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Jerry Newton
Jerry Newton, Chevy Technician
Category: Chevy
Satisfied Customers: 4508
Experience:  ASE Master Technician, L1, Master GM Technician. Over 20 years of bumper to bumper GM experience.
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I have a 2008 chevy suburban 5.3 flex fuel, my oil pressure

Customer Question

I have a 2008 chevy suburban 5.3 flex fuel, my oil pressure gauge showed no pressure so i ordered a new one, just replaced it and now vehicle cranks but wont start and i have a new code po25a, i disconnected the battery and waited a few minutes but no change still wont start
Submitted: 1 year ago.
Category: Chevy
Expert:  Mike S. replied 1 year ago.

I assume the fuel pump doesn't run when you first turn the key to run, is that correct?

Customer: replied 1 year ago.
correct
Expert:  Mike S. replied 1 year ago.

Do you have a voltmeter or 12 volts test light?

Customer: replied 1 year ago.
Expert:  Mike S. replied 1 year ago.

Are you trying to request a live phone call?

Customer: replied 1 year ago.
im good either way
Expert:  Mike S. replied 1 year ago.

let's begin by checking these 2 fuses, the fscm fuse #21 in the underhood fuse block. and the ecm/ign fuse #56.

Expert:  Mike S. replied 1 year ago.

This code sets if the fuel pump flow control module has no voltage to it, or has a short.

Customer: replied 1 year ago.
both fuses have power
Expert:  Mike S. replied 1 year ago.

Did you have the key on? Did you also check both sides of each fuse to make sure none is blown?

Customer: replied 1 year ago.
I checked with key on and yes both sides have power
Expert:  Mike S. replied 1 year ago.

I have a test that says to test for voltage to pin #13 of that fuel pump flow control module when using a scan tool to command the fuel pump on and off. If you don't have a scan tool with that function, I believe you could just use the key and turn it to run and off. remember though, the fuel pump is only supposed to run for the first 2 seconds, so you will need someone else to turn the key.

Disconnect that dark green/white wire at that module and then have someone turn the key to run. Is there voltage there with the key on run, for 2 seconds?

Expert:  Mike S. replied 1 year ago.

Here are the 2 wiring diagrams,

Expert:  Mike S. replied 1 year ago.

You should disconnect that harness at that module when testing.

Customer: replied 1 year ago.
where is the module located
Expert:  Mike S. replied 1 year ago.

Check with the key off, too. There should be no voltage wth the key off.

Expert:  Mike S. replied 1 year ago.

I believe it's back by the tank but I will look.

Expert:  Mike S. replied 1 year ago.

Before I forget this, if using a test lamp

If the test lamp is always ON, test the control circuit for a short to voltage. If the circuit tests normal, replace the ECM.

  • If the test lamp is always OFF, test the control circuit for a short to ground or an open/high resistance. If the circuit tests normal, replace the ECM.

  • If all circuits test normal, replace the FPCM.

Expert:  Mike S. replied 1 year ago.

It's bolted to the frame.

Customer: replied 1 year ago.
my truck was running great before I lost fuel pressure and drove it in the garage and just swapped oil pressure switch, im having trouble linking the 2 together, if the switch is bad and the computer is seeing no oil pressure will it cut the fuel circuit to protect motor?
Expert:  Mike S. replied 1 year ago.

Remove the 2 electronic brake control module shield retainers and remove the shield.

Expert:  Mike S. replied 1 year ago.

The fuel pump should still run when you first turn the key to run and you said it don't.

Customer: replied 1 year ago.
should the fuel pump fuse #20 on fuse block have power during crank or at start up, ive got no power at all
Customer: replied 1 year ago.
should fuse #20 the fuel pump fuse have power I don't have any when key is on nor when cranking
Expert:  Mike S. replied 1 year ago.

fuse #20? for the fuel pump? I don't see that, where do you see that?

Customer: replied 1 year ago.
same fuse block as the 21 and 56 you had me check, its number 20 and is a 20 amp fuse
Customer: replied 1 year ago.
same fuse block as 21 and 56, number 20 and it is a 20 amp fuse marked as fuel pump
Customer: replied 1 year ago.
same fuse panel as 21 and 56
Expert:  Mike S. replied 1 year ago.

Ok, I see it now. Give me a second

Expert:  Mike S. replied 1 year ago.

I am reading about a primary and secondary fuel pump. I was not aware of this.

Expert:  Mike S. replied 1 year ago.

I'm assuming because it's a flex fuel. That is exactly why I try to stay away from flex fuel questions. I am going to opt out now cause I cannot find any wiring diagram, that includes that fuel pump fuse #20.

Expert:  Jerry Newton replied 1 year ago.

Let me see if I can pick up on this. You have a fuse in socket #20, yes? Does it have power for two seconds when you turn the key on?

Customer: replied 1 year ago.
No power when key is on
Expert:  Jerry Newton replied 1 year ago.

OK. This fuse only gets power when the fuel pump is energized, as it's fed by the fuel pump relay. So, it's important to put a test light to this fuse with key off, then turn key ON to see if it's getting two seconds of power before shutting off again (normal operation).

Customer: replied 1 year ago.
I understand and that's what I did
Customer: replied 1 year ago.
Let me recap the situation I drove my truck into my garage this morning does my oil pressure sensor was inoperable I ordered a new one from Chevy swapped it out and went to start it and had no fuel pressure now one of the items you have to do when changing the sensor is pulled the fuel line off out of your way so I press the Schrader on the fuel rail to relieve the pressure took the hose off swap the sensor put the hose back on and now it won't run
Expert:  Jerry Newton replied 1 year ago.

That shouldn't have done anything to affect fuel pump operation.

Let's just start from scratch here. It cranks but won't start, yes?

Expert:  Jerry Newton replied 1 year ago.

Let's get your vin, too, because there are different engines with completely different fuel systems, I need to verify which one you have.

Expert:  Jerry Newton replied 1 year ago.

I don't see any correlation between the oil pressure sensor and this problem. I just don't. So, let's treat this as its own symptom, diagnose it by the book, and if it happens that we end up finding something that had to do with oil pressure sensor replacement, then that will be educational. Otherwise, we have to treat it separately, you'll pull your hair out trying to make the connection.

Customer: replied 1 year ago.
08 suburban 5.3 flex fuel
Expert:  Jerry Newton replied 1 year ago.

I understand that, but the factory service manual does not break it down that way. I need to know if it has an LY6 engine code, as the fuel pump wiring is different for that particular engine (and it's not exclusive to flex fuel). It's confusing, but if I had the engine code, there wouldn't be any confusion at all about it, and I could give you the most accurate information.

Customer: replied 1 year ago.
I understand I'll have to get back with you when I get back to my house I'm out running errands
Expert:  Jerry Newton replied 1 year ago.

OK, I understand. I'll research this further when you get back home and respond here. No hurry on my end, I just want to give you the best service.

Customer: replied 1 year ago.
Thanks
Expert:  Jerry Newton replied 1 year ago.

.

Customer: replied 1 year ago.
jerry, sorry it took me so long to get back to you, had to work and had to put this issue on the back burner....here is the vin number 1gnfc160x8r120725
Expert:  Jerry Newton replied 1 year ago.

OK, so it's still not running.

Did you ever perform the test that the previous expert gave you, regarding the circuit to the FPCM located on the frame rail?

Customer: replied 1 year ago.
i did, grounding the module had no effect