'90 Silverado electric fuel pump trouble

[rickcurtisjr]

I have a '90 Silverado 1500, 5.7L TBI that won't start. It ran fine the other day and stalled. Since then it won't start, but it turns over fine. I've checked, and I have spark. I think the problem is in the wiring for the electric fuel pump. I have no power to the fuel pump relay or the 20 amp fuse located on the firewall. When I run a jumper directly from the positive battery terminal to the fuse, the pump kicks on and runs continuously until I disconnect it, so I know the pump and relay are good. Plus, I had the tank and pump replaced a short time ago. Any ideas would be greatly appreciated.

[rjeffery]

Pick up a Hyanes manual. There is a procedure in there for troubleshooting this problem. I think it does tell you what the problem is if you jump the relay like you did and the pump runs, but I dont remember what it is off the top of my head.

[bracketshark]

I'm just guessin here, Ignition switch.

[2000CAYukon]

I have a 90 GMC Sierra and I also have the factory manuals. There is a oil pressure sensor at the back of the engine. If it thinks there is not enough oil pressure, it cuts off power to the fuel pump.

I would look at this sensor since it could be your issue. In the back of the engine by the dist you will find 2 oil pressure sensors. One is for the gauge and one is for the fuel.

Off hand, I can't remember which one is which; however, try this. Pull the electrical harness off one of them. Jumper the 2 wires together. If it starts, you found the problem. If the oil gauge is pegged or reads 0, try the other sensor.

//2000CAYukon

[rickcurtisjr]

Okay, here's an update. I tried replacing the fuel pump relay because it was only $24 and took 10 minutes to change. Still won't start. I have the Haynes manual and it's very ambiguous about troubleshooting the fuel system. It refers you from the troubleshooting guide to chapter 4 ( fuel system ) and from there to chapter 12 ( electrical system ). The wiring diagram is sort of confusing to me since I'm not an electrical engineer. All I need to know is how the power feeds from the battery to the fuel pump so I can trace the wiring. Next maybe I'll try the oil pressure sensor since it's the only other component in the system that can interrupt power to the pump. Thanks to everyone for your help and yes, I'm still open to ideas.

[webbch]

I think the problem is in the wiring for the electric fuel pump. I have no power to the fuel pump relay or the 20 amp fuse located on the firewall. When I run a jumper directly from the positive battery terminal to the fuse, the pump kicks on and runs continuously until I disconnect it, so I know the pump and relay are good.

Since the pump runs continuously when you jumper to this fuse, this would imply that under normal circumstances w/o the engine running, there would only be power to this fuse for ~2 seconds after you turned the ignition switch to the "on" position, wouldn't it (since the pump normally operates briefly when you turn on the ignition switch)? Is this how you tested for power to that fuse? I don't have a schematic to verify this must be the case, but assuming everything is working properly downstream of the fuse, this would seem to be the proper test to conduct.

Also, according to my FSM, the oil pressure switch forms a backup circuit to the fuel pump, in case the fuel pump relay isn't working. Scenario: fuel pump relay not working -- engine cranks over until it builds up a minimum of 4 psi of oil pressure -- oil pressure switch closes and sends power to fuel pump (assuming oil pressure switch is working) -- result = truck starts and runs fine, but takes longer to start since it doesn't have the ~2 seconds of fuel pumping prior to cranking. Obviously a bad oil pressure switch (or physically low oil pressure) could cause a no-start or brief-start-then-dies condition.

Just a dumb question, I'm assuming you've looked at the injectors and noticed that they weren't spraying fuel while cranking? The next thing I would really want to look for is a lack of fuel pressure (9-13psi) to really ensure a fuel delivery problem, and not some electrical problem at the injectors or other such silliness.

[Blue Bowtie]

webbch is correct. No domestic GM vehicle uses the oil pressure switch as a fuel cutoff device, or at least throught the 2003 model year, and possibly until today.

You can "discover" that for yourself by studying the schematics, or just simply reasoning it through.

