03 Chevy trailblazer 4.2l is misfiring cylinder 3 changed spark plugs still missfires now I'm going to change coil pack if this doesn't fix problem what my next step

SOURCE: 1998 Cavalier No Spark in cylinders 1 & 4

ignition mod. grounds,pcm or timing belt could all be it but, I would have to look at timing first.

SOURCE: I have a 2002 TrailBlazer with the straight

did you replace only one spark plug? that can cause issues because of differences in resistances between cylinders. also, did the map code stay off after replacing
?

SOURCE: 99 chevy metro 4 cylinder

white smoke is probably steam,do a compression test on that cylinder,and check for any evidence of a blown head gasket,(oil in water or water in oil)

SOURCE: I have a 2001 chevy

Did you replace the ignition wire? If you have replaced the wire litsen for a vacuum leak if your not sure take a can of wd40 and spray it at the intake manifold gasket if starts to run at a higher rpm your problem is here replace the gasket. Missfire codes are not always spark plugs , coils or wirers.

SOURCE: My 2000 Ford Expedition 4.6l had a manifold seal blown and was leaking fluid in the sparkplug sockets. Seal replaced and spark plugs changed, now I am having to change coil packs because Im getting mi

Faulty EGR valves do not cause "false" misfire codes. An EGR valve that is not closing properly can cause misfires (and misfire codes) but they are REAL misfires when this happens.

To understand this, you need to understand what a misfire is and what kinds of things can cause them. It is a very LONG list and I do not have the time or space to get into all that here.

You will also need to understand this if you are going to properly diagnose your misfire codes without wasting a whole bunch of money on parts that you do not need. MANY things can cause misfires other than spark plugs and ignition coils.

I do not know what EGR code you are getting, but I will tell you that in the 37 years that I have been diagnosing and repairing cars, there has only been once that I can remember that I have ever replaced an EGR valve on a Ford engine because the valve itself was faulty. Most EGR codes on Fords are caused by either clogged-up EGR ports or a bad DPFE sensor. Every so often you will find a bad EGR control solenoid or a broken vacuum line, but it is extremely rare that the EGR valve itself the cause. With the exception of leaking vacuum lines and on certain engines, clogged EGR ports (Like the 6 individual EGR ports in the 4.2L engine), none of these causes for EGR codes will cause any misfires.

Also, most of the time when people pull the trigger on the ignition coils on the 5.4L and 4.6L engines, all the thing really needs is new spark plug boots. They get contaminated with oil and water and can cause carbon-tracking on the spark plugs and/or arcing into the spark plug wells instead of sending the spark through the spark plug. It is pretty much the same thing as having bad spark plug wires on a conventional ignition system. You don't throw the coil and distributor away on a conventional system because it needs new wires. Neither should you replace a coil on a COP ignition system that just needs a boot. Some auto makers do not make the boots available seperate from the coil. Ford at least had the insight to understand that people cannot afford the cost of replacing all of their coils just because the boots are defective.