So my 99 4x4 auto blazer won't start when it rains. Just sits there and cranks. Tested and replaced most of the ignition system, which needed to be done anyways. Ran good until it rained again. Dropped the tank, cut lines, got gas sprayed in my face, much fun. So I got a fuel pressure gauge, and went on more diagnostic adventures. This is what I know so far: The fuel relay works the first few seconds of ignition on, as it should. No fuel pressure when trying to start or running. If I jumper the fuel pump, it will start and run fine. So it's not any restrictions in the fuel system. My next thought is the oil switch. But, I have a one wire oil sensor, that reads fine on the dash. If I unplug it, it pins the gauge. The only schematics I can find for my truck is a 3 wire oil sensor and fuel switch. So what turns on the fuel pump when running? Is there another sensor/switch? Is it the ECM reading the gauge?

 

Yuo know there are both fuses and relays for the pump

 

Both ECM fuses are fine. The pump works with the relay jumpered, so wouldn't that mean the fuel pump fuse is OK?

What I'd like to know is how the pump runs while the engine is running. It seems there was a design change from a 3 wire to a 1 wire oil sensor.