2004 GP Engine stumbles/hesitates at acceleration - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 12-05-2010, 07:32 PM   #1
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Default 2004 GP Engine stumbles/hesitates at acceleration

I have a 2004 GP GT with 104k. At 40-60 the engine will stumble when accelerating. It gets worse when driving long distances. Last month I replaced the trans filter and put in synthetic fluid. I have replaced the ignition coil packs this week but still have the same problem. I have also recently cleaned the throttle body and had a coolant flush. I replaced plugs and wires about 10k ago. I have not thrown a code yet. Any help on this problem would be appreciated.
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Old 12-05-2010, 07:40 PM   #2
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In most cases a stumble under load would have me looking at the coils. Generally this is caused by weak spark, so that'* coil, plug wires, and plugs. Check them all. You can also check the fuel pressure.
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Old 12-05-2010, 08:03 PM   #3
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I checked the fuel pressure today. Key on, engine not running about 45 psi, with car at idle 55-60 psi. The gauge needle did not level out on this but was bouncing while running. The haynes manual I have says the pressure should be 50-62 psi with key on engine off and 3-10 psi less when running. Not sure if this is a problem having less pressure with car off.

I am going to pull the plugs tomorrow to inspect them.
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Old 12-05-2010, 08:11 PM   #4
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Pull the vacuum line off the fuel pressure regulator and smell inside it. Do you smell gas? If so, the FPR is bad.
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Old 12-05-2010, 08:29 PM   #5
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The fuel pressure regulator is in the fuel tank. Is there a vacuum line under the hood to do this?
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Old 12-05-2010, 08:35 PM   #6
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The FPR is on the fuel rail. You will see it with a vacuum line on it.
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Old 12-06-2010, 03:38 PM   #7
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The fuel rail only has the line coming in for fuel and that is it.
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Old 12-06-2010, 04:05 PM   #8
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If there is only one line, I'm willing to bet that the fuel pressure regulator is down the line a little ways. Likely at the bottom of the firewall. I know that the 3500s used this setup as well.
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Old 12-06-2010, 07:01 PM   #9
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No i have done some searching and found that the pressure regulator is in the tank. Heres one link http://home.mchsi.com/~d.traub/fuel.htm.
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Old 12-06-2010, 07:22 PM   #10
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He'* right, the 2004+ GP'* have a returnless fuel system IIRC and therefore do not have an FPR on the rail.

I'm also leaning toward spark.
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