The engine is misfiring I changed spark plugs

Tiny
GARY BROOKS
  • MEMBER
  • 2004 CHEVROLET SILVERADO
  • 5.3L
  • V8
  • 4WD
  • AUTOMATIC
  • 160,000 MILES
Getting a po300 code, the truck is missing. I changed spark plugs, all coiL packs, and wires and is still doing it mostly after warming up the engine. The spark plugs down one side of engine are burning lean, and the other side are burning rich. Just starting doing it when driving home.
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Sunday, January 14th, 2018 AT 4:12 PM

68 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 42,468 POSTS
Yes, the plugs will act that way when the engine is misfiring. Can you pull all plugs and line then up in order? This will show us which cylinders are misbehaving. I believe you have an injector that is out but we need to see the plugs to start. It sounds like you might need a tune up or have a bad spark plug-coil (common), injector or low compression to be sure which one it is please follow these videos and guides.

https://youtu.be/QTE8k06MzwQ

and

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

and

https://www.2carpros.com/articles/how-to-tune-up-a-car-engine

Let us know what happens and please upload pictures or videos of the problem.

Please let us know what you find. We are interested to see what it is.
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Friday, March 12th, 2021 AT 10:02 AM
Tiny
GARY BROOKS
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  • 15 POSTS
It was an upper oxygen sensor bad.
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Friday, March 12th, 2021 AT 10:02 AM
Tiny
STEVE W.
  • MECHANIC
  • 12,869 POSTS
Thank you for the update.
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Friday, March 12th, 2021 AT 10:02 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
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Glad you could get it fixed, that kind of problem can be tough. Please use 2CarPros anytime we are here to help.

Cheers, Ken
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Friday, March 12th, 2021 AT 10:02 AM
Tiny
TXFIREFIGHTER
  • MEMBER
  • 2 POSTS
  • 2004 CHEVROLET SILVERADO
  • V8
  • 2WD
  • AUTOMATIC
  • 34,000 MILES
I have a 2004 silverado 1500 5.3L that I took to the dealership I purchased it at to have it checked. It was ideling very rough. When they ran diagnostics on it, it came back with P0300 code and missfire counts on cylinders 1, 3, 5, 7. (The entire left bank) Cylinder 1 had 40,000 missfires and they intermittenly decreased to 16,000 on cylinder 7.

This thing idles rough, at road speeds it's fine.
I took the throttle body off and cleaned it really well, checked for vaccum leaks on the intake manifold. No problems I could find there. Spark pllugs are all good.

The service techincian had no clue what to do with it.
I searched on the net and found that this is becoming a problem with this motor, but no one seems to be able to figure out how to fix it.

You guys have any clues?
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Wednesday, May 5th, 2021 AT 1:45 PM (Merged)
Tiny
BRIAN 1
  • MECHANIC
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I've had this problem before, try this info:Check Engine Light with a P0300 due to 2 - 4 cylinders misfiring on the same bank of the engine. DTCs P0171, P0172, P0174 or P0175 may also set for the misfiring bank.

Recommendation/Instructions:
If the published misfire diagnostic does not isolate the cause, perform the following suggestions as necessary:

Perform a fuel injector balance test for all 8 cylinders. If a fuel injector concern exists, it is possible to misfuel an entire bank of the engine, causing multiple cylinders on the same bank to misfire even though the root cause is a single fuel injector.
Inspect O2 sensor connections on the misfiring bank for corrosion or water intrusion. If water intrusion is found on the right bank, it may be due to the AC Evaporator Condensation dripping onto the O2 sensor harness. If this condition is found, reposition and shield the harness to prevent a repeat concern and repair the connections.
Check for excessive exhaust backpressure using the restricted exhaust diagnosis from SI.
Swap the Position 1 O2 sensors side to side to see if the misfires move to the other bank of the engine. If so, replace the O2 sensor. Also perform an exhaust back pressure test on that bank, a plugged cat converter can cause this also. Post back with what you find.
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Wednesday, May 5th, 2021 AT 1:45 PM (Merged)
Tiny
TXFIREFIGHTER
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  • 2 POSTS
Ok, Did like you said, started by flipping the o2 sensors. Bingo! New O2 sensor and bam. Problem fixed! Thanks!
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Wednesday, May 5th, 2021 AT 1:45 PM (Merged)
Tiny
TRAVIS00001
  • MEMBER
  • 1 POST
  • 2002 CHEVROLET SILVERADO
  • 250,000 MILES
My friend has the truck listed above, he is getting a P0300 code (random misfire) so I suggested on changing plugs and wires for starters but still no change. We also checked the compression and changed fuel pump also the filter so everything is good but still the misfire is still there there and I am starting to question maybe the ECM is defective or if the catalytic converter is clogged. I will also add when we clear the code for the check engine light to go off and drive it the light will come back on but flashes.
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Wednesday, May 5th, 2021 AT 1:45 PM (Merged)
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 42,468 POSTS
Hello,

