98 Silverado 5.7 Fuel Pump?

[maxwedge]

This indicates the vacuum pressure sensor on the tank is bad or disconnected or some leak at that area.

[BajaScout]

Cleared the codes and tried it again. This time I get a P0300 (random misfire) code. I replaced the distributor cap and rotor. Runs fine now and no service engine light. I tried to see if there were any fault codes, but the scan tool says all the diagnostic checks had not been completed. Not sure if I need to run the truck for a while of if there is a problem with the scan tool. I did turn the scan tool on while the truck was running. The book said not to do that. Hopefully that did not cause the scan tool to break. I'll try it again in a day or so. I can also double check it with another scan tool.

[BajaScout]

Drove the truck 30 miles tonight and everything seems to run fine and not service engine light. The scan tool still doens't run a complete check yet. Not sure what the issue is there. I'll do a scan with another scan tool to be sure.

[BajaScout]

Final thoughts:

(1) A complete scan was accomplished at around miles. Not sure if this means that it takes 100 miles for the computer to run a complete scan after clearing out all codes. The last check that needed to be accomplished was the EV (Evaporative System) check.
(2) I am really glad I chose to remove the bed instead of dropping the tank. Releasing the the fuel lines with the fuel line disconnect tool was a little bit of a pain due to the way the plastic fuel lines are molded into the fuel pump assembly. Having easy access from the top made it nice. Putting the bed back on took about 20 minutes with pneumatic tools. All 8 bolts were real easy to get to.
(3) Thanks for all the advice! These forums are great. I hope my documenting the process will help others who have similar issues with their truck.
(4) HAPPY HOLIDAYS!

[maxwedge]

The evap test is not done by the pcm if there is over 3/4 tank or under 1/4. When this is done you will see if an evap code pops up.

[BajaScout]

Thanks for the info! I would have never guessed that. It certainly makes sense though.

[old_master]

As you noticed on your scan tool, there are several monitors that need to run and complete their tests. Each monitor requires a unique set of driving circumstances called a "driving cycle" to run and complete its test. When the driver exactly duplicates the circumstances necessary, each test runs and completes its test. This link explains what must be done to run and complete all of the monitors. With practice, you can complete a drive cycle in about 20 minutes: http://www.obdii.com/drivecycle.html

Just an FYI on the fuel pump circuit that runs through the oil pressure switch: Current travels along the perimeter of a wire, not down the center. The larger the diameter, (or gauge) of the wire, the more surface area there is for the current to travel on resulting in less resistance and less heat generated. There are two reasons the circuit runs through the oil pressure switch: One is to allow more surface area for the current to travel on to the fuel pump. Secondly, is to allow the fuel pump to prime the fuel system for the next start. When the engine is shut down, residual oil pressure keeps the fuel pump running for a few seconds. When residual oil pressure drops below approximately 7psi, the contacts in the oil pressure switch open and fuel pump shuts off. That's why the fuel pump runs for a couple of seconds after you shut the engine down.

[jonnik]

Another reason to run the fuel pump through the oil pressure switch is to kill the pump in the event of an accident. When the engine dies, so does the pump and will not create a pool of gas if a fuel line is severed.

[old_master]

Originally Posted by jonnik

Another reason to run the fuel pump through the oil pressure switch is to kill the pump in the event of an accident. When the engine dies, so does the pump and will not create a pool of gas if a fuel line is severed.

There are no safety systems for turning off the fuel pump, (inertia switch etc). When the ECM stops receiving a signal from the crankshaft position sensor, it waits 2 seconds and then shuts off the fuel pump relay. The ECM will immediately shut off the fuel pump relay when the ignition is turned off. In both cases though, the fuel pump remains energized through the oil pressure switch until the oil pressure drops below 7psi. The circuit was not designed to be any kind of safety system. Also keep in mind that not all vehicles use the oil pressure switch to power the fuel pump. On the vehicles that do use it, once the engine is running, and oil pressure is above 7psi, the oil pressure switch is the last thing that turns off the fuel pump.