Check Engine Light (CEL) has been on a couple of days on my 2001 Mercedes E-430. I pulled the codes and they are as follows:
P0400: Exhaust Gas / P0308: Cylinder 8 Misfire.

The car seems to drive fine. I did have an AC repair (Compressor, condenser, fans, etc). Also, I filled up at BJs and wondered whether the fuel may have been the problem. Initially, I had the shop that did the AC repair clear the code and i put an additive in the gas tank and filled up with Chevron Premium fuel. I then drove the car 50 miles at interstate speeds. The check engine light stayed off for about 10 days. Now it is back on. Any suggestions?

All the best,

John T.
Newnan, GA

 

I would look at one of three things on cylinder number eight:

-Spark plugs (gap should be at 0.039" or less)

-Spark plug wires

-Ignition coil

Start with plugs and wires; they are easiest to change. Next up, ignition coil. If none of the previous fix the issue, you're gonna have to dig deeper.

If the spark plugs replacement fixes the mis-fire, you might as well change all of the plugs as the others will not be far behind in failure.

 

How old are spark plugs?
SP can last very long time on those engines and having 2 per cylinder makes misfire rare, but could happen.
Still my first suspect would be ignition coil.
Swapping them between cylinders would confirm the problem.

 

for the P0400: Exhaust Gas cleaning the EGR tube has worked for me. Also you could try clearing the codes and seeing what comes back.