The random misfire is due to mixture problems which is caused by the MAF or a vacuum leak, here are some guide to help you get the problem fixed.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

and

https://www.2carpros.com/articles/how-to-replace-a-mass-air-flow-sensor-maf

The MAF can be bad and not light the check engine light btw.

Please let us know what happens, we are interested to see what it is.

Cheers, Ken

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Wednesday, May 5th, 2021 AT 1:45 PM (Merged)
Tiny
ILOVECHEVY
  • MEMBER
  • 1 POST
  • 2002 CHEVROLET SILVERADO
  • V8
  • 2WD
  • AUTOMATIC
  • 115,000 MILES
Miss-fire in the morning when the engine is cold. Check Engine turns on. The truck runs fine after few minutes of running and it will not miss-fire for rest of the day. The code that I get is P0300 only. I have changed spark plugs back 30,000 miles. I have replaced PCV. I have checked for vacuum leaks and there aren’t any. Someone said that it could be fuel pump but I am not convinced since it only miss-fires in the morning and it runs fine afterwards. Besides the pump is $250.00 and I want to be sure be fore I change it.
Please help me if anyone has any ideas.
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Wednesday, May 5th, 2021 AT 1:45 PM (Merged)
Tiny
MASTERTECHTIM
  • MECHANIC
  • 4,749 POSTS
Although the fuel pumps are very common. i dont think yours is bad. i would check your fuel pressure regulator. its located on the fuel rail where the injectors are under the hood. it has a vacuum line going to it. usually you will find when you take off the vacuum hose you will see gas. if you see gas or the vacuum line is soft, replace the fuel pressure regulator. what happens is when the truck sits, the regulator leaks raw fuel into the intake, causing it to be slightly flooded when you first start it.

This guide can help us fix it

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

let us know what you find.
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Wednesday, May 5th, 2021 AT 1:45 PM (Merged)
Tiny
LONNIE BAILEY
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  • 1 POST
  • 2002 CHEVROLET SILVERADO
  • 4.3L
  • V6
  • 2WD
  • AUTOMATIC
  • 335,000 MILES
Had a multiple cylinder misfire code and right bank running rich code. Truck was very hard to start. Also, truck was burning up rotors and caps. Found somewhat worn distributor, changed it. New plugs and wires. Coil checked okay. Cleared codes. Codes came back, changed upper intake gasket and injector spider valve (upgraded model changes from poppet valves to actual injectors) truck now starts like a champ, but running worse than it ever has. No more multiple cylinder misfire code. Now shows misfire on number six and injector pressure code. EGR cleaned and works well. Found carbon buildup in intake. Cleaned best I could. Not sure where to go from here? Plugged catalytic converter? Dropped valve? Timing chain?
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Wednesday, May 5th, 2021 AT 1:45 PM (Merged)
Tiny
ASEMASTER6371
  • MECHANIC
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Good morning,

I would start by checking the compression. Make sure there is no internal issues with the engine.

https://www.2carpros.com/articles/how-to-test-engine-compression

Also, I would check the fuel pressure. That is the most common cause for a misfire code.

