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I'm new to the forum and really hope I'm posting properly. I have a 2003 Impala with 3.4 engine. When I purchased the car, it ran fine for a day and a half then starting missing horribly at idle, cutting out as if I turned the key off while driving then cutting back in hard, and will often rev itself up. When it's acting up, if I bump the throttle to keep it from choking out, it Will often run itself up to over 2,000 RPM and stay there for a minute. The codes that were thrown when I started to diagnose it were P1635 (5vref circuit 1) , P0300 (random misfire) and Cooling fan 1 control circuit ( I replaced a bad fuse and corrected that one.) After more tinkering and testing, the P1639 (5vRef circuit 2) code accompanied the P1635. Alldata says that if there is a short to voltage at fuel level sensor or intake air temperature sensor, the P1639 and P1635 will show together. I cant find a short to voltage at either place but I'm not positive that I've tested it 100% accurately. Alldata calls for 4.8v-5.2v at the sensors on the 5vref circuit. I have 5.3v on all of them except the fuel tank pressure sensor which shows 5.2v. I have a cheap meter so I'm not sure if I should call the 5.3v out of spec. The missing doesn't seem to be consistent as it will sometimes run for a solid half hour before acting up. It's been in my garage for a week so it doesn't seem to be related to weather. I noticed along the line that I did have a plug wire firing off from the boot against a bracket. I taped up the boot and later observed it firing off again seemingly from the plug itself or the very bottom of the boot. I taped up the porcelain on the plug and haven't observed it anymore. It has crossed my mind that the plug firing off could cause a "hot ground" situation since it was firing against the engine but I don't know how likely it is to cause the issues at hand. Ive also noticed that the fuel gauge jumps around quite a bit while the car is sitting still. It's not erratic, but it fluctuates often between half a tank and 3/4 Tank. I'm stumped and cant afford to throw a bunch of parts at it that it may not need. I have replaced the IAC for peace of mind. I have sprayed Ether around intake gasket areas and all vacuum hoses I could get to and didn't seem to make any change. I've thought about throwing a fuel pump assembly and plugs/wires at it but I don't know how likely it is that it will correct anything. Any help appreciated.
 

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Obviously you are going to have to change the spark plug wires. Clean and check all of the spark plug gaps while you are replacing the wires, one at a time. Resistance in the fuel pump wires allow for the voltage drop. Make sure those connections are clean and tight. Use dialectical grease on the connections. Seems like the fuel pump sending unit is failing and causing the intermittent fuel gauge readings. Use a fuel pressure tester at the fuel rail to see if the fuel pump is within specs for your model. If there is a separate wire harness in between the fuel pump and the fuel sending unit, make sure that you have it too when you have to replace the fuel pump.
 

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Obviously you are going to have to change the spark plug wires. Clean and check all of the spark plug gaps while you are replacing the wires, one at a time. Resistance in the fuel pump wires allow for the voltage drop. Make sure those connections are clean and tight. Use dialectical grease on the connections. Seems like the fuel pump sending unit is failing and causing the intermittent fuel gauge readings. Use a fuel pressure tester at the fuel rail to see if the fuel pump is within specs for your model. If there is a separate wire harness in between the fuel pump and the fuel sending unit, make sure that you have it too when you have to replace the fuel pump.
Thank you for your reply. Since posting I have replaced plug wires, plugs and fuel pump assembly. It did seem to run a bit smoother with the new plugs and wires, but the problem still persists. It seems that the problem always occurs when the engine is at operating temp but it can sometimes be at operating temp without problem.
 
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