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Fuel pump not working properly

Tiny
JERRY PLANCK
  • MEMBER
  • 1996 CHEVROLET 1500
  • 4.3L
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 120,000 MILES
Fuel pump not running changed it and still getting no fuel. I'm not getting any volts at pin 85 with the key on. Fuel pump will not run jumping pin 30 and 87. This started as an intermediate problem where I had to cycle the key (fuel pump) off and on before it would start. Fuel pressure would drop to 50 PSI, so I changed the fuel pump, but it didn't solve the problem, so I changed the fuel pressure regulator and the relay. Now I have no fuel pressure at all.
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Friday, April 8th, 2022 AT 4:04 PM

23 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi,

If you are jumping pins 30 and 87 and it won't run, either you have an open between the relay and fuel pump, a faulty pump, or a faulty ground. Those two pins are the secondary side of the relay and are what allow power to reach the fuel pump.

Also, the fuel pump operation on this vehicle is based on a functioning oil pressure switch. Make sure that it is connected.

I attached a wiring schematic below. First, confirm the 20amp ECM B fuse is good in the under-hood fuse box. That is the fuse that powers the pump after the relay is triggered.

Next, reconfirm if you are getting a signal to the primary side of the relay. Note that it will be a very brief signal, so have a helper turn the switch from off to on while you check.

If there is no signal, then you need to go to the PCM and check connector 3 pin 1 (dark green wire with a white tracer) to see if there is power to trigger the relay.

Let me know. Also, note that the grounds for the relay and VCM circuits are tied together at G119 near the thermostat housing. If that ground has failed, it would cause the problem.

Let me know what you find. Also, looking back over what I suggested, I would start by checking the ground. The fuel pump itself gets its ground from the same location (G119). I have a feeling that is where the problem may be especially since it won't run when you jump the relay.

Let me know what you find.

Joe

See pics below.
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Friday, April 8th, 2022 AT 8:54 PM
Tiny
JERRY PLANCK
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  • 14 POSTS
Thanks Joe, I checked the oil switch and its connected which reminded me my oil pressure was running around 75 almost pegged out. The double ground at thermostat seems okay and is reading good. I tested all pins on relay and all are good #30 12.5 #85 12.5 key on #86 tested good ground. This wouldn't be the first part (fuel pump) I bought that was bad or went bad. Not wanting to pull bed off to change fuel pump so if you have any more suggestions, I would appreciate it. Also, the 20amp fuse is okay.
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Saturday, April 9th, 2022 AT 12:22 PM
Tiny
JERRY PLANCK
  • MEMBER
  • 14 POSTS
Getting a resistance reading of 8.1 on pins 86 and 30. Is that good or bad on the fuel pump on my 4.3?
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Saturday, April 9th, 2022 AT 6:58 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Are you checking resistance in the relay box between pin 86 and ground and pin 30 and ground? If that is the case, that is a bit higher than I would like to see. Normally, I want to see 5 ohms or lower.

Are you certain the ground I mentioned is good? If you move the wiring at the connector around, is the connector good? I've seen them fall apart when moved.

I just want to confirm before we have to access the pump. If you are running 12v direct to it and nothing happens, as mentioned, either there is an open between the relay and pump, a faulty pump, or a faulty ground. The next step is to check for power and ground at the pump itself.

Let me know.

Joe
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Saturday, April 9th, 2022 AT 9:51 PM
Tiny
JERRY PLANCK
  • MEMBER
  • 14 POSTS
I double checked ground it's okay, but I'm not getting any volts at fuel pump connection with key on probably because I have no helper so by the time, I turn the key and climb under its off, but get only 10.3 volts when I jump pins 30 and 87. And yes I was checking resistance in box.
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Sunday, April 10th, 2022 AT 11:28 AM
Tiny
JERRY PLANCK
  • MEMBER
  • 14 POSTS
Joe, my new fully charged battery drop down to 11.5 volts overnight. So, I only lost 1 volt at the wire connector at the fuel pump.
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Sunday, April 10th, 2022 AT 2:57 PM
Tiny
JERRY PLANCK
  • MEMBER
  • 14 POSTS
What is the wiring on the fuel pump? I'm getting 11.5 volts on the grey, 7.6 volts on the purple, and black is grounded to the frame. So what is the black and white wire and how do I test it?
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Sunday, April 10th, 2022 AT 4:15 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Jerry,

According to my schematics, the ground is a black wire with a white tracer.

The gray wire you mentioned is the power supply for the pump. The purple wire is for the fuel level gauge.

Is there no black wire with a white tracer?

Let me know.

Joe

See pic below.
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Sunday, April 10th, 2022 AT 7:32 PM
Tiny
JERRY PLANCK
  • MEMBER
  • 14 POSTS
Joe, the black and white wire is what I need to test, do I test for continuity or resistance? I'm testing the plug at the fuel pump and if the black and white is not working (grounded) wouldn't the pump still run with the pump's black wire being grounded to the frame? Can I tie in a new wire to the black and white wire and ground it somewhere?
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Monday, April 11th, 2022 AT 10:42 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Jerry,

Check the black/white wire for continuity to ground. Also, if it has no ground via that wire, the pump won't run. And yes, you can provide a new ground.

