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New Fuel Pump with Sending Unit?


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#1 TexAg09

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Posted 27 April 2020 - 07:04 AM

Hello all! I have a few questions in regards to a 94 Firebird LT1. Ive changed every fluid on this car to include the gas. I had to use an external fuel pump to siphon the old gas through the fuel filter. I replaced with new gas and when I went to start, nothing. I used starter spray and the engine ran for a bit. Clearly no gas is moving. I can hear and feel the fuel pump relay clicking. I used the bypass to test the pump but no fuel comes through the lines out of the filter. I ordered a Spectra Premium Fuel Pump D3908H which comes with a sending unit. Ive tried shooting the wires, but since I have no experience with electrical I dont know if Im doing it right. Ive read some of the other articles that talk about the current at which wire. It seems wires are ok. So my questions are these: a) would siphoning fuel using an external fuel pump through the fuel filter cause the fuel tank pump to fail, b) using the siphoning, would an air pocket cause the fuel pump not to be able to send fuel, and c) is it better to replace the sending unit with the fuel pump even if its not OE like AC Delco? I will spend the extra money if need be for AC Deco parts. I have not I stalled the new pump as Im planning on dropping the tank. My skills to cut the sheet meta with the tank installed would most likely end up in cutting more than just sheet metal. Thanks for any info!

#2 Injuneer

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Posted 27 April 2020 - 09:57 AM

"Siphoning" the fuel through the fuel system with a pump is actually sucking the fuel through the in-tank pump.  Could have damaged the pump, pulled excessive debris against the sock, who knows what kind of damage could be done.  What kind of pump did you use to pull the old fuel through the fuel filter?   It is possible to actually siphon (not pump) the fuel out of the fill pipe.

 

As far as new pumps go, they must be purchased from a reliable source.  The market is saturated with counterfeit Chinese crap.  Some of the best-known pumps, like Walbro have been extensively counterfeited.   I would personally not trust anything from ebay, for example.

 

Replace the filter if you haven'y already done so.  You could have damaged the filter media by pulling fuel through it rather than pushing it.

 

You have to see if 12 volts is reaching the fuel pump connector at the rear of the car, and if the ground for the pump is good:

 

http://shbox.com/1/f...pump_wiring.jpg

 

http://shbox.com/1/f...p_connector.jpg

 

Check for 12 volts on the gray wire.  Red lead into the gray wire, black lead to a good chassis ground.  If you have 12 volts connected to the prime connector under the hood, you will check the wiring, but not the relay.  After verifying 12 volts from the prime connector and disconnecting at the prime connector, you will need someone to turn the key to "RUN" while you monitor the voltage at the rear fuel pump connector.  That checks the wiring from the relay.  When the PCM sees the key in the "RUN" position, it sends a signal to the relay to start the pump.  If the engine doesn't start in 2-3 seconds, it shuts down the signal to the fuel pump relay.  Happens very fast.

 

All this assumes there is not a problem with the PASS-Key (VATS) system.  It is possible to lose the signal the theft deterrent module sends to the PCM to allow it to start the pump.  A problem with that signal will set DTC 46.  Since you hear the fuel pump relay clicking, you don't have that problem.

 

To check the ground at the rear fuel pump harness connector, set meter to low ohms range, one lead to black wire at connector, other lead to a good chassis ground.  Reading should be very close to 0 ohms.


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#3 TexAg09

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Posted 27 April 2020 - 07:35 PM

I used a cheap universal 12v fuel pump into a 5 gallon gas container. It worked really quickly. I also was sure to use the old fuel filter for this and installed a new one afterwards. I tried sticking a couple of different siphoning tubes down the filler neck but I ran into some sort of blockage which I read online as an anti-siphon device. I didnt have the means at the time to lower the gas tank, nor did I remember how much gas was in it.

I went back and checked. The fuel pump is made from China. I bought it through Autozone. It has a sender unit attached to it as well. My thought here was since Im pulling it out, might as well try and replace all of it? Maybe I better stick with just replacing the fuel pending the electrical test.

I was able to use the bypass line to power the fuel pump directly. No fuel buts sparks were seen when I pressed the positive cable to the positive battery terminal. It almost looked like the wires were trying to fuse itself to the battery terminal. I thought that was strange. The car has not run in 7 years. Would sitting that long potentially cause the fuel pump to become corroded?

#4 Injuneer

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Posted 28 April 2020 - 09:13 AM

You need to do the electrical checks.  Sounds like there may be a short to ground in the system, producing sparks when you tried to power up the prime connector.    Or the fuel pump is drawing excessive amps.  Yes, 7 years immersed in fuel, and not running could have damaged the pump to the point where it is locked and drawing excessive amps. Surprised the fuse for the fuel pump/PCM has not blown if that's the case.  You should really include a fuse (15-amp suggested) in the line between the battery positive and the prime connector.  

 

 



#5 TexAg09

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Posted 29 April 2020 - 08:57 AM

Thank you for the advice and pictures. I wont be able to get around to it til the weekend. Ill update with what I find out with the electrics. Ill also return the new pump and exchange it for the ac Delco one. Again thank you for direction. Hopefully its just the fuel pump and an easy replacement to fix the issue.




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