Car struggles driving uphill?

Tiny
JR47
  • MEMBER
  • 2005 PONTIAC GRAND PRIX
  • 3.8L
  • V6
  • 2WD
  • AUTOMATIC
  • 238,935 MILES
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.
Monday, August 18th, 2025 AT 4:56 PM

8 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,137 POSTS
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.
Image (Click to make bigger)
Thumb
Was this
answer
helpful?
Yes
No
Tuesday, August 19th, 2025 AT 8:12 AM
Tiny
JR47
  • MEMBER
  • 114 POSTS
It gets around 57 to 58 psi and then it starts going down
Was this
answer
helpful?
Yes
No
Sunday, August 31st, 2025 AT 3:23 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,137 POSTS
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.
Was this
answer
helpful?
Yes
No
Sunday, August 31st, 2025 AT 9:52 AM
Tiny
JR47
  • MEMBER
  • 114 POSTS
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.
Was this
answer
helpful?
Yes
No
Monday, September 1st, 2025 AT 11:42 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,137 POSTS
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
Image (Click to make bigger)
Thumb
Was this
answer
helpful?
Yes
No
Tuesday, September 2nd, 2025 AT 8:16 PM
Tiny
JR47
  • MEMBER
  • 114 POSTS
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
Was this
answer
helpful?
Yes
No
Wednesday, September 3rd, 2025 AT 1:56 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 54,137 POSTS
Yep, the bad MAF sensor will cause a P0300 code. Good find, I would order a new one.
Was this
answer
helpful?
Yes
No
Wednesday, September 3rd, 2025 AT 1:57 PM
Tiny
JR47
  • MEMBER
  • 114 POSTS
Will do thanks for the help
Was this
answer
helpful?
Yes
No
Wednesday, September 3rd, 2025 AT 2:02 PM

Please login or register to post a reply.

Related Engine Misfire Content

Intermittent Misfire
VIDEO
Intermittent Misfire
Instructional repair video
Misfire Random Code P0300
GUIDE
Misfire Random Code P0300
The world of automotive diagnostics can be complex, with numerous codes and potential issues to keep track of. One of the more common engine diagnostic codes is P0300, which...
VIEW MORE