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

Roy
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Wednesday, May 5th, 2021 AT 1:45 PM (Merged)
Tiny
DEVIN MARSHALL
  • MEMBER
  • 1 POST
  • 2001 CHEVROLET SILVERADO
  • 5.3L
  • V8
  • 4WD
  • AUTOMATIC
  • 254 MILES
I have the truck listed above that's coming up code p0 300 which comes back is a multiple cylinder misfire. Not quite sure what to do to fix it. I've replaced plugs, wires and next step will be coils even though that I had them tested and they tested okay. Any ideas? There's no vacuum leaks and I changed my EGR valve as well.
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Wednesday, May 5th, 2021 AT 1:45 PM (Merged)
Tiny
CARADIODOC
  • MECHANIC
  • 33,689 POSTS
What's the true mileage? If it's over 100,000, a real common but elusive cause for multiple misfires on GM vehicles is the injectors. They are not defective, so don't waste your time trying to test them. (That's something professionals never waste their time on). The problem stems from different flow rates.

Most commonly the code will specify one or two cylinders that are having the misfires. In that case, you can switch two injectors, erase the fault code, then see if the code sets for the cylinder you moved the suspect injector to.

When you have a code 300, the misfires are occurring in more than one cylinder, so another good suspect is low fuel pressure.

Here's a link to a related article, if it will give you more ideas:

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

If it looks like injectors are the cause of the misfires, replace them with rebuilt injectors in a flow-matched set. There used to be a specialty shop near the Indianapolis Speedway that rebuilt injectors and matched them. It was owned by Jim Linder, and his last name was part of the business name. It appears he has retired, but I'm sure there are other sources for rebuilt injectors.

The EGR valve you mentioned was another good suspect. Rather than replacing it, you can loosen it, then slide in a thin metal shim to block the ports. If the misfires still occur, you can remove the shim and continue looking at other causes.
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Wednesday, May 5th, 2021 AT 1:45 PM (Merged)
Tiny
COYOTTE82
  • MEMBER
  • 7 POSTS
  • 2001 CHEVROLET SILVERADO
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 120,000 MILES
Vehicle will start and run fine when temperature is normal here in texas. When weather gets cold like in 50s vehicle will have long crank and once it starts it misfires causing CEL but once it warms up it will start and run no problems. Items that have been replaced include: sparkplugs, wires, cap, rotor, Coil, FPR, fuel filter, fuel pump. I need your opinion. PLease advise. Codes that it shows is multiple cylinder misfires, bank1 sensor 1 heater circuit malfunction.
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Wednesday, May 5th, 2021 AT 1:46 PM (Merged)
Tiny
MMPRINCE4000
  • MECHANIC
  • 8,549 POSTS
Have the ignition module tested at most autoparts stores for free. If good you may have bad crank/cam sensor.


https://www.2carpros.com/forum/automotive_pictures/108325_ign_module_1.jpg



The other code is the O2 sensor. These are heated sensors and it is indicating the sensor is not at operating temp. when it is supposed to be.
So the element that heats the sensor, is probably burnt out.
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Wednesday, May 5th, 2021 AT 1:46 PM (Merged)
Tiny
COYOTTE82
  • MEMBER
  • 7 POSTS
It was the camshaft sensor, thanx guys
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Wednesday, May 5th, 2021 AT 1:46 PM (Merged)
Tiny
TMH BURTON
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  • 1 POST
  • 2001 CHEVROLET SILVERADO
  • 5.3L
  • V8
  • 2WD
  • AUTOMATIC
  • 221,000 MILES
Check engine light keeps coming on after I clear codes after a few cycles. Has new coil packs, plugs, wires. But the speedometer went to 100 mph. Couldn't figure out what was going on. Then I beat on the PCM and it went to 0. Still get check engine light. Could the PCM terminal need cleaned?
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Wednesday, May 5th, 2021 AT 1:46 PM (Merged)
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 107,922 POSTS
Hi,

Anything is possible. What codes are you finding when you scan the computer? As far as the misfire, it could be many things such as a vacuum leak, fuel pressure, and with the mileage it could be a stretched timing chain or low compression.

First, let me know if you found any codes. Next, check to see if there are any vacuum leaks on the engine. Here is a link that explains how it is done:.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Next, if there are no leaks, check fuel pressure to make sure it isn't high or too low.

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

Do these things and let me know what you find.

Joe
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Wednesday, May 5th, 2021 AT 1:46 PM (Merged)

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