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2003 GMC Sonoma Fuel/P0171

Tiny
ARCHITUETHIS
  • MEMBER
  • 2003 GMC SONOMA
  • 2.2L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 140,000 MILES
I have a 2003 GMC Sonoma 4CYL 2.2L with 140K miles. It has had a recurring issue where it is hard to start when the fuel gauge reads 1/4 tank or less. Usually have to knock on the fuel tank to get it to turnover. Recently replaced fuel filter, still the same. I thought that is usually indicitive of a faulty pump that is seizing. Additionally, I am continually battling a P0171 code in the computer. Is this all related to the same issue, or do I have multiple issues (fuel pump, 02 sensor, etc)?
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Monday, March 24th, 2014 AT 10:41 AM

9 Replies

Tiny
JDL
  • MECHANIC
  • 16,098 POSTS
It sounds like an issue with fuel pump. Use a gage and check fuel pressure, 55--62 psi.

Make sure plenty of gas in the tank. There is a service bulletin for the fuel level sending card. The gage will show a 1/4 tank when the tank is empty.
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Monday, March 24th, 2014 AT 12:01 PM
Tiny
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JDL, thanks for the info. Looked into the service bulletin, I don't ever have a stall condition when driving, only see the issue when starting below 1/4 tank - can you explain how knocking the fuel tank "fixes" the issue? I'm just trying to wrap my head around it. I've never heard of a fuel pump being faulty (especially for 2 years as that's how long I've had this issue) - I thought they either worked or didn't work, no in between or low pressure. I will do as you suggested and check pressure - do you think this is also the reason for the P0171 code I am consistently seeing? Thanks!
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Tuesday, March 25th, 2014 AT 5:03 AM
Tiny
JDL
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I agree with you, two years is a long time to have that problem, if it is the fuel pump? When you first turn on the key, no crank, can you hear the pump run for a couple of seconds? If you cycle the key two or three times before you crank it, does it start faster?

If it isn't the pump, I don't know why banging on the bottom of the tank would help? Unless it was a problem with wiring or connector?
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Tuesday, March 25th, 2014 AT 8:24 AM
Tiny
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Can't say I hear the pump do anything over the engine trying to start. I've never tried cycling the key first to see if it starts faster. What would that do/indicate?
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Tuesday, March 25th, 2014 AT 8:34 AM
Tiny
JDL
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Cranking is starter motor action, don't crank, just turn the key till the dash lites come on, no further. Should prime for a couple seconds. No, I can't hear the fuel pump with the starter motor cranking the engine. My hearing is no good, anyway.
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Tuesday, March 25th, 2014 AT 8:45 AM
Tiny
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Gotcha. I will try and see, but am expecting it to prime. I will compare to when at a 1/4 tank as well. I'm really trying to find another issue besides the fuel pump. Aside from the obvsious cost impact, the symptoms just don't consistently point to it. I should've mentioned, I notice no driveability issues, which i've read should be pretty obvious if the fuel pump was degraded. I think I am going to concentrate on the P0171 code first (since I I am going to fail inspection), and hope that fixes the low tank starting issue, if not I'll revisit. This may sound silly, but have you ever heard of a fuel cap creating such issues? Just something I read, but not sure I believe it could be that simple.
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Tuesday, March 25th, 2014 AT 10:28 AM
Tiny
JDL
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I understand. If I suspect a fuel pump, I check voltage and ground at the pump connector. Gray wire is voltage, black wire is ground.

You could probably just jump the relay to see if pump will run, shouldn't need to use the key. I'll check your wiring.
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Tuesday, March 25th, 2014 AT 12:30 PM
Tiny
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As suspected, it primes. Watched the fuel gauge rise as it primed. I will see if it behaves any different when I'm at 1/4 tank. Thanks for the info, I'll be trying to narrow down the P0171 error code for now.
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Tuesday, March 25th, 2014 AT 12:38 PM
Tiny
JDL
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At the relay, use jumper wire between terminal 30 and 87, the pump should run, don't need the key. Terminal 30 is hot all the time.

Use a big enough gage wire for the jumper, maybe 14 gage?
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Tuesday, March 25th, 2014 AT 12:41 PM

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