According to the schematic, the pump doesn't have a black wire, only a black/white one for ground. The pump gets power from a gray wire. The ground is black/white.

Let me know.

Joe
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Monday, April 11th, 2022 AT 6:33 PM
Tiny
JERRY PLANCK
  • MEMBER
  • 14 POSTS
Joe, do I have to prime my fuel pump since I changed so many things (fuel pump, filter, fuel pressure regulator, plenum gaskets, crank sensor, coil, etc.), And if so how do I prime it?
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Tuesday, April 12th, 2022 AT 12:21 PM
Tiny
JERRY PLANCK
  • MEMBER
  • 14 POSTS
Joe, the black and white wire shows continuity to ground and still doesn't run, but if I splice into the black/white wire at the pump and ground it to the frame it will run. Is there a wire diagram so I can trace the ground system to the fuel pump? Does something send the ground to the pump like a relay or other component that may be bad?
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Tuesday, April 12th, 2022 AT 1:48 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Hi,

I attached the schematics above. If it runs, you have a weak ground that can't handle the load placed on it. At least, that is my theory.

You can do this two ways. First, recheck the grounds at the engine. Actually, remove them, clean and inspect and reinstall. Or you can simply run a new ground for the pump.

Let me know your thoughts. Also, I have the entire powertrain management circuit schematics if you need them. Just let me know.

Take care,

Joe
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Tuesday, April 12th, 2022 AT 8:03 PM
Tiny
JERRY PLANCK
  • MEMBER
  • 14 POSTS
Joe, it runs but now I'm getting a p0300 code and only 50 PSI fuel pressure. Has a lot of miss, what all can make it have low fuel pressure?
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Friday, April 15th, 2022 AT 1:36 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Hi,

I'm glad to hear it's running. What did you do to get the pump working? Did you have to change any of the wiring? If so, did you use a new wire that was of the same gauge?

As far as the fuel pressure, I'm asking about the wiring to eliminate power flow. If that isn't the case, then either the fuel filter or the pump itself is weak. Have you replaced the fuel filter? It's along the side of the frame. See pic 1 below.

Also, the P0300 is a random misfire code that is likely caused by the fuel pressure. Let's get that taken care of first, and then see if the code goes away.

Let me know.

Joe

See pic below.
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Friday, April 15th, 2022 AT 8:01 PM
Tiny
JERRY PLANCK
  • MEMBER
  • 14 POSTS
Joe, try to wrap your head around this, I ran a new ground from the b/w wire at the fuel pump to the thermostat housing bolt where diagram says is where pump grounds, and the pump wouldn't run, so I added a ground wire from thermostat to fire wall and it still wouldn't run. So, I ran the ground from the black and white at the pump straight to the battery ground and it still won't run, but if I touch the bare wire at the pump where I soldered the new ground to the b/w wire directly to the frame it will run, but fuel pressure is still too low. This has me scratching my head. Why does grounding it to the frame work where as grounding everywhere else didn't? And I never cut the original b/w wire I just skinned it back and soldered the new wire to it.
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Sunday, April 17th, 2022 AT 4:53 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Jerry,

Like you, that makes no sense to me, especially if you ran it to the battery ground. It can't get any more direct than that. LOL

Since it does with at the frame, let's keep that. All I recommend is that you used a wire of equal or heavier gauge than the original one. We need to determine why the pressure is reduced.

Was the fuel filter changed? Also, when the pump is running, are you getting 12+ volts to it, or is it reduced?

If you are getting 12v have a good ground, the fuel filter is new, the fuel pressure regulator is new, that leaves one thing, the "new" fuel pump is weak.

Let me know.

Joe
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Sunday, April 17th, 2022 AT 8:13 PM
Tiny
JERRY PLANCK
  • MEMBER
  • 14 POSTS
Joe, I'm getting 12.5v key on during prime of pump and 13.6v while truck is running. Fuel pressure drops from 52 key on to 48 running sometimes as low as 40PSI, and still only runs if I ground b/w wire to the frame by fuel pump. It sounds like I have a vacuum leak but spaying carburetor cleaner all over everywhere motor never stutters and that's the way I was showed how to find leaks.
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Wednesday, April 20th, 2022 AT 1:54 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Jerry,

It sounds like the pump is weak. We know it is getting power as you mentioned. If pressure drops that much (52 - 40), the pump can't keep up with the demand.

Is this a new pump?

Joe
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Thursday, April 21st, 2022 AT 3:37 PM
Tiny
JERRY PLANCK
  • MEMBER
  • 14 POSTS
Joe, yes, it's new, I even bought another one with the same result. As I said in an earlier post, I have four wires off fuel pump three go into harness connector 1 b/w, 1 gray, and 1 purple and 1 black made to bolt to frame. Since the b/w won't run pump even when hook to negative battery post but will run if it is touched to the black wire grounded at the frame. Is it possible that the problem isn't the black and white ground it's the black that grounds to the frame so when I touch the black and white to the frame its making a good or better ground for the b/w ground.
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Friday, April 22nd, 2022 AT 2:32 PM

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