The engine is misfiring I changed spark plugs

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.

Thank you for the update.

Glad you could get it fixed, that kind of problem can be tough. Please use 2CarPros anytime we are here to help.

Cheers, Ken

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.

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

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.

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

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.

Have the ignition module tested at most autoparts stores for free. If good you may have bad crank/cam sensor.

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.

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