I got a code for a misfire on my 1998 for contour. So I replaced the spark plugs, then the wires, then the ignition coil. The engine is still getting out of time and shaking progressively worse (i.e. when I start it up it sounds somewhat ok, but over several minutes it shakes worse and worse).

Diagnosing Mercedes Benz Misfire DTCs Auto Service World
Thanks

A code P0303 (misfire #3 cyl) 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

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. I've also heard in other cases the problems were faulty fuel injectors.


Keep us updated.

It could possibly be one of the four ignition wires, or the engine is misfiring. A cheap method of finding out would be to try to swap or replace the plug wires. If that doesnt solve it I recommend taking it in to a mechanic to find out what is causing your misfire.

Chris, these cars are notorious for misfires.
The coils may be firing but often they will leak the spark off a cracked tower or internally.
Also check each spark plug carefully for carbon tracks on the outside of the porcelain insulator.
These carbon tracks are caused by repeated ignition sparks traveling down the outside of the spark plug. This is what I see most of the time on these cars.
If you see this, replace the plug that is tracked and the ignition wire as well.
Use di-electric grease on the inside of the new wires to help prevent tracking on the new wires.

This advise should lead you to repairing your misfire.
Thank you for using FixYa.

KL

If there is a misfire, the cylinder that it is misfiring in will be recorded as a code in the computer.

Have the car scanned to determine which cylinder is bad. Then, replace the coil pack going to that cylinder.

Ignition Coil Pack Removal & Installation 2V Engine To Remove:

1. Twist the spark plug wires off of the ignition coil towers. Mark the spark plug wire positions so they can be installed in their original positions.
2. Disconnect the electrical connector from the ignition coil. CAUTION
   When removing the spark plug wires, a slight twisting motion will break the seal and ease removal.
   
   
   
3. Remove the mounting screws and remove the ignition coil.

To Install:

1. Position the ignition coil pack on engine.
2. Install mounting screws and torque to 53 in-lbs. (6 Nm).
   
   
3. Connect ignition coil pack electrical connector.
4. Match previously marked spark plug wires to ignition coil and install.

4V Engine To Remove:

1. Remove the RH cowl grille
   
2. Remove the screws and the water shield
3. Mark spark plug wire positions
4. Disconnect the spark plug wires from the ignition coil CAUTION
   When removing the spark plug wires a slight twisting motion will break the seal and ease removal.
   
5. Disconnect the ignition coil electrical connector
   
   
6. Remove the mounting screws and remove the ignition coil

To Install:

1. Position ignition coil pack. Install mounting screws and torque to 56 in-lbs. (6Nm)
   
   
   
2. Connect ignition coil pack electrical connector
3. Match previously marked spark plug wires to ignition coil and install
4. Position water shield and install screws
5. Install RH cowl panel

It's misfiring. Replace spark plugs and plug wires.

1. Bad or fouled Spark Plug at #1 Cylinder.
2. Bad or misfiring Wire going from the #1 on the Distributor/Electronic Ignition Coil to the #1 Cylinder.
3. Misfiring at the #1 on the Distributor/Electronic Iginition Coil.

a. Unplug the #1 Wire at the #1 on the Distributor/Electronic Ignition Coil.
b. Using an Auto electrical testing: Probe, wire and ground clip electrical, stick the Probe into the opening at the #1 Distributor/Electronic Ignition Coil. Use the grounding clip - clipped to a portion on the engine.
c. Start the Vehicle and check for rhythmic spark.
d. If there is NOT rhythmic spark, the Distributor/Electronic Ignition Coil needs to be replaced.
e. Turn off the engine.
f. If there is rhythmic spark: Reconnect the #1 Wire at #1 on the Distrubutor/Electronic Igntion Coil.
g. Disconnect the #1 Wire from the #1 Spark Plug.
h. Again using an Auto electrical testing: Probe, wire and ground clip. Insert the Probe into the end of the #1 Wire and clip the ground clip onto the engine.
i. Start the engine again.
j. Again Check for rhythmic spark.
k. If there is NOT rhythmic spark the #1 Wire needs to be replaced. (Replace all Wires as a Set).
l. If there is rhythmic spark then the #1 Wire is fine.
m. Turn off the engine,
n. Remove and Inspect the #1 Spark Plug. Clean if necessary and Regap.
o. Reinstall the Spark Plug and reconnect the #1 Wire.
p. Restart the engine.
q, If the code reappears:
r. Replace all the Spark Plugs.
Let me know if this helped or if you have additional information or questions. Feel free to contact me at FixYa.Com.

This Sounds Like a COIL PACK Problem

A multi misfire may mean that one or more of the following has happened:
Faulty spark plug or wires, Faulty coil (pack), Faulty oxygen sensor(s), Faulty fuel injector(s), Stuck/blocked EGR valve / passages, Faulty camshaft position sensor, Defective computer.

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 wire, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the the problems were faulty fuel injectors. 

Random misfires that jump around from one cylinder to another (read: P030x codes) also will set a P0300 code. The underlying cause is often a lean fuel condition, which may be due to a vacuum leak in the intake manifold or unmetered air getting past the airflow sensor, or an EGR valve that is stuck open.

Good luck and hope this information helps, keep me posted, be glad to help you get the truck running 100% again. Not sure if your running a K&N air filter, and if you are there may be a flim of oil on the MAF sensor that is making you run lean. You may want to spray down the MAF sensor with a can of MAF cleaner. 

Check for trouble codes and check spark control module and coils