Multiple misfire

Tiny
STIVALE91
  • MEMBER
  • 2002 GMC SONOMA
  • 4.3L
  • V6
  • 2WD
  • AUTOMATIC
  • 142,000 MILES
I have replaced my spark plugs, my wires, my cap and rotor. Got the check engine light to turn off and passed inspection. Then today check engine light came back on and truck was running rough like the multiple misfire came back. What should I do next or is there nothing else I can do?
Friday, February 7th, 2020 AT 5:36 AM

9 Replies

Tiny
ASEMASTER6371
  • MECHANIC
  • 52,796 POSTS
Good morning,

I would check the fuel pressure. Low fuel pressure is the most common cause for this issue.

https://www.2carpros.com/articles/engine-misfires-or-runs-rough

I attached a flow chart for the fuel system fr you to follow and you need a gauge to hook up to test the pressure. It could be the fuel pump or the spider assembly.

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

Roy
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Friday, February 7th, 2020 AT 6:32 AM
Tiny
STIVALE91
  • MEMBER
  • 1 POST
I replaced the coil earlier today and disconnected the battery to reset the code. Now I’m just waiting to see if the code comes back. And I don’t know if it’s a fuel issue because it only runs rough at idle when I hit the gas and let off it’s fine but when it’s at idle it wants to shut off.
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Sunday, February 9th, 2020 AT 11:47 AM
Tiny
JOETECHPRO
  • MECHANIC
  • 705 POSTS
Hey STIVALE91,

Can you please check for engine management fault codes and report back with what you have stored?

Guide to checking for fault codes below.

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

Regards, Joe
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Sunday, February 9th, 2020 AT 11:27 PM
Tiny
STIVALE91
  • MEMBER
  • 1 POST
So I plugged in my truck in and I ended up with five codes Three of the codes won’t harm the vehicle one is a missing left wheel speed sensor not working left wheel speed sensor and ads sensor. The other two codes it’s throwing is P0300 which I know is the multiple misfire and P0174 which says it could be my mass airflow sensor. Online it says if the mass airflow sensor is bad and leaning out the plugs will foul out is that true. Just want to make sure before I spend the 80 and change on the part. Very sorry for the long paragraph thank you for everybody’s help
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Monday, February 10th, 2020 AT 10:52 AM
Tiny
JOETECHPRO
  • MECHANIC
  • 705 POSTS
Hey STIVALE91,

Can you please unplug the MAF sensor and test drive?
Are you still experiencing the misfires?

You can run the vehicle with the MAF disconnected and it will run on a reference value. If this clears up the fault generally the MAF sensor itself is the problem.

Regards, Joe
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Monday, February 10th, 2020 AT 9:31 PM
Tiny
STIVALE91
  • MEMBER
  • 1 POST
I’ll try that on my way into work today.
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Tuesday, February 11th, 2020 AT 11:32 AM
Tiny
JOETECHPRO
  • MECHANIC
  • 705 POSTS
Hey STIVALE91,

Perfect, let us know what you find.

Best of luck.

Regards, Joe
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Tuesday, February 11th, 2020 AT 1:21 PM
Tiny
STIVALE91
  • MEMBER
  • 1 POST
I disconnected the mass airflow sensor drove around a little bit yesterday. Same result nothing got better. I wanted to wait a little bit before I gave an update but I replace the O2 sensor bank two so it’s the downstream sensor ever since I replaced it check engine light hasn’t come on I’m 135 miles in. I looked it up online and they said a faulty O2 sensor can throw off the wrong air ratio to the computer and will foul out your plugs an give you a multiple misfire and will also say you have a mass airflow problem. What a fun lesson lol thank you everybody
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Thursday, February 13th, 2020 AT 9:48 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 51,710 POSTS
Glad you could get it fixed, that kind of problem can be tough. Please use 2CarPros anytime we are here to help.
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Monday, February 17th, 2020 AT 2:02 PM

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