Car struggles driving uphill?

2005 PONTIAC GRAND PRIX
238,935 MILES • 3.8L • V6 • 2WD • AUTOMATIC
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JR47
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The car starts to hesitate/struggle when going uphill to the point where it turns the cruise control off and flashes the cel it'll also do it when accelerating up to highway speeds. It did this before with other miss fire codes and a code for the catalytic converter which i had asked about on here and was told the converter was bad so I replaced it and two spark plugs that I had gotten miss fire codes for and it ran fine for a few days after that but now it's doing it again and giving the p0300 code. What else should I look for or check.
Aug 18, 2025 at 4:56 PM
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STRAILER
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This sounds like the fuel pump is going out when it heats up, lets check the fuel pressure which you can do by following this guide and using the instructions in the images below.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Fuel Pressure (Key ON, Engine OFF) .................... 384-425 kPa (56-62 psi)

You can rent a fuel pressure gauge set at the auto parts store for free. Check out the images (below). Please let us know what happens.
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Aug 19, 2025 at 8:12 AM
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JR47
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It gets around 57 to 58 psi and then it starts going down
Aug 31, 2025 at 3:23 AM
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STRAILER
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Does it go down when the engine is running or just after the fuel pump stops? If it goes down after the pump stops it seems like it is okay. I have seen the coil pack short out on these cars FYI, there is no codes for this problem. Have you replaced the spark plug wires? Also, I would replace all of the spark plugs as well.
Aug 31, 2025 at 9:52 AM
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JR47
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Only after the fuel pump stops, the pump assembly itself is only about a year and a half old same with spark plug wires but I have put a little over 30k miles on it since all that was replaced back then. I did replace all the plugs yesterday. Plugs 1, 3 and 5 were the worst looking and 2, 4 and 6 didn't look too bad but the plug for cylinder 6 had the ceramic part of it cracked. Did gap and tighten them to spec when I replaced them both times. It does seem to run a little better but still has some hesitation like before.
Sep 1, 2025 at 11:42 PM
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STRAILER
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One popular problem with low power issues is a bad MAF sensor, also I would double check the catalytic converter. This guide can help, also go over the engine lower power guides as well.

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

and

https://www.2carpros.com/articles/engine-has-low-power-output

Here is the location of the MAF sensor, if the sensor is weak cleaning it will not help, it must be replaced. Check out the images (below). Let us know how it goes
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Sep 2, 2025 at 8:16 PM
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JR47
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I think it's the maf sensor. I unplugged it after doing some research and drove to work didn't have any issues going uphill where I usually do. Only thing is it gave me the p0300 code again when accelerating up to highway speeds but that was it
Sep 3, 2025 at 1:56 PM
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STRAILER
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Yep, the bad MAF sensor will cause a P0300 code. Good find, I would order a new one.
Sep 3, 2025 at 1:57 PM
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JR47
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Will do thanks for the help
Sep 3, 2025 at 2:02 PM