After I drive it, it begins missing on one or two cylinders?

Tiny
HALYES
  • MEMBER
  • 2003 CHEVROLET TRAILBLAZER
  • 4.2L
  • 6 CYL
  • 4WD
  • AUTOMATIC
  • 197,000 MILES
When I start the car, it runs smoothly when it is cold. After I drive it, it begins missing on one or two cylinders and the check engine soon light comes on. I continue driving and the miss clears, then comes back in minutes. When it first began, it would clear up after a few minutes and be fine for the rest of the trip. It is now doing it more frequently. I changed plugs, coils and ran additives through the system with no result. Could this be wires? What other probabilities would you suggest for resolution?
Thursday, April 3rd, 2025 AT 9:27 AM

5 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 50,584 POSTS
I would definitely replace the plug wires. Also, I have seen weak fuel pressure cause this issue, but we should run the codes to see if any are stored, it could be an EVAP purge valve that is stuck open.

Here are two guides that can help us find out what's going on:

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

and

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

Fuel Pressure .................... 334 - 375 kPa (48 - 54 PSI)

Fuel Pressure Drop (After 1 minute) .................... 34 kPa (5 PSI)

Check out the images (below). Let us know what you find.

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Friday, April 4th, 2025 AT 10:54 AM
Tiny
HALYES
  • MEMBER
  • 3 POSTS
Ken, thanks for the reply. The code is PO302, which means it is in the #2 cylinder (of course I know you know that code). I will replace the plug and see if that resolves it, if it doesn't, I will replace the coil and I will check the wires as well when I am in there. I will get back to you. Thanks again!
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Friday, April 4th, 2025 AT 11:07 AM
Tiny
KEN L
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  • 50,584 POSTS
Sounds good, I would do the wires as well. Also take note of the color of the spark plug and if it is dark or black you may have a fuel injector going bad. Have a great weekend!
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Saturday, April 5th, 2025 AT 10:33 AM
Tiny
HALYES
  • MEMBER
  • 3 POSTS
Ken, I do appreciate the input. I suspected the issue causing the miss in #2 cylinder was electronic. I had changed the plugs and coils about a year ago and have put less than 5,000 miles on it as I don't drive it except for long drives and rainy weather. My other car is an Escalade, and I keep it pristine, so rain and dirt are not frequent for it. Anyway, I took the intake cover off to expose the coils and removed the #2 coil. There is not much to see but the wires (3-16 gauge) seemed to be good. I removed the spark plug and it still looked good, no oil and no sign of misfiring or wear. Just to be sure, I changed the plug and replaced the coil. I started it up when I had it back together and it was perfect, but the car would start and run well when it was cold prior to the replacement. I drove it about 30 miles today and it ran without missing or acting up. The problem is solved, and it was the coil in #2, I am sure. Thank you, again,
Hal
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Tuesday, April 8th, 2025 AT 1:37 PM
Tiny
KEN L
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  • 50,584 POSTS
Nice work Hal, yep sometimes you can't see the failure in electrical parts, you just got to replace them. Glad you could get it fixed. Thanks for letting us know.
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Wednesday, April 9th, 2025 AT 10:26 AM

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