I have been searching the forums and reading manuals trying to figure out what to do next.

When I check the voltage on the plug hole that corresponds with terminal 86 of the fuel pump relay with the key on, i get some very low erratic readings.

But nothing that is strong enough to trigger the relay.

When I start the truck after sitting for a few hours it will run for a very short while. Maybe 30 seconds. Then it will not run anymore or start. I don't hear the fuel pump when I turn on the key and when I disconnected the fuel line from the rail I didn't get but a few drops of fuel.

However now that I'm writing this I'm realizing that I have never checked to see if I hear the fuel pump after it's been sitting, nor have I checked the pressure before I tried starting it. Maybe I should disconnect the battery and then reconnect and listen for the fuel pump again......

Because if the fuel pump is not working, then how is the truck priming it's self? Sheer weight of the gas? It does have a full tank.

Does the voltage for terminal 86 on the fuel relay come from the ignition? Or perhaps the PCM? Can the PCM shut down that voltage and only that voltage if it does not like something that is going on in the fuel system or other sensors?

I traced the terminal 86 fuel relay wire all the way back to the PCM connector via continuity check. However I did not pull the wires off the firewall to check if it trailed off somewhere else. It look as though it's connected to a lot of things according to the control schematic in the Haynes manual. For instance, should I not have continuity between terminal 86 of the fuel relay and terminal 86 of the ASD?

Also, I'm getting full 12.5 volts coming into the fuse box for terminal 85??? I thought Terminal 85 is supposed to be the ground for terminal 86, why would it have voltage. This may be chalked up to my inexperience with electronics, but it seems odd to me that a ground would have voltage.

I cannot read schematics very well or presumably I would know the answers to these questions. Especially considering I stared at the control schematic for hours.

I've tested the ASD, it is getting proper voltage at all times and the relay works fine.

Side note about the relay: I tested it and it its working, however I'm only getting about 68 ohms of resistance between 85 and 86.

Side Questions: is there a trick to checking the fuel pressure if you don't have the test port and you don't have the OEM fuel line connector? Maybe between the fuel rails?

I really feel like I'm at my ends with this, and it sucks because I thought I was going to be able to figure it out.

Regards,

Patrick

 

Pin 86 looks like it gets power from Fuse number 9, in the fuse block in the drivers side of the dash. (light green with with black trace wire.)

 

In a nutshell: B+ is applied through the ignition switch and fuse F9 to the relay coil, and the PCM provides a current path from ground (on the other side of the coil) when it wants to energize the relay. That current path is via the collector of a transistor, so you can't just measure continuity to ground with an ohmmeter. If you really want to know whether or not the PCM is doing the right thing while the relay is removed, stuff a 10k 1/4W resistor between pins 85 and 86 of the relay socket in the PDC and measure the voltage across it. If it's near battery voltage when the ignition is in the Start or Run positions, and near zero when the ignition is in the Off position, the transistor is switching just fine and you can stop thinking about the PCM.

If you suspect the relay, just swap it out to save yourself some time and energy.

I'd be more inclined to suspect a faulty fuel pump, a plugged filter, or a bad pressure regulator. If you don't have a test port, you're going to have to open the system and tee in a pressure gauge. All the usual gasoline safety caveats apply.