would be in spec for the first 45 min, then slowly drop off to nothing. If I let it sit over night, i would have another 45 min before the car would die again. The best way to tell is measure the current draw at the pump while the pressure is low. Bentley spec is 5 amps max. Anything higher, especially with corresponding low pressure would be a worn pump.