Presuming that the oil pressure switch WAS a safety shut-off:

1. In order for the switch to effectively shut off the fuel pump, it would have to interrupt power to the fuel pump either before or after the pump relay contacts;
2. After having been parked for hours, an engine would have NO oil pressure;
3. Even if the ECM operated the pump relay, the fuel pump COULD NOT run without oil pressure, so the engine would NEVER start.

It's an issue which probably needs to be an episode of "Myth Busters" or get more exposure somehow. Far too many people misunderstand the system, and believe the urban legend of the oil pressure switch as a safety shut-off for the fuel pump. From 1983 until at least 2003,that simply isn't the case.

[bigj-dog]

I would lean towards the ignition switch too. It's not as common a problem on these as it is on the '94-'98 models, but I have seen a few. I just replaced one on a '91 that always took forever to start. It took a starter burning up to figure out the problem.

[webbch]

No domestic GM vehicle uses the oil pressure switch as a fuel cutoff device

That's interesting to know, because I'd read that the oil pressure switch functioned as a fuel cutoff device numerous times here and other places, but could never verify it in my FSM.

That myth probably got started by someone who had a bad fuel pump relay, in which case the oil pressure switch WOULD function as a fuel cutoff if the oil pressure got low.

[rickcurtisjr]

It's not the oil pressure switch.

[webbch]

I don't think we were necessarily implying a problem with the oil pressure switch - just got side tracked. Any updates?

[jdl]

I'm not sure if I'm looking at the correct wiring diagram? The wiring colors to the relay show, green wire with a white tracer, that wire is voltage for the coil side of the relay, controlled by the PCM. Black wire with white tracer, that is constant ground for the coil side of the relay. Red wire, that is for the prime connector, find that connector, run B+ to the connector and the pump runs. Orange wire, that carries voltage to the load side of the relay, that circuit shows to be fused by 10 amp ecm B fuse, the oil pressure switch in the fuel pump circuit gets its voltage through the same fuse. Gray wire, carries voltage to the fuel pump when the relay is energized. That wiring circuit changes color after a ways, to tan/white, I believe. If your wiring colors don't match-up, then I'm looking at the wrong diagram.

I believe that 20 amp fuse you mentioned, carries voltage to the hot fuel module, goes hot when you turn the key on.

If that oil pressure switch in the fuel pump circuit has voltage and is working correctly, when you crank it enough to build oil pressure and the switch closes, the pump should run. It won't matter what the relay is doing.

[rickcurtisjr]

I replaced the oil pressure switch and it still won't start. Next I'm on to the ignition switch, which is only $12 and in stock. By the way, no fuel from the injectors and even tested the 20 amp fuse as the key was turned on, and no power. I had power at the oil pressure switch, which led me to believe that was the problem, but unless the new one's bad, that's not it. Made sure there was spark after reinstalling the distributor just in case, and that's ok.

[webbch]

Don't have a schematic in front of me, how does battery power get to the relay contact (not the coil). It would be logical that it would depend on the ignition switch position, but can't verify w/o a schematic.

If schematic shows this dependency, then I would test the ignition switch to see if it was the culprit.

[jdl]

There appears to be two oil pressure switches on this vehicle, one for the lite or gage on the dash and the other switch is in the fuel pump circuit. The wires going to the switch in the fuel pump circuit are gray and orange. You can take the connector loose, put a jumper between the two terminals on the connector and the pump should run.

That cheap wiring diagram I was looking at, doesn't show the circuit going to the fuse. It shows the circuit going from the fuse to the load or switch side of the fuel pump relay. If that fused circuit is hot all the time, I don't believe it is controlled by the ignition switch. If that circuit goes hot only in the crank or run position, then I believe it is controlled by the ignition switch.

I did look up a better wiring diagram on a similar vehicle, different year. The circuit going to the load side/ switch side of the fuel pump relay is hot all the time. Because the other diagram on the vehicle in question didn't show the circuit going to the fuse, I can't say for sure.