Fuel Pump Fails to Prime Intermittently

[Chris Diersen]

Good morning!! Excited to discover this forum, and hoping someone here can help me solve this riddle!!

2003 Mazda B4000. I’m the original owner, 125k miles, and have done all services and repairs on the truck over the years except when the AC compressor failed. Took that to the shop. Truck has been super reliable, until a couple of months ago. Went on a short drive, then an hour later tried to start the truck and it would crank but not start. Ended up having to come back the next day, and it eventually started. This has happened a number of times, and I’ve never been able to figure out exactly what it is I do that gets the pump priming again. I have:

1. Removed the battery cables from the battery for a few minutes, assuming it might be something funky with the circuitry that controls the fuel system. No effect.
2. Unplugged the inertia switch and shunted the wires, assuming it may be intermittently faulty. No effect.
3. Checked all the fuses related to the fuel system. all fine.
4. reached up above the fuel tank and wiggled all the connectors I could get to. nothin.

When it fails, I can hear that the fuel pump is not running, i.e., no high-pitched whir of the electric pump motor in the gas tank from under the truck.

The fuel pump has never, ever, failed in any way while the vehicle is running. It has only failed to prime in the pre-ignition one-second worth of operating the pump does before you crank the engine. The last time it happened, I did all of the above and after all those failed, rocked the truck hard back and forth, and it started. (?!?!) Came *that close* to being stuck at a snowed-in trailhead in the mountains with no cell signal after a hike. So i’m done playing roulette with this intermittent failure and need to fix it.

What initiates the 1-second operation of the fuel pump to prime the injector rails? I cannot find a discussion of this anywhere, and have to assume there’s a relay somewhere? something in the ignition system? a 1-second timer? Some simple computer? Is it possible that the fuel pump operates fine once it’s running but has some defect that causes it to “lock up” to where the eclectic motor lacks the toque to get it spinning (but can keep it spinning once it’s moving?)

I’m stumped, and until I fix it, I can’t really drive the truck any further from home than I’m willing to walk. Ha!

Thanks in advance for any help or tips.

Chris

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