Cylinder 3 mifire, I have new plugs and wires. Haven't changed any coils yet, but I hear it might be a valve problem. Should I change a coil first? When I hooked a timing light to it it showed it had a spark, but it seems to a hit and miss problem, it was running smooth when the timing light was put on it. Do the coils go out slow sometimes? I put some JB engine tuneup in the oil that seemed to help for a few days.

Here's a link on how to replace your coil pack.

Have you run a compression check on that cylinder? Have you verified the fuel injector is working?

Haven't played with any modern Caddies,but guessing this is a distributorless setup.If you're not getting a misfire code,it's probably a weak but not dead cylinder.If you have plug wires,you can take a standard test light,grease the tip,and slip it between the boot+wire to ground out one cylinder at a time to find the weak one(will cause least amount of idle drop off).If you find a weak one,try swapping the coils to see if it goes with the coil or not.If you don't have plug wires(coil on plug set up), disconnect/reconnect the primary to find the culprit cylinder.If swapping the coils doesn't change anything,you may have low compression in that cylinder(leaky rings or broken valve spring?)(Or maybe next try swapping injectors)

Vehicles: Any vehicle with the above symptoms.

You have a bad spark plug wire on #3, or possibly a clogged/malfunctioning fuel injector on Cylinder #3. Or, less likely, some of the other possible culprits mentioned below (exhaust valve/O2 sensor/Catalytic converter/ECC).

I'm presuming the OBD-II DTC code from the Expedition's computer is P0303.
Always try resetting the code and see if it returns - your new spark plug and new coil pack may have fixed the problem, but if the P0303 code isn't reset, it'll remain stored,and the CEL (Check Engine Light) will remain lit.

If I were standing in front of the vehicle right now, the 1st thing I'd do is put my "mechanic's stethoscope" on the fuel injector with the motor running to verify correct operation. Why would I do this first? Its free, and informative. You'll hear both the explosion in the cylinder, and the "tic" sound of the fuel injector solenoid as the injector operates. Auscultate the properly functioning cylinders first, so you know what to listen for.

If all you have is a medical/Sprague-Rappaport stethoscope. remove the auscultation bell, and replace it with a 12-inch (36 cm) long wooden dowel or solid/hollow metal rod as a substitute. Sound will transmit just fine through the wood/metal. Prefer metal, if you have a choice.

Here's the "official" P0303 write-up:
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P0303 Code - Cylinder #3 Misfire
Technical Description

Cylinder #3 Misfire Detected

A P0303 code means that the the car's computer has detected that one of the engine's cylinders is not firing properly. In this case it's cylinder #3.
Symptoms
Symptoms may include:

the engine may be harder to start
the engine may stumble / stumble, and/or hesitate
other symptoms may also be present

Causes

A code P0303 may mean that one or more of the following has happened:

Faulty spark plug or wire
Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector
Burned exhaust valve
Faulty catalytic converter(s)
Running out of fuel
Poor compression
Defective computer (ECC)

Possible Solutions

If there are no symptoms, the simplest thing to do is to reset the code and see if it comes back.

If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. In other cases the problems were faulty fuel injectors.

Hello mikew564.The first diagnosis was number 2 or 3 cylinder not firing. Once you changed the coil, the check engine light went off. When the light came back on again, did you get it diagnosed again to find out if the problem was the same?... 2 or 3 cylinder not firing????

First of all, there is NO extra wire/ground on the coil/ignition wiring (see images below). The spark plugs and ignition plug wiring can be checked for arcing, corrosion, resistance and can be checked for cracks in the insulators or worn electrodes (and spark gap) by a competent mechanic.
If you are having the same issue (2 - 3 misfire), there are several techniques for eliminating the plug wires or spark plugs. Spray water on the plug wires to see if they are arcing. Swap plugs from cylinders 2 & 3 with cylinders 1 & 4. If you still have a 2-3 misfire, check/inspect/examine the coil connector pins & coil harness to the coil(s) themselves.

The issue is with only one cylinder, the 2nd cylinder to be specific. You may have to swap coils to check if the issue transfers to another cylinder. If it does you will have to replace the coils. You may have to inspect the spark plug if there is oil. It may also be a failed gasket.

I would start with checking your plugs and plug wires. Check the wires for worn or burnt spots. You can take a little spray bottle and mist your plug wires. Then see if it starts to miss or you might hear it arcing which will be a snapping noise. I believe your car has a coil pack that all of your wires go to. I would start with the plugs and wires if they haven't been changed in a while.

CYLINDER 3 MISFIRING. MEANS NOT FIRING .CYLINDER 3 IS MISSING FIRE.COULD BE FAULTY OR DIRTY FUEL INJECTOR.YOU COULD HAVE BAD SPARK PLUG OR COIL AND WIRE.CLEAN FUEL INJECTORS CHANGE PLUGS AND COIL TO NUMBER 3 CYLINDER .WHEN YOU DO A LITTLE TUNE UP.THAT WAY YOU WILL KNOW IF YOU HAVE A MECHANICAL PROBLEM TO 3 CYLINDER LIKE LOW COMPRESSION.LEAKING INTAKE AT 3 CYLINDER OR BURNED OUT VALVE.

either a coil ,wire, fuel injector. if you have wires check for white spots for arcing out on ground.

hi there, if you can feel the misfire at an idle but not at a higher rpm than the fuel pressure regulator is bad im working on one right now that has mifire code and random misfire codes coming up