Fuel pump electrical issue

WESFITZ

MEMBER
2007 GMC ENVOY
6 CYL
2WD
AUTOMATIC
147,000 MILES

History: Vehicle was idling in the driveway when it stalled out. No successful restart since that time.
Checked all electrical to fuel pump, electrical checks out. Replaced fuel pump - no change.
Bypassed the fuel pump(pin 30 to pin 87) relay and pump works. Voltage at the pump is twelve volts.
Pin 86 shows ground. Pin 85 shows twelve volts with ignition in run position.
Pin 30 shows twelve volts.
Dumping a small amount of fuel into the carburetor allows the truck to rumble for around two to four seconds.
I have tested the relay by swapping out with high beam relay, all relays work properly in the high beam slot.

6 Replies

STEVE W.

MECHANIC
13,461 POSTS

Do you hear the pump kick on during the 2 second prime when you first turn on the key?
If you jump the relay so the pump turns on does it run then? I'll guess it doesn't.

Pick an injector and test the pink wire to see if you have full battery voltage on it. If you do you might want to unplug one injector at a time and see if it starts. A shorted injector could cause the PCM to shut off the injectors as protection.

Test to see if the PCM is pulsing the injectors by pulling an injector wire off. Now connect a test light to battery positive and probe the other wire in the connector. While cranking the engine the light should flash every time the PCM grounds the control side of the system.

You said you swapped the pump, have you checked the actual pressure at the fuel rail? You should have 50-58 psi at the rail. I have seen a bad pump out of the box before.

I hate to ask but how much fuel is in the tank and is it good gas and not E85?

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Monday, October 9th, 2017 AT 3:48 AM

WESFITZ

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4 POSTS

I can get the pump to run when I bypass the relay. The flow isn't what I would expect, but I have no baseline to go by. There is no pump prime when the ignition is turned on. I don't have access to a way to test the pressure :(
The initial test, Pin 85 (relay power from the PCM) shows 12 volts with the ignition on. Testing with the rely in place, the voltage drops to zero on Pin 85. This is not what I would expect. I have swapped relays with the same behavior.
My next test will be to reconnect the fuel lines, bypass the relay and attempt to start the engine. I should have the results tomorrow.

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Tuesday, October 10th, 2017 AT 7:34 AM

STEVE W.

MECHANIC
13,461 POSTS

The relay on signal at pin 85 should turn on for about 2 seconds then go off every time you turn the key on. That is the initial prime. That power will stay off until you turn the key off and on again or until the PCM receives a signal from the engine that shows it is running after a cranking attempt.

The pump should shoot a solid stream of fuel. From your description I would check the ground for the pump and the connector pigtail. They can get corroded and cause a voltage drop.

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Tuesday, October 10th, 2017 AT 1:02 PM

WESFITZ

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4 POSTS

I'll check pin 85 again. I know when I was testing I had battery voltage to the pump end of the wires.
The bypass test worked as expected. The engine fired and ran until the ignition was shut off.
Could there be an issue with the PCM specifically related to the fuel pump relay circuit?

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Thursday, October 12th, 2017 AT 9:44 AM

WESFITZ

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4 POSTS

More on the PCM (powertrain Control Module), what other things might this control?
We just previously had to override the shifter release & key lock to shift out of park & place the key into position to remove it.